Natural rhythm: evidence for occult 40 Hz gamma oscillation in resting motor cortex.

Fast gamma oscillations, often at 40 Hz, have been demonstrated throughout the brain including the thalamus, auditory, visual and motor cortices. The function of gamma rhythms is elusive, but several authors have hypothesized that they contribute to the "binding" of diverse information into a single coherent percept, and to the synchronization of movement. In skeletal muscle a "Piper rhythm" around 40 Hz is commonly observed during maximal voluntary contraction, and has been shown to correlate with activity of similar frequency in a limited area of contralateral motor cortex. Gamma rhythms are detected primarily during complex cortical activity, and are seldom recorded at rest or coherently over wide areas. Here we use bihemispheric transcranial magnetic stimulation (TMS) to study time-dependent correlations between evoked motor potentials from non-homologous muscles in opposite limbs of normal volunteers. The results suggest the presence of an occult, synchronous 40 Hz rhythm across broad areas of resting motor cortex in both hemispheres.