Investigating Differences in Reaction Time and Preparatory Activation as a Result of Varying Accuracy Requirements

Abstract

The preparation and initiation of movement has previously been described using a neural accumulation model; this model involves an increase of neural activation in the motor cortex (M1) from baseline to a subthreshold level following a warning signal, which is maintained until presentation of an imperative stimulus (IS). Activity then increases until reaching movement initiation threshold. This model predicts that variability in activation during preparation may influence reaction time (RT) and its variability. The purpose of this thesis project was to determine whether differences in RT/variability of RT during the completion of tasks with varying levels of complexity may be attributable to differences in neural excitability in M1. To test this prediction, transcranial magnetic stimulation (TMS) delivered concurrently with an IS was used to determine neural excitability for movements with different accuracy demands. It was hypothesized that higher accuracy demands would result in lowered amplitude and/or greater variability of neural activation, and consequently slower/more variable RT. Fifteen healthy participants completed a simple RT task involving a targeted wrist extension movement under three different accuracy conditions (easy, moderate, difficult). TMS was delivered concurrently with the IS on 50% of trials during each condition. While pilot testing showed RT differences between accuracy conditions (Appendix A), the data presented here failed to detect significant differences in RT latency (F(2, 28) = .074, p = .929) or variability (F(1.432, 20.053) = .633, p = .538) between conditions . Similarly, no difference in MEP amplitude was observed between difficulty conditions (F(2, 28) = 2.439, p = .106). However, a subset of participants (n = 7) did show significant RT increases between easy and hard conditions (t(6) = 2.531, p = .045), but this subset still failed to show differences in MEP amplitude (t(6) = 1.157, p = .291) or variability (t(6) = 1.545, p = .173), suggesting that preparatory levels at the IS may be similar for movements involving both high and low accuracy demands.