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Examining cortical involvement in the StartReact effect using transcranial magnetic stimulation Stevenson, Andrew James


The goal of this thesis was to examine the subcortical storage and triggering hypothesis proposed by Valls-Solé et al. (1999) and Carlsen et al. (2004b). This hypothesis suggests movements that can be prepared in advance of an imperative stimulus are stored and triggered from subcortical areas without cortical involvement by a startling acoustic stimulus (SAS). The rapid release of prepared movements by a SAS has been termed the StartReact effect, and premotor reaction times (PMTs) <70 ms are often observed. We used transcranial magnetic stimulation (TMS) to probe cortical involvement in the StartReact effect. In Experiment 1 we examined whether a TMS-induced cortical silent period could delay the release of movement by a SAS. Thirteen participants performed 20° wrist extension movements as fast as possible in response to either a control tone (82.3 dB) or a SAS (123.2 dB). During selected control and startle trials, suprathreshold TMS was delivered to the contralateral primary motor cortex (M1) 50 or 70 ms prior to mean PMT. Startle PMTs were faster than control PMTs, while TMS significantly delayed movement onset in control and startle trials compared to No TMS or Sham TMS conditions. Additionally, TMS facilitated the size of the first agonist burst (AG1) in both control and startle trials. Experiment 1 provided evidence for the involvement of M1 in the StartReact effect, however, it was possible that suprathreshold TMS disrupted subcortical startle reflex pathways. Experiment 2 utilized subthreshold TMS to M1, which has been shown to reduce ongoing EMG activity during a voluntary contraction by activating inhibitory cortical interneurons and not descending motor pathways (Davey, et al., 1994). TMS was delivered to the contralateral M1 during AG1 activity in random control and startle trials. Despite all seven participants displaying TMS-induced suppression in isometric wrist extension, EMG suppression was observed in only one participant in control and startle trials. Data are discussed with respect to a recent model of preparation and initiation proposed by Carlsen et al. (in press).

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