UBC Theses and Dissertations
How do "invisible" stimuli influence action? : Visuomotor processing in the absence of conscious awareness Cressman, Erin K.
The goal of the current research was to examine the properties of visuomotor processing occurring in the absence of conscious awareness. Specifically, we investigated the ability of a subliminal stimulus to influence the on-line control of an action (Studies 1 and 2) and the extent to which the same subliminal stimulus could influence action when the probability of it predicting the upcoming response was manipulated (Studies 3 and 4). In order to display stimuli subliminally, stimuli were presented through the psychophysical procedure of metacontrast masking - a form of backward masking in which the visibility of a briefly displayed visual stimulus (the prime ) is greatly reduced when it is followed by a second visual stimulus (the mask ). Thus in the present research we were interested in how the primes would influence performance. Results revealed that (1) unconscious visuomotor processing can result in the modification of an overt response, such that a goal-directed movement is adjusted in response to a subliminal stimulus and (2) the visuomotor system can be modified in response to manipulations of the prime-mask sequence presented at an unconscious level. These results imply that subliminal stimuli are not processed in a conditionally automatic manner. In order to explain the ability of subliminal stimuli to influence behaviour we propose an accumulator model, in which adaptations to the state of the system arising due to task constraints are reflected at the level of response activation (i.e. at the accumulators). An "accumulator" is tuned to a specific stimulus-response mapping such that if participants are instructed to make a left or right response, two separate accumulators are established with one collecting neural evidence for stimuli mapped to the left response and the other collecting neural evidence for stimuli mapped to the right response. Both primes and masks are equally effective at driving the accumulators and a response is initiated as soon as the accumulated neural evidence for one response versus the alternative response reaches a critical threshold. The level of this threshold can be set strategically, or modified without awareness, depending on the prime-mask sequence displayed.
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