Influence of area 5 on primary motor cortex: a paired-pulse TMS investigation in healthy adults

The neural correlates that underpin fine motor control of the hand and their connections with the primary motor cortex (M1) require further investigation. Brodmann’s area 5 located in the superior parietal lobule (SPL) is suggested to be an important cortical area involved in the processing of somatosensory input important for precision movements. Area 5 is present in monkey species capable of opposable thumb movements and it is proposed that this area evolved with the ability to execute manual behaviours such as pinch grip. Further, area 5 is dominated by the representation of the hand and forelimb, and has direct connectivity with M1 implicating its role in the control of hand movements. Few studies have investigated the function of area 5 in humans and none have examined the connectivity between area 5 and ipsilateral M1. This thesis presents a novel approach to study the influence of area 5 on M1 output in healthy and awake humans during the processing of somatosensory inputs and during performance of motor tasks involving the hand. Using paired pulse transcranial magnetic stimulation over left area 5 and ipsilateral M1, the connections between the two cortical loci was probed. It was hypothesized that area 5 would facilitate M1 output at short and long latencies during the processing of tactile inputs and during the performance of motor tasks compared to rest. The current results demonstrate that changes in M1 output are task and temporally specific. Facilitation of the motor evoked potential (MEP) was present at short latency of 6 ms during the processing of somatosensory input whereas inhibition was present during conditions where the hand was performing a task with the thumb and index finger. Further, an inhibitory effect was seen at 40 ms during cutaneous stimulation. In experiments 1 and 2, there was no net influence of area 5 on M1 output observed at rest. The findings presented may have revealed a novel path with which to alter the motor output, and possibly movement of hand muscles

Bilateral somatosensory evoked potentials following intermittent theta-burst repetitive transcranial magnetic stimulation

<p>Abstract</p> <p>Background</p> <p>Intermittent theta-burst stimulation (iTBS) is a form of repetitive transcranial magnetic stimulation that may alter cortical excitability in the primary somatosensory cortex (SI). The present study investigated the effects of iTBS on subcortical and early cortical somatosensory evoked potentials (SEPs) recorded over left, iTBS stimulated SI and the right-hemisphere non-stimulated SI. SEPs were recorded before and at 5, 15, and 25 minutes following iTBS.</p> <p>Results</p> <p>Compared to pre-iTBS, the amplitude of cortical potential N20/P25 was significantly increased for 5 minutes from non-stimulated SI and for 15 to 25 minutes from stimulated SI. Subcortical potentials recorded bilaterally remained unaltered following iTBS.</p> <p>Conclusion</p> <p>We conclude that iTBS increases the cortical excitability of SI bilaterally and does not alter thalamocortical afferent input to SI. ITBS may provide one avenue to induce cortical plasticity in the somatosensory cortex.</p