Observations on the variability of corticospinal tract excitability during the reaction time period for simple human finger movements

There is extensive evidence that movements are prepared prior to their release. Transcranial magnetic stimulation, and in particular the motor evoked potential produced when stimulating over the primary motor cortex, has given a great deal of insight into the processes involved in preparation for voluntary movements. The excitability of the primary motor cortex remains in a state of dynamic fluctuation even when in the “resting” state, with the TMS MEP being exquisitely sensitive to this as evidenced by its tremendous trial to trial variability. Interestingly there is growing body of evidence to suggest that modulation of signal noise can provide insight into biological processes including movement preparation – indeed the output of the corticospinal tract would logically need to adapt to resting variability to enable the precise reproduction of movements. While much of the TMS literature has addressed MEP variability as a “noisy” signal, this thesis aims to assess whether elements of this “noise” can be utilized as a marker of biologic process during the reaction time period for simple human finger movements. Through successive chapters we demonstrate that the variability of corticospinal tract output, as evidenced by the TMS MEP, declines during the process of preparation for simple human finger movements. We demonstrate that the reaction time decline in variability is focal to muscles directly involved in the task. Furthermore, the rate of decline in MEP amplitude variability during the reaction time period appears intimately linked to the speed of movement initiation. Additionally, the changes we see here precede changes in mean excitability in agonists, and indeed are seen to be associated with a decline in mean excitability when surround muscles are tasked with deliberate inactivity. Finally, observations in stroke patients suggest an alteration in variability control during movement preparation and appear to be associated with concordant changes in task performance.