Health and Social Development, Faculty of (Okanagan)
Health and Exercise Sciences, School of (Okanagan)
DSpace
UBCO
Trotman, Megan
Master of Science - MSc
University of British Columbia
While most reactive balance control studies have used whole-body perturbations, research is limited regarding how whole-body balance adjustments are integrated with continuous voluntary reaching movements when a hand perturbation is encountered. Further, when two tasks are performed simultaneously through a dual-task paradigm, performance of one or both tasks is often disrupted owing to attentional resources being divided. However, it is unclear how reactive whole-body balance responses to hand perturbations adjust when paired with a cognitive-motor task. The purpose of this study was to assess the effects of increased cognitive processing on reactive whole-body balance responses to perturbations of the hand during a continuous voluntary reaching task. Sixteen participants (8 females, 22.9 ± 4.5 years) stood and held the handle of a KINARM, a robotic-controlled manipulandum paired with an augmented visual display. Participants completed 10 trials of 100 continuous mediolateral arm movements while receiving 20 randomly interspersed anteroposterior hand perturbations throughout each trial. Participants moved their right hand at a pace of 60 beats per minute between two targets, with the hand moving towards the next target at each beat (single-task). An auditory n-back task was performed during half of the trials (dual-task), and during a quiet standing baseline trial (cognitive-motor task only). Peak centre of pressure (COP) displacement and velocity as well as time to COP displacement onset and peak following the hand perturbation were evaluated. During dual- compared to cognitive-motor task condition, n-back response times increased and percentage of correct responses decreased, indicating an increase in cognitive processing. Only peak COP displacement upon posterior perturbations during leftward reaching increased, and time to COP displacement onset upon anterior perturbations during rightward reaching decreased during dual- compared to single-task. However, there were no other differences between tasks for the remaining COP variables. Overall, there were limited differences in the COP variables during the dual- compared to single-task condition, but negative effects on the cognitive-motor task. The findings indicate that increased cognitive processing provided a minimal effect on reactive balance responses to a hand perturbation, therefore indicating balance is likely prioritized during the reaching task used here.
THE EFFECTS OF INCREASED COGNITIVE PROCESSING ON REACTIVE BALANCE CONTROL FOLLOWING PERTURBATIONS TO THE UPPER LIMB by Megan Trotman BHK, The University of British Columbia, 2016 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Health and Exercise Sciences) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) December 2020 © Megan Trotman, 2020 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis entitled: The effects of increased cognitive processing on reactive balance control following perturbations to the upper limb submitted by Megan Trotman in partial fulfillment of the requirements of the degree of Master of Science Dr. Brian H. Dalton, School of Health and Exercise Science Supervisor Dr. Jennifer M. Jakobi, School of Health and Exercise Science Supervisory Committee Member Dr. Paul van Donkelaar, School of Health and Exercise Science Supervisory Committee Member Dr. Mark G. Carpenter, School of Kinesiology, University of British Columbia, Vancouver University Examiner iii Abstract While most reactive balance control studies have used whole-body perturbations, research is limited regarding how whole-body balance adjustments are integrated with continuous voluntary reaching movements when a hand perturbation is encountered. Further, when two tasks are performed simultaneously through a dual-task paradigm, performance of one or both tasks is often disrupted owing to attentional resources being divided. However, it is unclear how reactive whole-body balance responses to hand perturbations adjust when paired with a cognitive-motor task. The purpose of this study was to assess the effects of increased cognitive processing on reactive whole-body balance responses to perturbations of the hand during a continuous voluntary reaching task. Sixteen participants (8 females, 22.9 ± 4.5 years) stood and held the handle of a KINARM, a robotic-controlled manipulandum paired with an augmented visual display. Participants completed 10 trials of 100 continuous mediolateral arm movements while receiving 20 randomly interspersed anteroposterior hand perturbations throughout each trial. Participants moved their right hand at a pace of 60 beats per minute between two targets, with the hand moving towards the next target at each beat (single-task). An auditory n-back task was performed during half of the trials (dual-task), and during a quiet standing baseline trial (cognitive-motor task only). Peak centre of pressure (COP) displacement and velocity as well as time to COP displacement onset and peak following the hand perturbation were evaluated. During dual- compared to cognitive-motor task condition, n-back response times increased and percentage of correct responses decreased, indicating an increase in cognitive processing. Only peak COP displacement upon posterior perturbations during leftward reaching increased, and time to COP displacement onset upon anterior perturbations during rightward reaching decreased during dual- compared to single-task. However, there were no other iv differences between tasks for the remaining COP variables. Overall, there were limited differences in the COP variables during the dual- compared to single-task condition, but negative effects on the cognitive-motor task. The findings indicate that increased cognitive processing provided a minimal effect on reactive balance responses to a hand perturbation, therefore indicating balance is likely prioritized during the reaching task used here. v Lay Summary It is common for humans to use their upper limbs to perform everyday functional tasks while standing. However, research is limited surrounding how reactive balance operates when parts of the body are interfered with during these tasks, such as when the arm is bumped while reaching for an object. Further, humans perform tasks without complete focus and are often distracted with other thoughts or conversations, resulting in divided attention. The effects of divided attention on reactive balance responses to perturbations of the hand were investigated by examining standing balance control during a continuous reaching task paired with a working memory task. This study will advance our understanding of how balance responses to perturbations of the hand adjust when attention is divided. vi Preface The University of British Columbiaâs Clinical Research Ethics Board granted ethical approval for this research on December 18th, 2019 prior to any data collection. The certificate approval number is H19-01982. This certificate includes four studies to be conducted concurrently. The research conducted in the following study, to date, has not been submitted for publication. The author declares that no conflicts of interest are present. The composition and findings of this study are original and without dishonest data fabrication, manipulation, or falsification while presented honestly and without bias. vii Table of Contents Abstract ................................................................................................................................... iii Lay Summary ............................................................................................................................. v Preface ...................................................................................................................................... vi Table of Contents ..................................................................................................................... vii List of Tables ............................................................................................................................ ix List of Figures ............................................................................................................................ x List of Abbreviations ................................................................................................................. xi Acknowledgements .................................................................................................................. xii 1. Introduction ........................................................................................................................... 1 1.1 Reaching and Balance Control .................................................................................. 1 1.2 Reactive Balance Control .......................................................................................... 2 1.3 Dual-Task Paradigms ................................................................................................ 3 1.4 Cognitive Tasks ........................................................................................................ 4 1.5 Purpose and Hypothesis ............................................................................................ 6 2. Methods ............................................................................................................................... 7 2.1 Participants ............................................................................................................... 7 2.2 Experimental Equipment ........................................................................................... 7 2.3 Cognitive-motor Task ............................................................................................... 8 viii 2.4 Experimental Design ................................................................................................. 9 2.5 Single-Task ............................................................................................................. 11 2.6 Cognitive-motor Task Only .................................................................................... 11 2.7 Dual-Task ............................................................................................................... 12 2.8 Data Analysis ......................................................................................................... 12 2.9 Statistical Analysis .................................................................................................. 14 3. Results ............................................................................................................................... 16 3.1 Centre of Pressure ................................................................................................... 16 3.2 Hand Kinematics .................................................................................................... 22 3.3 Cognitive-motor Performance ................................................................................. 25 4. Discussion .......................................................................................................................... 27 4.1 Dual-Task Effect on Cognitive and Balance Tasks .................................................. 28 4.2 Reaching and Balance Control ................................................................................ 29 4.3 The Influence of Reaching Direction ....................................................................... 31 4.4 De-coupling Between the Hand and Centre of Pressure Responses ......................... 32 4.5 Limitations and Future Directions ........................................................................... 34 5. Conclusion ......................................................................................................................... 37 Bibliography ............................................................................................................................ 38 ix List of Tables Cognitive-motor Task Performance .......................................................................................... 26 x List of Figures Figure 1. Auditory N-back Task Protocol ................................................................................... 9 Figure 2. Experimental Setup and KINARM Displays with Hand Movements ......................... 10 Figure 3. Evaluation Protocol of Hand Kinematic and COP Parameters ................................... 14 Figure 4. Peak Anteroposterior Displacement of COP .............................................................. 19 Figure 5. Peak Anteroposterior Velocity of COP ...................................................................... 20 Figure 6. Time to COP Displacement Onset ............................................................................. 21 Figure 7. Time to COP Peak Displacement ............................................................................... 22 Figure 8. Hand Anteroposterior Peak Displacement ................................................................. 24 Figure 9. Hand Anteroposterior Peak Velocity ......................................................................... 25 xi List of Abbreviations 2D two-dimensional ANOVA analysis of variance AP anteroposterior bpm beats per minute COP centre of pressure cm centimeters ERP event-related potential Hz Hertz kg kilograms m meters ms milliseconds s seconds xii Acknowledgements First, I would like to thank my supervisor, Dr. Brian Dalton, for providing continual guidance and support throughout my degree. I truly appreciate all of the valuable insight you have given me, and for having faith in my ability as a masterâs student. I would also like to thank my committee members, Dr. Jennifer Jakobi and Dr. Paul van Donkelaar, for contributing your time and advice towards my thesis. I am also very grateful for all of my amazing lab mates in the SPIN Lab; Paige Copeland, Mathew Debenham, Mike Kennefick, Hogun Kang, and Lisa Ha. Thank you for enhancing my experience throughout my degree by providing endless support and many laughs. To my family and friends who have always been there for me, I am overwhelmingly appreciative of your continuous love and encouragement as I accomplish my goals and pursue my career in academics. 1 1. Introduction Maintaining balance equilibrium following an external perturbation is achieved through the integration of multiple sensory systems in order to produce a compensatory motor output to keep the body upright (Rasman et al., 2018). Although whole-body reactive balance control has been well studied, humans constantly engage their upper body to perform daily tasks while standing and much less is known about how reactive standing balance control is prioritized when the hands are perturbed during engagement of an upper limb manual task. The central nervous system must use strategies to maintain balance equilibrium when only portions of the body are perturbed (e.g., limb movements disrupted by an obstacle or external force) or integrate postural control while performing voluntary upper limb tasks such as reaching or grasping (Massion 1998). Further, dual-task paradigms involving reactive postural responses to whole-body perturbations and a concurrent cognitive task involve increased cognitive processing, or divided attention, and often result in decreased performance of one or both tasks (Lajoie et al., 1993; Shumway-Cook & Woollacott, 2000; Pellecchia 2003). However, it is unclear whether increased cognitive processing influences whole-body postural responses to perturbations of the hand. 1.1 Reaching and Balance Control In order to produce a voluntary arm movement, such as reaching for an object or target, postural adjustments of the trunk and lower limb muscles must precede upper limb motion in order to maintain upright equilibrium (Gahery 1987; Massion 1992). These anticipatory postural adjustments not only act to predict and minimize a disruption to balance control, they can also contribute to the performance of the reaching task (Bouisset & Zattara 1987; Bouisset et al., 2000; Massion, 1992). While the intention of the present study is not to examine anticipatory 2 postural adjustments, these responses highlight the importance of postural control for the planning and execution of voluntary reaching tasks. The interaction between arm movements and anticipatory postural adjustments is well known (Friedli et al., 1984; Aruin & Latash, 1995; Stapley et al., 1998; Kennefick et al., 2018), yet reactive whole-body postural responses following unexpected perturbations to the hand while reaching are less understood, and therefore are the primary focus of this thesis. 1.2 Reactive Balance Control External perturbations applied to the body require subsequent compensatory adjustments from the postural muscles of the trunk and lower limbs in order to keep the body upright (Horak & Nashner, 1986). Balance responses to whole-body perturbations, through the use of surface translations and rotations, have been studied thoroughly (Nashner 1976; Horak & Nashner, 1986; Horak et al., 1990; Day et al., 1993; Dietz et al., 1984; Allum et al., 2001; Allum et al., 2002; Lin & Woollacott, 2002, Adkin et al., 2006). However, Lowrey and colleagues (2017) demonstrated that perturbations directly to the hand during a goal-directed reaching task evoked muscular responses in the lower limb and responses in centre of pressure (COP) in order to maintain upright equilibrium. The study (Lowrey et al., 2017) also reported that reactive postural adjustments were modified to consider the goal of the upper-limb task (reaching towards the centre of a small circle compared to a larger rectangle). In this thesis, we extend previous findings (Lowrey et al., 2017) to investigate reactive whole-body balance adjustments while performing a continuous voluntary reaching task rather than performing a discrete, ballistic goal-oriented reaching task that is often used in the literature (Trivedi et al., 2010; Leonard et al., 2011; Lowrey et al., 2017; Dierijck et al., 2020; Kennefick et al., 2020). In the present study, 3 participants performed continuous mediolateral reaches between two circular targets with a larger diameter (4 cm) in comparison to the circular target (1.5 cm) used in Lowrey et al. (2017), which suggests that the present task demands less accuracy of the hand to reach the target. Further, during goal-directed reaching movements, participants are informed to perform the task as quickly and accurately as possible (Lowrey et al., 2017; Dierijck et al., 2020). In contrast, participants in the present study were asked to maintain a steady 60 beats per minute (bpm) cadence while moving between targets; speed and accuracy was not stressed. 1.2 Dual-Task Paradigms Studies using a dual-task paradigm concluded that cognitive processing is involved in maintaining standing balance (Woollacott & Shumway-Cook, 2002), although alternative arguments exist (Murnaghan et al., 2014; McGeehan et al., 2017). Cortical activity related to postural adjustments seems to decrease during a dual-task paradigm involving an attentional motor task. For example, the N1 event-related potential (ERP) amplitude, as recorded from the motor and somatosensory cortical areas, decreased upon pairing a reactive balance task with a cognitively demanding task compared to when reactive balance was evaluated in isolation without divided attention (Quant et al., 2004, Adkin et al., 2006, Little & Woollacott, 2015; Bogost et al., 2016). The addition of a cognitive task reduced performance of the reactive balance tasks, as demonstrated by an increase in COP parameters such as peak COP displacement, greater area under the COP trajectory, and peak COP trajectory (Quant et al., 2004; Little & Woollacott, 2014; 2015). A dual-task paradigm involves the performance of two tasks simultaneously (Lajoie et al., 1993; Woollacott & Vander Velde, 2008) and the attentional capacity of the brain is limited 4 (Kahneman, 1973; Wickens, 1983). Therefore, when performing multiple tasks simultaneously, the attentional resources of the brain are divided and the performance of one or both tasks are decreased (Kahneman, 1973; Wickens, 1983). Numerous theories have been proposed to explain the dual-task interference effects, including the cross-talk, neural-structure, and bottle-neck theories (Maki & McIlroy, 2007). The cross-talk theory proposes that processing of one task creates noise that interferes with the performance of the other (Hommel & Eglau, 2002; Logan & Gordon, 2001; Logan & Schulkind, 2000). The neural-structure theory assumes that performance of both tasks compete for specific neural pathways within the brain and as such, leads to an interference effect (Just et al., 2001; 2008). Lastly, the bottle-neck theory proposes that both tasks utilize one information-processing channel, and performance of one task is delayed until the first task is complete (Pashler 1994). Typically, dual-task paradigms involving standing balance and a cognitive task result in interference of the cognitive task performance (Maki & McIlroy, 2007). However, performance of the balance task can also be altered. When the simultaneous balance task leads to decreased performance in the cognitive task, it is inferred that the balance task demands attention (e.g., prioritizing the balance task) or greater cognitive resources (Maki & McIlroy, 2007). 1.4 Cognitive Tasks A variety of cognitive tasks have been used to assess the attentional demands required for upright postural control (Lajoie et al., 1993; Brauer et al., 2001; Brown et al., 1999; Siu et al., 2008), including simple reaction time tasks (Lajoie et al., 1993; Teasdale et al., 1993; Dierijck et al., 2020), mathematical subtraction (Brown et al., 1999; Yardley et al., 1999; McGeehan et al., 2017) sentence completion (Shumway-Cook et al., 1997) and working memory tasks (Swan et 5 al., 2004; Vander Velde et al., 2005). Working memory tasks are often used during dual-task paradigms with balance control, as they require both online information storage and processing (Huxhold et al., 2006; Woollacott & Vander Velde, 2008). Introduced by Kirchner (1958), the n-back task demands rule-governed as opposed to familiarity-based decisions (Monk et al., 2011). By requiring participants to actively store, process, and respond to a sequence of consecutive stimuli, the n-back task is designed to inflict a continuous parametrically variable load upon working memory (Braver et al., 1997). Therefore, the n-back task can serve as an effective method of dividing attention and exploring the effects of increased cognitive processing on reactive postural control. It has been previously demonstrated that varying n-back task levels of difficulty within a given cognitive task type provides a limited effect on postural sway measures, such as comparing a 1-back task to a 2-back or 3-back task (Woollacott & Vander Velde, 2008). However, research surrounding dual-task effects on balance control indicate that performance of the postural task may be influenced by the type of cognitive task used (Logie et al., 1986; Cocchini et al., 2002). The addition of a cognitive task has been shown to decrease (Andersson et al., 1998; Woollacott & Shumway-Cook, 2002), enhance (Stroffregen et al., 1999; 2000; Swan et al., 2004; Huxhold et al., 2006), or provide no influence on postural control (Norrie et al., 2002; Redfern et al., 2002). Interference of postural control is often demonstrated by increased postural sway (Andersson et al., 1998) or heightened reactive balance responses (Quant et al., 2004; Little & Woollacott., 2014; 2015), and likely reflects a competition for attentional resources between the cognitive and postural task (Little & Woollacott, 2015). Facilitation of postural control, as demonstrated by an attenuation in COP measures (Stroffregen et al., 1999; 2000; Swan et al., 2004; Huxhold et al., 2006), may occur as a result of using a cognitive task to shift attentional resources away from the automatic process of balance control, enabling the postural system to 6 self-organize automatically (Wulf et al., 2001; Huxhold et al., 2006). In studies where the addition of a cognitive task does not alter postural responses (Norrie et al., 2002; Redfern et al., 2002), it is possible that increased cognitive processing provided a minimal effect on balance control, or attentional resources were diverted away from the cognitive to the postural task to prioritize maintaining upright postural control (Little & Woollacott, 2015). 1.5 Purpose and Hypothesis The overall purpose of this thesis was to examine the influence of increased cognitive processing on reactive whole-body balance responses to perturbations of the upper limb during a repetitive, continuous voluntary reaching task. It was hypothesized that if attentional demand is required for reactive balance control to hand perturbations, then upon increased cognitive processing (i.e., reactive balance combined with the n-back task) the accuracy would decrease and response times increase for the cognitive-motor task. It was also hypothesized that with increased cognitive processing, whole-body reactive balance responses to a hand perturbation would be greater, similar to that observed for whole-body perturbations (Quant et al., 2004; Little & Woollacott, 2014; 2015). If cognitive task performance decreased and reactive balance control was unaltered or improved (faster and smaller postural adjustments), the findings would indicate a prioritization of standing balance control over the cognitive task (Little & Woollacott, 2015), or alternatively a shift of attentional resources from the postural to the cognitive task (Wulf et al., 2001; Huxhold et al., 2006). 7 2. Methods 2.1 Participants Sixteen participants completed the experiment (eight females, age: 22.9 ± 4.5 years, mass: 75.6 ± 9.7 kg, height: 173.9 ± 7.8 cm). All participants were self-declared right-handed with normal hearing and vision or corrected-to-normal vision, with no known history of cardiovascular, neurological, musculoskeletal, or balance impairments, or used medications with side effects of dizziness, lack of motor control, or slowed reaction time. Participants provided written and oral informed consent prior to participation. The study procedures were approved by the University of British Columbiaâs Clinical Research Ethics Board (CREB approval: H19-01982). 2.2 Experimental Equipment Data were sampled using the KINARM End-Point robot (BKIN Technologies Ltd., Kingston, Canada) and a True Impulse force plate (NDI, Waterloo, ON). The KINARM is a graspable manipulandum via the handles paired with a 2D augmented reality display. The KINARM software and data acquisition board recorded hand displacement and acceleration at each robotic joint angle, as well as force/torque and kinematics in 6 degrees of freedom. The KINARM software and data acquisition board also collected forces and moments in the X (mediolateral), Y (anteroposterior), and Z (vertical) axes through the force plate. Data were sampled at 1000 Hz and stored offline for analysis. 8 2.3 Cognitive-motor Task A working memory task was used in the form of an auditory n-back task (N-backer; Monk et al., 2011). The participant wore a headset with built-in headphones and a microphone (Logitech H390 USB Headset, Switzerland). The headset was worn throughout the entire experiment, and was connected to a laptop with speech recognition software that presented the task (Monk et al., 2011). A sequence of numbers between 1-9 was presented in random order, with a pseudo random interstimulus interval of ~1500 ms (range: 1200-1800 ms), through the headset and the participant verbally stated the number presented n steps prior. For the current study, the size of n was adjusted to 1 (Figure 1). Speech recognition software was used to record the timing of when the number was presented from the computer (âSpeak timeâ) and when the participant stated the number (âReceive timeâ). The software also recorded the number presented by the computer (âSpokenâ) and the number spoken by the participant (âReceivedâ). Time boundaries (range: 1200-3300 ms) were set to determine whether the participant was able to respond between the two numbers in the sequence following each presented number. The participantâs response was deemed correct if it matched the number presented from the computer 1 step prior, and if answered within the time boundaries. 9 Figure 1. Auditory n-back task protocol. The speech recognition software on the computer presented a consecutive sequence of numbers (black font), to which the participant would repeat the number presented 1-step prior (blue font). Each number from the computer was presented with an interstimulus interval of ~1500 ms (range: 1200-1800 ms). Participants were required to respond to each number between the time boundaries of 1200-3300 ms to ensure the response was given between the two numbers in the sequence following each presented number. 2.4 Experimental Design The study took place during a single session in a dimly lit room. Although additional data were collected from the participants for other studies within the same testing session, data collection for the current thesis always occurred first, and therefore should not have been impacted by experimental procedures of the other studies. The thesis study involved three conditions: single-task, cognitive-motor task only, and dual-task. The single-task condition involved the performance of a continuous voluntary reaching task, while receiving random perturbations to the hand. The cognitive motor-task condition required participants to perform the n-back task in isolation, whereas the dual-task condition incorporated performance of both tasks simultaneously. Further explanation is provided in following paragraphs. For all three conditions, participants stood and faced the KINARM, held the right handle linked to the robotic motors with their right hand, and their elbow positioned at an ~90° angle (Figure 2A). The feet were positioned together on the force plate so the medial malleoli were touching. Although 10 maintaining consistent arm movement only applied to the single- and dual-task conditions, a metronome maintained an audible cadence of 60 bpm during all conditions. The cognitive-motor task in isolation was always performed directly prior to the dual-task, and this combination of conditions was performed in a randomized order with the single-task condition. Figure 2. Experimental setup and KINARM displays with hand movements. Experimental setup of a participant standing and holding the handle of the KINARM (A). KINARM display screen presenting two targets (circles) and performance of voluntary reaching to the right (B) and left (C) with anterior and posterior perturbations as well as when reaching in both directions with no perturbations (D). Black arrows represent rightward (lateral) hand movements, and grey arrows represent leftward (medial) hand movements. Upward deflections in B and C represent anterior perturbations, whereas downward deflections in B and C depict posterior perturbations. 11 2.5 Single-Task Participants began with a familiarization trial consisting of 40 continuous mediolateral reaching movements (20 leftward; 20 rightward) including eight randomly interspersed anteroposterior hand perturbations (four anterior; four posterior). For the single-task condition, participants performed five trials, each consisting of 100 continuous mediolateral reaching movements (50 leftward; 50 rightward) with 20 randomly interspersed anteroposterior perturbations (10 anterior, 10 posterior), for a total of 500 reaches and 100 perturbations per participant (Figure 2B; 2C; 2D). Voluntary reaches were performed in the horizontal plane between the two adjacent green circular targets of the same size (visual radius: 2 cm) presented 30 cm apart on the display screen. The perturbation occurred as soon as the participantâs hand left the target, consisting of a 25 ms ramp up, with an external 60 N force being applied for a duration of 40 ms, and a 25 ms ramp down (90 ms total). Prior to the familiarization trial, participants were advised that they would receive random perturbations from the manipulandum, but were instructed to perform a continuous hand movement between the two targets, and avoid pausing at each target when changing hand direction. An experimenter provided verbal instructions throughout the trials to ensure participants were keeping a consistent pace with the metronome, so as to move rightwards to the target for 1-s and leftwards for 1-s. However, speed and accuracy of the reaching task were not stressed. Each trial lasted ~100-s. 2.6 Cognitive-motor Task Only Participants first familiarized themselves with the n-back task by practicing until they felt comfortable with the protocol. In order to determine whether cognitive processing would be altered by the dual-task paradigm, participants completed one 120-s trial of the cognitive-motor 12 task in isolation as a baseline measure. The KINARM display screen presented a green circular target (visual radius: 2 cm). The participants held their hand in the centre of the target throughout the entire trial, and performed the cognitive-motor task as accurately as possible while standing quietly. 2.7 Dual-Task Participants performed a similar familiarization trial as the single-task condition paired with the cognitive-motor task prior to the experimental trials. The dual-task condition combined the performance of the single- and cognitive-motor task simultaneously. Participants completed the same number of trials as outlined in the single-task condition. 2.8 Data Analysis All COP and hand kinematic data analyses were performed with MATLAB software (The MathWorks, Inc., Natick, MA, USA). All COP signals were processed using a fourth-order, dual-pass digital Butterworth filter with a 5 Hz cut-off frequency. Postural responses were assessed by calculating the peak COP displacement and velocity following the hand perturbation onset. For both COP displacement and velocity, maximum values were examined for anterior perturbations, while minimum values were used for posterior perturbations as negative values represented a posterior shift. In addition, onset of COP displacement was measured by taking the time point at which the COP displacement surpassed 3 standard deviations from the baseline value, defined as the mean COP displacement 300 ms prior to the hand perturbation onset (Figure 3). Time to COP displacement onset was calculated by subtracting onset of hand perturbation from time of COP displacement onset. Time to peak COP displacement was 13 calculated by subtracting onset of hand perturbation from time of peak COP displacement. Hand kinematics were assessed by calculating peak hand displacement and velocity following the hand perturbation. Maximum values were examined for perturbations in the anterior direction, while minimum values were used for perturbations in the posterior direction given that negative values represented a posterior shift. Owing to technical difficulties, some files from the data collected were not long enough to capture the perturbation profile, and could not be analysed. Therefore, two dual-task trials (40 perturbations) from one participant (leaving 60 perturbations for analysis), and one dual-task trial (20 perturbations) each from two other participants (leaving 80 perturbations per participant for analysis) were excluded. Individual participant data for each dependent variable were averaged across all trials within each condition. For the cognitive-motor task data, response times were calculated by subtracting âSpeak timeâ from âReceive timeâ. The time between the points at which the participant began to vocalise the answer and at which voice recognition from the software was complete, termed the recognition delay, of 850ms was also taken into consideration when determining timings of the response, and therefore subtracted from the calculations. Performance of the cognitive-motor task was also assessed by calculating the percentage of correct responses per trial. Individual participant data were averaged across the five dual-task trials. 14 Figure 3. Evaluation protocol of hand kinematic and COP parameters. Hand perturbation onset (A; black unfilled circle) occurred once the hand left the target. Displacement onset of COP (grey unfilled circle) was taken when COP displacement surpassed 3 standard deviations from the baseline value (BL), defined as the mean COP displacement 300ms prior to the hand perturbation onset. Peak hand displacement is represented by a black filled circle, and peak COP displacement is represented by a grey filled circle. 2.9 Statistical Analysis All statistical analyses were conducted using SPSS version 23 (SPSS Inc., Chicago, IL, USA). All data were normally distributed (Jarque-Bera test). The COP and hand kinematic data were analyzed using a two-way 2 (single vs dual task) Ã 2 (leftward vs rightward) repeated-measures analysis of variance (ANOVA). Effect sizes were evaluated using the partial-eta squared (ηp2) metric. Post hoc analyses were performed using Bonferroni-corrected paired samples t-tests where appropriate, with effect sizes reported using Cohenâs d. The cognitive-motor task data were analyzed using paired t-tests in order to compare the cognitive-motor task only values to the cognitive-motor task values performed during the dual-task trials. Effect sizes 15 were reported using Cohenâs d. A p-value of < 0.05 was considered significant. Values are reported as mean Â± standard deviation. 16 3. Results 3.1 Centre of Pressure For anteroposterior peak COP displacement there was no interaction for anteriorly directed hand perturbations (F(1,15) = 0.196, p=0.664, ηp2 = 0.013) nor a main effect (Task: F(1, 15) =0.117, p=0.737, ηp2 = 0.008; Hand Direction: F(1, 15) =0.327, p = 0.576, ηp2 = 0.021) (Figure 4A). For perturbations in the posterior direction, an interaction was detected (F(1,15) = 20.050, p<0.001, ηp2 = 0.572) with a main effect for task (F(1, 15) =5.576, p = 0.032, ηp2 = 0.271), but no main effect for hand direction (F(1, 15) =0.552, p = 0.469, ηp2 = 0.035). Post hoc analyses revealed that posterior COP peak displacement during the dual-task condition was 18% greater than the single-task when the hand was moving leftward (t(15) = 3.430, p = 0.004, d = 0.857). However, there were no detectable differences between the dual- and single-task conditions with rightward hand movements (t(15) = 0.185, p = 0.856, d = 0.046) (Figure 4B). During rightward hand movements, COP peak displacement in response to perturbations directed posteriorly was 6% greater than leftward hand movements during the single-task (t(15) = 2.768, p = 0.014, d = 0.692), but 9% less during the dual-task (t(15) = -2.413, p = 0.029, d = 0.603) (Figure 4B). For anteroposterior COP peak velocity in response to anterior perturbations, there were no main effects (Task: F(1,15) = 1.119, p= 0.307, ηp2 = 0.069; Hand Direction: F(1,15) = 1.521, p = 0.236, ηp2 = 0.092) nor an interaction (F(1,15) = 1.243, p = 0.282, ηp2 = 0.077) (Figure 5A). When perturbations were directed posteriorly (Figure 5B), an interaction (F(1,15) = 5.747, p = 0.030, ηp2 = 0.277) and hand direction effect (F(1,15) = 5.395, p = 0.035, ηp2 = 0.25) was detected with no task effect (F(1,15) = 0.717, p = 0.410, ηp2 = 0.046). Post hoc analyses revealed 17 no detectable differences in posterior COP peak velocity between dual- and single-tasks when the hand was moving leftward (t(15) = 1.274, p = 0.222, d = 0.318) or rightward (t(15) = 0.367, p = 0.719, d = 0.092). When the hand moved rightward, COP peak velocity in response to posterior perturbations was 7% greater compared to leftward movements during single-task (t(15) = 2.810, p = 0.013, d = 0.703), but there was no detectable difference between hand directions during the dual-task condition (t(15) = 1.306, p = 0.211, d = 0.327). Time to onset of COP displacement following anterior directed perturbations exhibited an interaction (F(1,15) = 10.233, p=0.006, ηp2 = 0.406), but no detectable main effects for task (F(1,15) = 0.925, p = 0.351, ηp2 = 0.058) nor hand direction (F(1,15) = 0.580, p = 0.458, ηp2 = 0.037). Displacement onset of the COP during the dual-task condition was 5% faster than the single-task during rightward hand directions (t(15) = 2.849, p = 0.012, d = 0.712). However, there was no detectable difference between tasks when moving the hand leftward (t(15) = -1.293, p = 0.216, d = 0.323) (Figure 6A). Time to COP displacement onset when the hand was moving rightward was 5% less than moving leftward during the dual-task condition (t(15) = -2.156, p = 0.048, d = 0.539), but there were no detectable differences between hand directions during the single-task condition (t(15) = 1.184, p = 0.255, d = 0.296) (Figure 6A). For posterior directed perturbations, there was no interaction (F(1,15) = 3.885, p=0.067, ηp2 = 0.206) nor task effect (F(1,15) = 0.132, p= 0.721, ηp2 = 0.009), but there was a hand direction effect (F(1,15) = 9.693, p = 0.007, ηp2 = 0.393). When the data were collapsed over task, time to COP displacement onset during hand movements directed rightward was 6% greater than leftward hand movements (Figure 6B). For time to peak COP displacement in response to anterior perturbations, there was no interaction (F(1,15) = 2.769, p = 0.117, ηp2 = 0.156), task effect (F (1,15) = 2.097, p= 0.168, ηp2 = 18 0.123) nor hand direction effect (F(1,15) = 3.060, p = 0.101, ηp2 = 0.169) (Figure 7A). When perturbations were directed posteriorly, there was no interaction (F(1,15) = 0.866, p = 0.367, ηp2 = 0.055) nor main effect for task (F(1,15) = 0.309, p=0.586, ηp2 = 0.020) detected. However, there was a main effect for hand direction (F(1,15) = 6.232, p = 0.025, ηp2 = 0.294). When the data was collapsed over task, time to peak COP displacement when the hand was moving in the rightward direction was 3% less than the leftward direction (Figure 7B). 19 Figure 4. Peak anteroposterior displacement of COP. Squares represent mean values (Â± standard deviation), while dots represent individual data. Single-task values are represented by unfilled shapes, while dual-task values are represented by filled shapes. Differences (p < 0.05) between single- and dual-task conditions are indicated by an asterisk (*). Differences (p < 0.05) between leftward and rightward hand movements are indicated by a dagger (â ). Grey lines link individual data points between single- and dual- tasks. Positive values denote responses to anterior perturbations (A), and negative values denote responses to posterior perturbations (B). 20 Figure 5. Peak anteroposterior velocity of COP. Squares represent mean values (Â± standard deviation), while dots represent individual data. Single-task values are represented by unfilled shapes, while dual-task values are represented by filled shapes. Differences (p < 0.05) between leftward and rightward hand movements are indicated by a dagger (â ). Grey lines link individual data points between single- and dual- tasks. Positive values denote responses to anterior perturbations (A), and negative values denote responses to posterior perturbations (B). 21 Figure 6. Time to COP displacement onset. Values denote responses to perturbations in the anterior (A) and posterior (B) directions. Squares represent mean values (Â± standard deviation), while dots represent individual data. Single-task values are represented by unfilled shapes, while dual-task values are represented by filled shapes. Differences (p < 0.05) between single- and dual-task conditions are indicated by an asterisk (*). Differences (p < 0.05) between leftward and rightward hand movements are indicated by a dagger (â ). Grey lines represent changes between tasks within individuals. 22 Figure 7. Time to COP peak displacement. Values denote responses to perturbations in the anterior (A) and posterior (B) directions. Squares represent mean values (Â± standard deviation), while dots represent individual data. Single-task values are represented by unfilled shapes, while dual-task values are represented by filled shapes. Differences (p < 0.05) between leftward and rightward hand movements are indicated by a dagger (â ). Grey lines represent changes between tasks within individuals. 3.2 Hand Kinematics For anteroposterior peak hand displacement in response to anterior perturbations, there was no interaction (F(1,15) = 1.295, p = 0.273, ηp2 = 0.079) nor task effect (F(1,15) = 2.792, p= 0.115, ηp2 = 0.157). However, there was a main effect for hand direction (F(1, 15) = 44.516, p<0.001, ηp2 = 0.748). When the data were collapsed over task, peak hand displacement during rightward movement was 10% greater than leftward movement (Figure 8A). When perturbations were directed posteriorly, an interaction (F(1,15) = 7.775, p = 0.014, ηp2 = 0.341) with main effects (Task: F(1,15) = 11.400, p = 0.004, ηp2 = 0.432; Hand Direction: F(1,15) = 7.306, p = 0.016, ηp2 = 0.328) were detected. Post hoc analyses revealed that peak hand displacement during dual-task decreased by 9 and 4% compared to single-task during leftward (t(15) = -3.844, p = 23 0.002, d = 0.961) and rightward hand movements (t(15) = -2.266, p = 0.039, d = 0.567), respectively (Figure 8B). Posterior peak hand displacement while directing the hand rightward was 6 and 11% greater than directing the hand leftward during single- (t(15) = 2.274, p = 0.038, d = 0.568) and dual-task (t(15) = 2.922, p = 0.011, d = 0.730), respectively (Figure 8B). For anteroposterior peak hand velocity, there was no interaction for anteriorly directed perturbations (F(1,15) = 2.072, p = 0.171, ηp2 = 0.121). A main effect for task was detected (F(1,15) = 5.375, p = 0.035, ηp2 = 0.264). When the data were collapsed over hand direction, peak hand velocity during the dual-task condition decreased by 3% compared to the single-task (Figure 9A). Further, there was also a main effect for hand direction (F(1,15) = 50.023, p < 0.001, ηp2 = 0.769). When the data were collapsed over task, peak hand velocity during rightward directed hand movements was 11% greater than during leftward hand movements (Figure 9A). For posterior directed perturbations, peak hand velocity exhibited an interaction (F(1,15) = 5.188, p = 0.038, ηp2 = 0.257) and a hand direction effect (F(1,15) = 28.845, p < 0.001, ηp2 = 0.658), but no task effect (F(1,15) = 0.821, p = 0.379, ηp2 = 0.052) (Figure 9B). Post hoc analyses revealed that there were no differences in posterior peak hand velocity between tasks when the hand was moving leftward (t(15) = -1.775, p = 0.096, d = 0.444) or rightward (t(15) = 1.100, p = 0.289, d = 0.275). Peak hand velocity following posteriorly directed hand perturbations when the hand was moving in the rightward direction was 13 and 18% greater than the leftward direction during the single-task (t(15) = 5.723, p < 0.001, d = 1.431) and the dual-task (t(15) = 4.890, p < 0.001, d = 1.223), respectively (Figure 9 B). 24 Figure 8. Hand anteroposterior peak displacement. Squares represent mean values (Â± standard deviation), while dots represent individual data. Single-task values are represented by unfilled shapes, while dual-task values are represented by filled shapes. Differences (p < 0.05) between single- and dual-task conditions are indicated by an asterisk (*). Differences (p < 0.05) between leftward and rightward hand movements are indicated by a dagger (â ). Grey lines represent changes between tasks within individuals. Positive values denote responses to anterior perturbations (A), and negative values denote responses to posterior perturbations (B). 25 Figure 9. Hand anteroposterior peak velocity. Squares represent mean values (Â± standard deviation), while dots represent individual data. Single-task values are represented by unfilled shapes, while dual-task values are represented by filled shapes. Differences (p < 0.05) between single- and dual-task conditions are indicated by an asterisk (*). Differences (p < 0.05) between leftward and rightward hand movements are indicated by a dagger (â ). Grey lines represent changes between tasks within individuals. Positive values denote responses to anterior perturbations (A), and negative values denote responses to posterior perturbations (B). 3.3 Cognitive-motor Performance Response times to the n-back task were 8% slower during the dual-task condition than when performing the cognitive-motor task only (t(15) = -6.711, p<0.001, d = 1.678). The 26 percentage of correct responses were reduced by 11% during the dual-task condition than when performing the cognitive-motor task alone (t(15) = 4.877, p<0.001, d = 1.219) (Table 1). Table 1. Cognitive-motor task performance. Cognitive-motor Task Only Dual-Task Response Times (ms) 2286.90 ± 165.82 2464.41 ± 176.70* Correct Responses (%) 89.82 ± 10.10 79.98 ± 9.41* Response times (ms) and percentage of correct responses (%) of the cognitive-motor task and dual-task. Differences (p < 0.05) between cognitive-motor task and dual-task conditions are indicated by an asterisk (*). Values are reported as mean Â± standard deviation. 27 4. Discussion The overall purpose of the current thesis was to investigate the effects of increased cognitive processing on reactive balance control when perturbations are encountered by the hand performing a continuous voluntary reaching task. Through pairing a standing balance task involving upper limb perturbations with an auditory working memory task, we explored how reactive balance control adjusts in the presence of divided attention (e.g., increased cognitive processing) via various COP and hand kinematic variables. Performance of the cognitive-motor task during the dual-task condition was compared to when completing the n-back task alone to demonstrate that the dual-task paradigm effectively increased cognitive processing, and divided attentional demand away from the cognitive task. During the dual-task condition, response times were slower and less accurate (Table 1) compared to the cognitive-motor task performed alone while standing quietly. Our findings revealed that peak COP displacement was greater with increased cognitive processing upon posterior perturbations during leftward hand movements. Further, COP displacement onset following anterior perturbations was faster for the dual- than single-task when the hand was moving rightward. However, there were no other detectable differences between tasks for the remaining COP variables. When perturbations were directed posteriorly, peak hand displacement during dual-task decreased compared to single-task for both hand movement directions. With anterior directed perturbations, peak hand velocity decreased during the dual-task condition. Overall, the present dual-task paradigm increased cognitive processing, as indicated by a decrease in performance of the cognitive-motor task. However, given that there were limited differences in the COP variables between the single- and dual-task condition, it appears that increased cognitive processing had minimal effect on whole-body reactive balance adjustments to perturbations of the hand while performing the continuous 28 reaching task. The main findings likely indicate that attentional resources were allocated to the reactive balance task to prioritize the maintenance of upright postural control during the dual task paradigm. 4.1 Dual-Task Effect on Cognitive and Balance Tasks Cortical resources have been shown to play a role in postural responses through the use of electroencephalography during dual-task paradigms. An attenuated N1 ERP amplitude, recorded from the motor and somatosensory cortical areas, occurs when adding a cognitive demanding task to a whole-body reactive balance paradigm. The N1 response likely reflects the sensory processing of the balance disturbance at the level of the cortex (Quant et al., 2004; Little & Woollacott, 2015; Bogost et al., 2016). Quant et al. (2004) observed a decreased ERP amplitude when combining a visuomotor tracking task with transient anteroposterior translations while standing. Compared to the reactive balance task in isolation, the addition of a cognitive task increased anteroposterior COP peak displacement magnitude and lower limb muscle activity combined with a decreased N1 ERP amplitude. The dual-task paradigm may have also altered visuomotor tracking performance, however changes in the cognitive task were not evaluated in that study (Quant et al. 2004). Little & Woollacott (2015) reported a decreased N1 ERP amplitude through the use of a visual working memory task paired with anteroposterior support surface translations, compared to performing the reactive balance task alone. The dual-task condition reduced visual working memory capacity, as represented by a decreased efficiency in the cognitive task performance, as well as increased COP trajectory peak amplitude, which could reflect a less efficient whole-body postural adjustment. Therefore, the authors (Little & Woollacott, 2015) proposed that decreased performance of both the cognitive and reactive 29 balance tasks occurred as a result of competition, diverting attentional resources away from the processing of sensory input associated with postural control. If a reduction in only the cognitive task performance had occurred, as seen in the current study, the authors (Little & Woollacott, 2015) suggested that it was likely a result of a re-allocation of resources from the cognitive to the postural task, emphasizing the importance of maintaining standing balance. 4.2 Reaching and Balance Control Prior to the onset of a voluntary hand movement, anticipatory postural adjustments are initiated to stabilize posture and equilibrium, and contribute to reaching task performance (Massion 1992). Recently, Dierijck et al. (2020) examined the role of attention in the interaction between goal-directed anterior reaching movements and upright postural control. A discrete, ballistic movement task was performed while standing, and executed either concurrently with a button press simple reaction time task, or in isolation. During the dual-task paradigm, performance of both the reaching and simple reaction time task decreased, as demonstrated by a slower reaching execution and delayed button presses. Anteroposterior whole-body postural responses during the dual-task paradigm were also altered, as demonstrated by a slowed time to peak COP displacement. In addition, the study (Dierijck et al., 2020) incorporated mediolateral perturbations to the hand while reaching in the anterior direction to assess whole-body reactive postural responses. Mediolateral peak COP velocity following the perturbation decreased during the dual- compared to single-task condition. Taken together, the authors (Dierijck et al., 2020) concluded that divided attention influenced both the reaching task and postural adjustments, and that attentional resources were required for both reaching and standing balance control. 30 The present study extends these findings to evaluate cognitive contributions to reactive postural responses following hand perturbations during a continuous upper limb movement task. While some of our COP variables following perturbations were altered with implementation of a dual-task paradigm compared to a single-task (i.e. increased posterior COP maximum displacement during leftward hand movements, faster anterior COP displacement onset during rightward hand movements), these changes were inconsistent. Overall, our results demonstrated minimal changes in COP between the single- and dual-task conditions. While these findings contrast those of Dierijck et al. (2020), it is important to take into consideration the differences between the reaching tasks used in each study. A ballistic, goal-oriented reaching task demands certain speed and accuracy, and may require greater attention than the continuous reaching movements used in this thesis. Further, while goal-directed reaching movements involve preceding anticipatory postural adjustments (Lowrey et al., 2017; Dierijck et al., 2020) owing to the planning of the proceeding hand movement (Kennefick et al., 2018), the continuous movements performed in the present study require minimal or no anticipatory postural adjustments, demonstrated by the lack of change in COP displacement prior to hand perturbation onset (Figure 3). Therefore, we suggest that a continuous movement task could involve less attentional demand and planning than a discrete, goal-oriented reaching task, allowing the central nervous system to prioritize more cognitive resources towards reactive postural control. Thus, whole-body reactive balance was largely unaltered by the cognitive-motor task implemented here. Another methodological difference between the present study and the work of Dierijck et al. (2020) involves the standing posture of participants. In Dierijck et al. (2020), participants stood with feet hip width apart, with their non-reaching hand holding on to the button press 31 resting on the platform below the manipulandum when responding to unexpected mediolateral perturbations. Stability is increased when stance is widened, as indicated by decreased mediolateral peak COP and body displacement and velocity (Day et al., 1993). Contacting a stable surface with a fingertip (light touch and force contact) also increases stability as demonstrated by reduced mean COP and body sway amplitude in the mediolateral direction (Jeka & Lackner, 1995). In contrast, the current study required participants to stand with feet together, while the non-reaching hand rested by their side, facilitating a less stable posture. It is possible that a less stable posture requires greater attentional demand when responding to unexpected perturbations compared to a wider stance, therefore drawing more attentional resources away from the cognitive task and remaining unaltered by the dual-task paradigm compared to the single-task. 4.3 The Influence of Reaching Direction Although not the primary aim of the study, hand kinematic and COP variables demonstrated many detectable differences between voluntary leftward and rightward hand movements. Overall, more hand and COP values were greater when the hand was moving in the rightward direction than the leftward direction. Specifically, peak hand displacement and velocity collapsed over both tasks following anterior perturbations demonstrated greater values with rightward compared to leftward hand direction, as well as COP peak displacement and COP peak velocity during single task, time to COP displacement onset collapsed over both tasks, and hand displacement and hand velocity during both tasks following posterior perturbations. During performance of the arm movement task, the two targets were presented on the right side of the display screen in order to align closer to the right hand. Horizontal movements toward the right 32 target enabled the hand to move away from the body, while leftward movements brought the hand toward the body. An increase in COP parameters, such as peak COP displacement and velocity, is often associated with reduced stability of upright postural control (Day et al., 1993). Therefore, the body likely became less stable when the hand is voluntarily moved away from the body, and may rely on heightened responses to counteract the hand perturbation. Interestingly, about half of the values of the hand kinematic variables decreased during the dual-task condition compared to the single-task, while the other half displayed no detectable changes. Specifically, peak hand displacement in response to posterior perturbations decreased during both leftward and rightward hand movements, and peak hand velocity decreased following anterior perturbations during the dual-task compared to the single-task. It is unclear as to why only some hand kinematic variables, but not others, were altered. Surface electromyography of the deltoids, biceps brachii and triceps brachii of the right arm was not evaluated for this study. However, data regarding muscle activation of the upper limb could provide further insight into how the arm movements respond to hand perturbations during a dual-task paradigm. For example, co-contraction of the elbow flexors and extensors may decrease velocity and displacement to ensure accurate pacing with the metronome during the dual- compared to single-task. 4.4 De-coupling Between the Hand and Centre of Pressure Responses Given the lack of consistency between the hand kinematics and COP results when comparing single- to dual-task, it is likely that there is a disconnect between the upper and lower limbs when responding to perturbations of the hand with increased cognitive processing. During a goal-directed reaching task, Lowrey et al. (2017) reported that evoked upper limb muscle 33 responses proceeded hand perturbations by 50-ms, which was followed by lower limb activity around 60-70-ms, and COP adjustments after 100-ms. The authors (Lowrey et al., 2017) suggested that the upper limb reaction likely drives the lower-limb postural responses, consequently generating COP adjustments. However, the present study demonstrates inconsistent responses between the hand and COP values with increased cognitive processing, indicating a disconnect between the upper limbs and subsequent balance responses. Specifically, during dual-task compared to the single task, peak hand displacement decreased while peak COP displacement increased following posterior perturbations during leftward hand movements. While posteriorly directed peak hand displacement also decreased during rightward hand movements, there was no detectable differences in the COP responses. Further, peak hand velocity following anterior hand perturbations decreased during the dual-task compared to single-task condition, but there were no detectable differences for the COP parameters. Evaluation of upper and lower limb muscle activity would serve to provide further insight into the de-coupling between the hand kinematics and COP variables. A possible factor that may contribute to the disconnect between the upper limb and postural responses is a change in reactive balance strategies. When examining postural responses to whole-body perturbations, two distinct whole-body muscle activation patterns are commonly characterized to restore equilibrium: an ankle or hip strategy. The ankle strategy, intended for smaller perturbations, utilizes a single-link inverted pendulum model around the ankle joints, while the hip strategy is used to respond to larger perturbations and under circumstances when the support surface is smaller than the base of support (Horak & Nashner, 1986). The hip strategy takes the form of a double-link pendulum as it involves hip motion in addition to movement about the ankles. The response to a large perturbation not only requires a 34 larger amplitude of synergistic muscle responses, but also alternative activation patterns (Horak & Nashner, 1986). For example, a whole-body anterior perturbation results in activation of the gastrocnemius, hamstrings, and paraspinals if using an ankle strategy, driving the centre of mass (COM) backwards (Horak & Nashner, 1986). However, using a hip strategy in response to an anterior perturbation would first activate the abdominals, quadriceps, and tibialis anterior, driving the COM forwards as a result of dorsiflexion (Horak & Nashner, 1986). For the present study, the de-coupling between the hand and COP responses with increased cognitive processing may be occurring as a result of participants utilizing different strategies, possibly the ankle or hip strategy, or a combination of both. Assessment of trunk and other postural muscles in combination with motion analysis may provide a better understanding to the reactive balance control strategies used in the present study and to further explore the integration between hand perturbations and reactive postural adjustments. 4.5 Limitations and Future Directions As previously mentioned, future studies should examine muscle activity of the upper limbs, trunk, and lower limbs to provide further insight into how increased cognitive processing affects the integration between perturbations to the hand during a continuous reaching task and subsequent lower limb and whole-body responses. In addition, assessing whole-body kinematics using a motion capture system may also serve to provide insight into hand perturbations and whole-body postural responses. This technique has previously been used to analyze ankle and hip strategies following simulated support surface translations (Versteeg et al., 2016), and to quantify the effect of various arm swing and walking conditions on gait stability when recovering from a sudden slip perturbation (Gholizadeh et al., 2019). Therefore, future studies 35 examining whole-body kinematics with electromyography and COP parameters may provide novel and comprehensive information surrounding reactive balance responses during our dual-task paradigm. During the present study, performing a continuous voluntary movement task while standing decreased performance in the cognitive task. It would be interesting to isolate the influence of standing balance control by comparing our standing dual-task paradigm to an altered paradigm that involved performance of the arm movement and cognitive tasks while seated. To further explore dual-task effects on balance control, future studies could also compare the cognitive-motor task in isolation to quiet standing without a cognitive task. Upon examining performance of the cognitive-motor task and subsequent balance control, we did not assess how increased cognitive processing influences the performance of the reaching task. Analyzing the exact timings and accuracy of the reaching task would provide greater insight into the responses of the arm following the perturbation, as well as the participantâs ability to maintain hand movements with the cadence of the metronome during the dual- compared to single-task. Investigating the extent of which increased cognitive processing altered performance of the arm movement task would allow us to further understand the division of attentional resources used towards this reactive balance control paradigm. Lastly, owing to research curtailment resulting from the COVID-19 pandemic, the number of participants included in this thesis (n = 16) did not satisfy the a priori sample size calculation (n = 34, power = 0.80). Therefore, conclusions regarding the results examined in this study may not be truly accurate or represent those of the population. Despite these limitations, by pairing an auditory n-back task with a continuous reaching task while standing, this thesis still 36 provides novel insight into the effects of increased cognitive processing on reactive balance responses to perturbations of the hand. 37 5. Conclusion To examine the effects of increased cognitive processing on reactive postural responses to perturbations of the hand, a continuous reaching task while standing was paired with an auditory, working memory task to create a dual-task paradigm. As demonstrated by a decrease in the responses and accuracy of the cognitive-motor task, cognitive processing was effectively increased, dividing attentional demand away from the working memory task. Despite increased cognitive processing, COP responses were mostly unaffected by the addition of a dual-task paradigm. We suggest that with divided attention, cognitive resources are reduced from the cognitive task in order to prioritize upright postural control and maintain reactive balance performance. The minimal differences of COP responses between the single- and dual-task conditions may have also occurred due to the disassociation between reactive balance and the less attention-demanding nature of the continuous hand movement task, in comparison to a discrete, goal-oriented, ballistic reaching task. The hand kinematics and COP variables demonstrated a lack of consistency with increased cognitive processing, suggesting a de-coupling between responses of the hand and postural responses following the perturbation. Findings from this thesis provides a direction for future studies to investigate the mechanisms underlying reactive balance control strategies to hand perturbations during a continuous voluntary movement task. To conclude, increased cognitive processing had limited effects on reactive balance responses in this dual task paradigm, which indicates standing balance was likely prioritized over the cognitive-motor task to maintain upright postural control in response to a hand perturbation during a continuous reaching task. 38 Bibliography Adkin, A. L., Quant, S., Maki, B. E., & McIlroy, W. E. (2006). Cortical responses associated with predictable and unpredictable compensatory balance reactions. Experimental Brain Research, 172(1), 85-93. doi:10.1007/s00221-005-0310-9 Allum, J. H. J., Carpenter, M. G., & Adkin, A. L. (2001). Balance control analysis as a method for screening and identifying balance deficits. Annals of the New York Academy of Sciences, 942(1), 413-427. doi:10.1111/j.1749-6632.2001.tb03763.x Allum, J. H. J., Carpenter, M. G., Honegger, F., Adkin, A. L., & Bloem, B. R. (2002). Age-dependent variations in the directional sensitivity of balance corrections and compensatory arm movements in man. The Journal of Physiology, 542(2), 643-663. doi:10.1113/jphysiol.2001.015644 Andersson, G., Yardley, L., & Luxon, L. (1998). A dual-task study of interference between mental activity and control of balance. The American journal of otology, 19(5), 632-637. Aruin, A. S., & Latash, M. L. (1995). Directional specificity of postural muscles in feed-forward postural reactions during fast voluntary arm movements. Experimental brain research, 103 (2), 323-332. Bogost, M. D., Burgos, P. I., Little, C. E., Woollacott, M. H., & Dalton, B. H. (2016). Electrocortical sources related to whole-body surface translations during a single-and dual-task paradigm. Frontiers in human neuroscience, 10, 524. Bouisset, S., Richardson, J., & Zattara, M. (2000). Are amplitude and duration of anticipatory postural adjustments identically scaled to focal movement parameters in humans?. Neuroscience letters, 278(3), 153-156. Bouisset, S., & Zattara, M. (1987). Biomechanical study of the programming of anticipatory postural adjustments associated with voluntary movement. journal of Biomechanics, 20(8), 735-742. Brauer, S. G., Woollacott, M., & Shumway-Cook, A. (2001). The interacting effects of cognitive demand and recovery of postural stability in balance-impaired elderly persons. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 56(8), M489-M496. Braver, T. S., Cohen, J. D., Nystrom, L. E., Jonides, J., Smith, E. E., & Noll, D. C. (1997). A parametric study of prefrontal cortex involvement in human working memory. Neuroimage, 5(1), 49-62. Brown, L. A., Shumway-Cook, A., & Woollacott, M. H. (1999). Attentional demands and postural recovery: the effects of aging. Journals of Gerontology Series A: Biomedical Sciences and Medical Sciences, 54(4), M165-M171. 39 Cocchini, G., Logie, R. H., Della Sala, S., MacPherson, S. E., & Baddeley, A. D. (2002). Concurrent performance of two memory tasks: Evidence for domain-specific working memory systems. Memory & Cognition, 30(7), 1086-1095. Day, B. L., Steiger, M. J., Thompson, P. D., & Marsden, C. D. (1993). Effect of vision and stance width on human body motion when standing: Implications for afferent control of lateral sway. The Journal of Physiology, 469(1), 479-499. doi:10.1113/jphysiol.1993.sp019824 Dierijck, J., Kennefick, M., Smirl, J., Dalton, B. H., & van Donkelaar, P. (2020). Attention is required to coordinate reaching and postural stability during upper limb movements generated while standing. Journal of Motor Behavior, 52:1, 79-88. doi:10.1080/00222895.2019.1587351 Dietz, V., Quintern, J., & Berger, W. (1984). Cerebral evoked potentials associated with the compensatory reactions following stance and gait perturbation. Neuroscience Letters, 50(1), 181-186. doi:10.1016/0304-3940(84)90483-X Friedli, W. G., Hallett, M., & Simon, S. R. (1984). Postural adjustments associated with rapid voluntary arm movements 1. Electromyographic data. Journal of Neurology, Neurosurgery & Psychiatry, 47(6), 611-622. Gahery, Y. (1987). Associated movements, postural adjustments and synergies: some comments about the history and significance of three motor concepts. Archives italiennes de biologie, 125(4), 345-360. Gholizadeh, H., Hill, A., & Nantel, J. (2019). Effect of arm motion on postural stability when recovering from a slip perturbation. Journal of biomechanics, 95, 109269. Hommel, B., & Eglau, B. (2002). Control of stimulus-response translation in dual-task performance. Psychological Research, 66(4), 260-273. Horak, F. B., & Nashner, L. M. (1986). Central programming of postural movements: Adaptation to altered support-surface configurations. Journal of Neurophysiology, 55(6), 1369-1381. doi:10.1152/jn.1986.55.6.1369 Horak, F. B., Nashner, L. M., & Diener, H. C. (1990). Postural strategies associated with somatosensory and vestibular loss. Experimental Brain Research, 82(1), 167-177. doi:10.1007/BF00230848 Huxhold, O., Li, S. C., Schmiedek, F., & Lindenberger, U. (2006). Dual-tasking postural control: aging and the effects of cognitive demand in conjunction with focus of attention. Brain research bulletin, 69(3), 294-305. 40 Jeka, J. J., & Lackner, J. R. (1995). The role of haptic cues from rough and slippery surfaces in human postural control. Experimental Brain Research, 103(2), 267-276. doi:10.1007/BF00231713 Just, M. A., Carpenter, P. A., Keller, T. A., Emery, L., Zajac, H., & Thulborn, K. R. (2001). Interdependence of nonoverlapping cortical systems in dual cognitive tasks. Neuroimage, 14(2), 417-426. Just, M. A., Keller, T. A., & Cynkar, J. (2008). A decrease in brain activation associated with driving when listening to someone speak. Brain research, 1205, 70-80. Kahneman, D. (1973). Attention and effort. Englewood Cliffs, N.J: Prentice-Hall Kennefick, M., Wright, A. D., Smirl, J. D., & van Donkelaar, P. (2018). Anticipatory postural adjustments as a function of response complexity in simple reaction time tasks. Neuroscience letters, 684, 1-5. Kennefick, M., McNeil, C. J., Burma, J. S., Copeland, P. V., van Donkelaar, P., & Dalton, B. H. (2020). Modulation of vestibular-evoked responses prior to simple and complex arm movements. Experimental Brain Research, 1-13. Lajoie, Y., Teasdale, N., Bard, C., & Fleury, M. (1993). Attentional demands for static and dynamic equilibrium. Experimental brain research, 97(1), 139-144. Leonard, J. A., Gritsenko, V., Ouckama, R., & Stapley, P. J. (2011). Postural adjustments for online corrections of arm movements in standing humans. Journal of Neurophysiology, 105(5), 2375-2388. doi:10.1152/jn.00944.2010 Lin, S., & Woollacott, M. H. (2002). Postural muscle responses following changing balance threats in young, stable older, and unstable older adults. Journal of Motor Behavior, 34(1), 37-44. doi:10.1080/00222890209601929 Little, C. E., & Woollacott, M. (2015). EEG measures reveal dual-task interference in postural performance in young adults. Experimental brain research, 233(1), 27-37. Little, C. E., & Woollacott, M. (2014). Effect of attentional interference on balance recovery in older adults. Experimental brain research, 232(7), 2049-2060. Logan, G. D., & Gordon, R. D. (2001). Executive control of visual attention in dual-task situations. Psychological review, 108(2), 393. Logan, G. D., & Schulkind, M. D. (2000). Parallel memory retrieval in dual-task situations: I. Semantic memory. Journal of Experimental Psychology: Human Perception and Performance, 26(3), 1072. 41 Logie, R. H. (1986). Visuo-spatial processing in working memory. The Quarterly Journal of Experimental Psychology Section A, 38(2), 229-247. Lowrey, C., Nashed, J., & Scott, S. (2017). Rapid and flexible whole body postural responses are evoked from perturbations to the upper limb during goal-directed reaching. Journal of Neurophysiology, 117(3), 1070-1083. doi:10.1152/jn.01004.2015 Maki, B. E., & McIlroy, W. E. (2007). Cognitive demands and cortical control of human balance-recovery reactions. Journal of Neural Transmission, 114(10), 1279-1296. Massion, J. (1992). Movement, posture and equilibrium: interaction and coordination. Progress in neurobiology, 38(1), 35-56. Massion, J. (1998). Postural control systems in developmental perspective. Neuroscience & Biobehavioral Reviews, 22(4), 465-472. McGeehan, M. A., Woollacott, M. H., & Dalton, B. H. (2017). Vestibular control of standing balance is enhanced with increased cognitive load. Experimental brain research, 235(4), 1031-1040. Monk, A. F., Jackson, D., Nielsen, D., Jefferies, E., & Olivier, P. (2011). N-backer: An auditory n-back task with automatic scoring of spoken responses. Behavior research methods, 43(3), 888. Murnaghan, C. D., Squair, J. W., Chua, R., Inglis, J. T., & Carpenter, M. G. (2014). Cortical contributions to control of posture during unrestricted and restricted stance. Journal of Neurophysiology, 111(9), 1920-1926. Nashner, L. M. (1976). Adapting reflexes controlling the human posture. Experimental Brain Research, 26(1), 59-72. doi:10.1007/BF00235249 Norrie, R. G., Maki, B. E., Staines, W. R., & McIlroy, W. E. (2002). The time course of attention shifts following perturbation of upright stance. Experimental brain research, 146(3), 315-321. Pashler, H. (1994). Dual-task interference in simple tasks: data and theory. Psychological bulletin, 116(2), 220. Pellecchia, G. L. (2003). Postural sway increases with attentional demands of concurrent cognitive task. Gait & posture, 18(1), 29-34. Quant, S., Adkin, A. L., Staines, W. R., Maki, B. E., & McIlroy, W. E. (2004). The effect of a concurrent cognitive task on cortical potentials evoked by unpredictable balance perturbations. BMC neuroscience, 5(1), 18. 42 Rasman, B. G., Forbes, P. A., Tisserand, R., & Blouin, J. S. (2018). Sensorimotor manipulations of the balance control loopâbeyond imposed external perturbations. Frontiers in Neurology, 9, 899. Redfern, M. S., Müller, M. L., Jennings, J. R., & Furman, J. M. (2002). Attentional dynamics in postural control during perturbations in young and older adults. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 57(8), B298-B303. Shumway-Cook, A., & Woollacott, M. (2000). Attentional demands and postural control: the effect of sensory context. Journals of Gerontology-Biological Sciences and Medical Sciences, 55(1), M10. Shumway-Cook, A., Woollacott, M., Kerns, K. A., & Baldwin, M. (1997). The effects of two types of cognitive tasks on postural stability in older adults with and without a history of falls. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 52(4), M232-M240. Siu, K. C., Chou, L. S., Mayr, U., Donkelaar, P. V., & Woollacott, M. H. (2008). Does inability to allocate attention contribute to balance constraints during gait in older adults? The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 63(12), 1364-1369. Stapley, P., Pozzo, T., & Grishin, A. (1998). The role of anticipatory postural adjustments during whole body forward reaching movements. Neuroreport, 9(3), 395-401. Stoffregen, T. A., Smart, L. J., Bardy, B. G., & Pagulayan, R. J. (1999). Postural stabilization of looking. Journal of Experimental Psychology: Human Perception and Performance, 25(6), 1641. Stoffregen, T. A., Pagulayan, R. J., Bardy, B. G., & Hettinger, L. J. (2000). Modulating postural control to facilitate visual performance. Human Movement Science, 19(2), 203-220. Swan, L., Otani, H., Loubert, P. V., Sheffert, S. M., & Dunbar, G. L. (2004). Improving balance by performing a secondary cognitive task. British Journal of Psychology, 95(1), 31-40. Teasdale, N., & Simoneau, M. (2001). Attentional demands for postural control: the effects of aging and sensory reintegration. Gait & posture, 14(3), 203-210. Trivedi, H., Leonard, J. A., Ting, L. H., & Stapley, P. J. (2010). Postural responses to unexpected perturbations of balance during reaching. Experimental Brain Research, 202(2), 485-491. doi:10.1007/s00221-009-2135-4 Vander Velde, T. J., Woollacott, M. H., & Shumway-Cook, A. (2005). Selective utilization of spatial working memory resources during stance posture. Neuroreport, 16(7), 773-777. 43 Versteeg, C. S., Ting, L. H., & Allen, J. L. (2016). Hip and ankle responses for reactive balance emerge from varying priorities to reduce effort and kinematic excursion: A simulation study. Journal of biomechanics, 49(14), 3230-3237. Wickens, CD. (1983). Processing resources in attention, dual task performance and workload assessment. In: R. Parasuraman & R. Davies (Ed.), Varieties of attention (pp. 63-102). New York, NY: Academic Press. Woollacott, M., & Shumway-Cook, A. (2002). Attention and the control of posture and gait: a review of an emerging area of research. Gait & posture, 16(1), 1-14. Woollacott, M. & Vander Velde, T. (2008). Non-visual spatial tasks reveal increased interactions with stance postural control. Brain research, 1208, 95-102. Wulf, G., McNevin, N., & Shea, C. H. (2001). The automaticity of complex motor skill learning as a function of attentional focus. The Quarterly Journal of Experimental Psychology Section A, 54(4), 1143-1154. Yardley, L., Gardner, M., Leadbetter, A., & Lavie, N. (1999). Effect of articulatory and mental tasks on postural control. Neuroreport, 10(2), 215-219.
Thesis/Dissertation
eng
Health and Exercise Sciences
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
The effects of increased cognitive processing on reactive balance control following perturbations to the upper limb
Text