The “Third Rhythm”: Alpha-like Activity over the Midtemporal Region

Abstract

.In addition to the posterior alpha and the rolandic mu rhythm, there is a third alphoid rhythm, which is located over the midtemporal region. This rhythm cannot be detected in the scalp EEG unless there is a bone defect over the midtemporal region. It is easily demonstrable with epidural electrodes. With the use of magnetoencephalography, the third rhythm is readily recorded through the intact skull.This rhythm is of physiological nature: it is impaired or absent in the presence of major temporal lobe pathology. Its frequency is 8–12/sec and may slow down to 6–7/sec. It is essentially a rhythm of wakefulness, declining (but still lingering on) in drowsiness and early NREM sleep. Its blocking mechanism is still debatable.The third rhythm is likely to arise from the neocortical portions of the midtemporal region. A deep origin in limbic structures is unlikely; this rhythm is not found in depth recordings from limbic structures.