The effect of increasing the complexity of a movement on the motor pathway

Abstract

In their seminal experiment, Henry and Rogers (1960) sought to understand how the complexity of a movement affected reaction time (RT). They demonstrated that increasing the number of response elements leads to longer RTs; however, the reason for lengthened RTs has remained controversial. While this phenomenon has been interpreted using neural activation models (e.g. Hanes & Schall, 1996), few studies have examined how changes within the motor pathway may contribute to RT differences. Transcranial magnetic stimulation (TMS) is used to examine responsiveness of the motor pathway by recording motor evoked potentials (MEPs) at the target muscle. Therefore, the purpose of this study was to examine how MEPs were affected by the complexity of a movement in a RT paradigm. Participants (n=12) were seated at a KINARM End-Point Lab and completed a ballistic, simple RT task, in which they directed a robotic handle to one, two or three targets. Across the three levels of complexity, participants completed 8 trials at each TMS point for a total of 144 trials. During each trial, TMS was delivered at 0, 50, 60, 70, 80 or 90% of each participant's mean RT at the stimulator intensity which yielded a triceps brachii MEP equivalent to 10% the maximal M-wave. As intended, RTs increased with increasing movement complexity (p<0.05). There was an interaction between the complexity of the movement and the stimulation points (p=0.016). These findings suggest that there is an increase in neural excitability as a ballistic movement becomes more complex.

Acknowledgments: Supported by NSERC