Is it possible to compare inhibitory and excitatory intracortical circuits in face and hand primary motor cortex?

Face muscles are important in different functions such as feeding, speech and communication
of non-verbal affective states, which require quite different patterns of activity than those of a
typical hand muscle. We ask whether there are differences in their neurophysiological control
that might reflect this. Fifteen healthy individuals were studied. Standard single- and pairedpulse TMS methods were used to compare intracortical inhibitory (short interval intracortical
inhibition, SICI; cortical silent period, CSP) and excitatory circuitries (short interval
intracortical facilitation, SICF) in two typical muscles, the depressor anguli oris (DAO), a
face muscle, and the first dorsal interosseous (FDI), a hand muscle. TMS threshold was
higher in DAO than in FDI. Over a range of intensities, resting SICF was not different
between DAO and FDI, while during muscle activation SICF was stronger in FDI than DAO
(p=0.012). At rest, SICI was stronger in FDI than DAO (p=0.038) but during muscle
contraction, SICI was weaker in FDI than DAO (p=0.034). We argue that although many of
the difference in response to the TMS protocols could be result from the difference in
thresholds, some, such as the reduction of resting SICI in DAO may reflect fundamental
differences in the physiology of the two muscle groups.