Excitability of subcortical motor circuits in Go/noGo and forced choice reaction time tasks

The size of the response to a startling auditory stimulus (SAS) may reflect the excitability of the reticulospinal tract. In this study, we examined whether there was any excitability change in the reticulospinal tract during preparation for execution of two types of choice reaction time task: a forced choice reaction time task (fCRT) and a Go/no-Go task (GnG). In 13 healthy volunteers we used three types of trials: control trials in which subjects were requested to perform ballistic wrist movements during fCRT or GnG tasks; test trials in which a SAS was presented with the visual cue, and baseline trials in which SAS was presented alone. Latency and area of the responses to SAS were measured in the orbicularis oculi and in the sternocleidomastoid (SCM) muscles. The results obtained in baseline trials were used to calculate the mean resting baseline excitability level of the reticulospinal tract for each individual, and the values obtained in test trials were expressed as percentages of mean baseline for normalization of data from all individuals. The area of the responses to SAS in SCM was significantly larger in fCRT than in GnG (p=0.002). There was a significant shortening of reaction time in test trials that was inversely correlated with area of SCM (p<0.05). We conclude that the subcortical motor tracts activated by SAS are more excitable during fCRT tasks than GnG tasks. Changing the excitability of subcortical motor structures may be a strategy used by the central nervous system in motor control that is selected according to the motor program.