Human area 5 modulates corticospinal output during movement preparation.

Neuroimaging evidence suggests that human Brodmann area 5 (BA5) within the superior parietal lobule contributes to movement planning. However, a causal role for the contribution of BA5 to preparatory processes has yet to be reported. We used paired-pulse transcranial magnetic stimulation to investigate the influence of human BA5 on corticospinal excitability during movement preparation in the context of a GO/NO-GO task. Functional connectivity between BA5 and the ipsilateral primary motor cortex (M1) was investigated by probing corticospinal output to the first dorsal interosseous muscle of the right hand. Results indicate that BA5 influences M1 during movement preparation in a task-specific manner: motor-evoked potentials are suppressed in the context of a NO-GO versus GO task. These findings provide evidence that human BA5 participates in movement preparation and differentiates between whether a movement is withheld or executed.

Metaplasticity in human primary somatosensory cortex: effects on physiology and tactile perception.

Theta-burst stimulation (TBS) over human primary motor cortex evokes plasticity and metaplasticity, the latter contributing to the homeostatic balance of excitation and inhibition. Our knowledge of TBS-induced effects on primary somatosensory cortex (SI) is limited, and it is unknown whether TBS induces metaplasticity within human SI. Sixteen right-handed participants (6 females, mean age 23 yr) received two TBS protocols [continuous TBS (cTBS) and intermittent TBS (iTBS)] delivered in six different combinations over SI in separate sessions. TBS protocols were delivered at 30 Hz and were as follows: a single cTBS protocol, a single iTBS protocol, cTBS followed by cTBS, iTBS followed by iTBS, cTBS followed by iTBS, and iTBS followed by cTBS. Measures included the amplitudes of the first and second somatosensory evoked potentials (SEPs) via median nerve stimulation, their paired-pulse ratio (PPR), and temporal order judgment (TOJ). Dependent measures were obtained before TBS and at 5, 25, 50, and 90 min following stimulation. Results indicate similar effects following cTBS and iTBS; increased amplitudes of the second SEP and PPR without amplitude changes to SEP 1, and impairments in TOJ. Metaplasticity was observed such that TOJ impairments following a single cTBS protocol were abolished following consecutive cTBS protocols. Additionally, consecutive iTBS protocols altered the time course of effects when compared with a single iTBS protocol. In conclusion, 30-Hz cTBS and iTBS protocols delivered in isolation induce effects consistent with a TBS-induced reduction in intracortical inhibition within SI. Furthermore, cTBS- and iTBS-induced metaplasticity appear to follow homeostatic and nonhomeostatic rules, respectively.