The effect of primary motor cortex stimulation on the timing of motor responses in a timed-response task

The generation of a motor action requires both the preparation and execution of a motor plan that specifies the necessary motor commands. Previous research has demonstrated that transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) can advance the onset of a motor response in a reaction time (RT) task when delivered early in the response latency (Pascual‐Leone, Valls‐Solé, et al., 1992; Sawaki, Okita, Fujiwara, & Mizuno, 1999). It has been hypothesized that this advancement reflects the facilitation of preparatory motor processes necessary for the generation of the response (Pascual‐Leone, Valls‐Solé, et al., 1992; Sawaki et al., 1999; Soto, Valls‐Solé, & Kumru, 2010), although the precise mechanisms for this effect remain unclear. The interpretation of these findings is complicated by the occurrence of stimulus recognition, motor preparation, and motor execution processes in close proximity to each other, which makes it difficult to disentangle the effects of TMS on different perceptual and motor processes (Miller, 1982; Sawaki et al., 1999).

The experiments reported in this thesis investigated the effect of TMS on the timing of motor responses in a timed‐response task. The task required the onset of a pinch to be synchronized with the last tone in a sequence of four evenly spaced tones, which were presented at an interstimulus interval of 1000 ms. This timed‐response task permits the advanced preparation of a motor response (Carlsen & MacKinnon, 2010; Hening, Favilla, & Ghez, 1988; Steglich, Heuer, Spijkers, & Kleinsorge, 1999), which temporally separates preparation and execution‐related processes and may help disentangle the effects of TMS on different stages of motor processing. M1 TMS advanced both ipsilateral and contralateral pinches by a constant amount, without affecting the kinematics of the response, when administered over a wide range of intervals preceding the target response time. Left M1 TMS was found to advance pinch onsets more than right M1 TMS, consistent with the hypothesis that the left M1 has more influence than the right M1 over the control of the limbs ipsilateral and contralateral to each M1 (Haaland, 2006). TMS similarly advanced unimanual and bimanual pinches, suggesting that the each hemisphere makes similar contributions to the control of the timing of unimanual and bimanual responses. Similar levels of corticomotor excitability were also observed prior to the execution of unimanual and bimanual pinches, suggesting that interhemispheric interactions involved in the control of bimanual pinches do not affect corticomotor excitability during motor preparation.

The specificity of M1 stimulation to the advancement of pinches, produced by TMS, was investigated by comparing the effects of M1 and occipital TMS on the timing of these responses. M1 and occipital TMS were both found to advance responses to a similar extent, indicating that non‐specific mechanisms, such as a startle‐like response, could mediate this effect. However, occipital TMS was also found to increase the force of the response, indicating that occipital TMS might not be an appropriate control for non‐specific effects of M1 TMS. In the final study reported in this thesis, TMS was found to advance the onset of responses that required muscle contractions but not responses which required the relaxation of a sustained contraction. These findings suggest that TMS does not facilitate preparatory processes that are shared between responses that require a muscle contraction and those that require a muscle relaxation.

The findings of this thesis suggest that TMS advances the execution of muscle contractions by increasing M1 excitability, thereby abbreviating a period of rising M1 excitability that precedes the execution of a response. The results also suggest that M1 can make small modifications to the timing of a motor response by altering the time taken to execute the prepared motor plan. Overall, the results of this thesis demonstrate the importance of neural processes separable from the motor plan in cued sensorimotor tasks.