Single pulse TMS during preparation for lower limb movement: Effect of task predictability on corticospinal excitability

Motor preparation involves inhibitory and excitatory processes that influence the scaling and efficiency of movement. Understanding the modulation of these processes when predictability is altered can provide insight as to how individuals prepare for temporally-urgent scenarios where the lower limbs are engaged. This study aimed to determine the influence of task predictability on preparatory corticospinal excitability during a 3 s foreperiod. It was hypothesized that during preparation for an unpredictable choice (go/no-go) reaction time task, corticospinal excitability would be facilitated compared to the responses measured for the predictable (simple) reaction time task. Twenty-eight healthy young adults participated in two reaction time tasks (predictable and unpredictable) using a go/no-go paradigm. During the foreperiod, transcranial magnetic stimulation was applied over the left primary motor cortex to elicit a motor-evoked potential in the right tibialis anterior muscle. Task predictability had a significant effect on corticospinal excitability when controlling for task order [F(1,25) = 7.124, p = 0.013], with the unpredictable task augmenting corticospinal excitability to a greater extent than the predictable task. When the predictable condition was performed first, MEP amplitude was 109.2 ± 14.3% of baseline for the predictable task and 124.0 ± 15.4% for the unpredictable task. In contrast, when the unpredictable condition was performed first, MEP amplitude was 119.5 ± 14.4% for the unpredictable task and 126.6 ± 10.1% for the predictable task. These results suggest that unpredictability may require individuals to have a heightened attention to incoming stimuli, thereby increasing corticospinal excitability, which can be maintained in subsequent predictable scenarios.