Speed-Dependent Contribution of Callosal Pathways to Ipsilateral Movements

Abstract

Transcallosal inhibitory interactions between primary motor cortices are important to suppress unintended movements in a resting limb during voluntary activation of the contralateral limb. The functional contribution of transcallosal inhibition targeting the voluntary active limb remains unknown. Using transcranial magnetic stimulation, we examined transcallosal inhibition [by measuring interhemispheric inhibition (IHI) and the ipsilateral silent period (iSP)] in the preparatory and execution phases of isotonic slower self-paced and ballistic movements performed by the ipsilateral index finger into abduction and the elbow into flexion in intact humans. We demonstrate decreased IHI in the preparatory phase of self-paced and ballistic index finger and elbow movements compared to rest; the decrease in IHI was larger during ballistic than self-paced movements. In contrast, in the execution phase, IHI and the iSP increased during ballistic compared to self-paced movements. Transcallosal inhibition was negatively correlated with reaction times in the preparatory phase and positively correlated with movement amplitude in the execution phase. Together, our results demonstrate a widespread contribution of transcallosal inhibition to ipsilateral movements of different speeds with a functional role during rapid movements; at faster speeds, decreased transcallosal inhibition in the preparatory phase may contribute to start movements rapidly, while the increase in the execution phase may contribute to stop the movement. We argue that transcallosal pathways enable signaling of the time of discrete behavioral events during ipsilateral movements, which is amplified by the speed of a movement.

Long‐term progressive motor skill training enhances corticospinal excitability for the ipsilateral hemisphere and motor performance of the untrained hand

“…A recent imaging study found increased i M1 activation with the demand for precision during a pointing task suggesting that the involvement of i M1 relates to task difficulty (Buetefisch et al ., ). Also, a lesion to the i M1 caused by cerebral infarction impairs fine motor control of the hand (Parkin, ), whereas temporarily disrupting iM1 with repetitive TMS in able‐bodied subjects can impair execution of complex piano sequences (Chen et al ., ) and alter timing of muscle recruitment probably through transcallosal influences (Davare et al ., ; Tazoe & Perez, ). This interpretation is supported by the sparse ipsilateral corticospinal connections seen in non‐human primates (Soteropoulos et al ., ).…”

Polarity Specific Effects of Transcranial Direct Current Stimulation on Interhemispheric Inhibition

“…Conceivably, it might be that a functional role of the decreased transcallosal inhibition can be observed in a time-specific motor event like movement initiation. Transcallosal inhibition is gradually decreased according to the time course of movement initiation [50] , [51] . Therefore, a sustained reduction of transcallosal inhibition could contribute to such a situation of motor performance rather than a static enhancement of corticospinal excitability.…”

Assessment of Inter-Hemispheric Imbalance Using Imaging and Noninvasive Brain Stimulation in Patients With Chronic Stroke

“…Despite the difference in EMG activity it has recently been shown that level of EMG does not influence the degree of inhibition. 51 Results are reported as ISP ratio, which is defined as the Contralesional to Ipsilesional iSP/Ipsilesional to Contralesional iSP. The subject shown had an iSP ratio of .75.…”