Investigating the Role of the Primary Motor Cortex in the StartReact Effect Using Transcranial Magnetic Stimulation

• Main Author: Smith, Cora
• Other Authors: Carlsen, Anthony
• Language: en
• Published: Université d'Ottawa / University of Ottawa 2017
• Subjects: Startle; Reaction time
• Online Access: http://hdl.handle.net/10393/36638; http://dx.doi.org/10.20381/ruor-20918

It is well-established that the presentation of a startling acoustic stimulus (SAS) simultaneous with the go-signal in a simple reaction time (RT) task results in significant RT reductions, while leaving movement kinematics essentially unaltered. While this phenomenon, termed the StartReact effect, has been extensively studied, cortical involvement in the neural mechanism underlying the RT-facilitation effects of a SAS remains widely debated. Applying sub-threshold TMS to motor areas results in increased cortical excitability and reductions in control RT. When this technique was used in a startle paradigm no RT benefits were seen, providing evidence that the cortex may not be involved in the StartReact effect; however, these results may also have been due to a floor effect of startle RT. It has been shown that RT in response to a SAS is significantly slower for complex movements, providing a possible method of distinguishing between these hypotheses. As such, the purpose of the experiments in this thesis was to determine if the application of sub-threshold TMS following a SAS when preparing to react with a complex movement would facilitate startle RT. If so, it would provide evidence for cortical involvement in the RT-facilitation effects of startle. The first experiment revealed that the task employed did not lead to an increase in RT in startle conditions, limiting the ability to make conclusions regarding the StartReact effect. In the second experiment the timing complexity of the task was increased, with the goal of increasing startle RT; however, startle RT was again not significantly slower for the complex movement than the simple movement. Furthermore, there was again no effect of TMS stimulation condition on startle RT. These results suggest that either the cortex does not play a role in the StartReact effect, or a floor effect of RT was reached in startle conditions; thus, alternative methods of investigating the neural mechanism underlying the RT-facilitation effects of startle are warranted.