Influence of area 5 on interhemispheric inhibition

Brodmann's area 5 is implicated in the sensorimotor control of hand movement in humans and nonhuman primates. However, little is known about the influence of area 5 on the neural circuitry within the primary motor cortex that underpins hand control. The present study investigated the neural circuitry of interhemispheric inhibition (IHI) that exists between homologous muscle representations in the motor cortex. Using paired-pulse transcranial magnetic stimulation, IHI was probed from the left-to-right hemisphere and vice versa for the first dorsal interosseous muscle of the hand at short (10 ms) and long (40 ms) latencies before and for up to 1 h after continuous θ-burst stimulation over left hemisphere area 5. The results indicate that continuous θ-burst over area 5 increases IHI at short latencies in the left hand (left-to-right inhibition) from 5-20 and 45-60 min after stimulation. Short latency inhibition in the right hand and bilateral long latency inhibition remain unaltered. The data indicate that area 5 influences the IHI that exists between the representations of the hand muscles. This effect occurs ipsilateral to the left area 5, suggesting that effects are mediated through changes in the excitability of transcallosal neurons originating in the left motor cortex.