FIRING OF INFERIOR COLLICULUS AUDITORY NEURONS IS PHASE-LOCKED TO THEHIPPOCAMPUS THETA-RHYTHM DURING PARADOXICAL SLEEP AND WAKING

The activity of 52 single auditory units in the central nucleus of the inferior colliculus (IC) was recorded along with cortical and hippoca mpal (CA1) electrograms and neck muscle electromyograms in behaving, h ead-restrained guinea pigs during paradoxical sleep (PS) and wakefulne ss. Sixteen (30%) of the IC auditory Units showed positive correlation with the hippocampal theta (theta) rhythm: 8 (15%) were theta rhythmi c with theta phase-locking (type 1), 8 (15%) showed only theta phase-l ocking with no rhythmicity (type 2), while 70% did not show any correl ation to hippocampal theta rhythm (type 3). During wakefulness IC neur ons (4 of 13) showed a higher synchrony with hippocampal theta when so und-stimulated at the unit's characteristic frequency. During PS all I C auditory neurons recorded presented some hippocampal theta correlati on: 40% were rhythmic and phase-locked to the theta frequency and 60% were nonrhythmic maintaining the theta phase-locking. Shifts in the an gle of phase-locking to the theta rhythm were observed during PS. It i s suggested that the hippocampal theta rhythm may play the part of an internal clock, adding a temporal dimension to the processing of audit ory sensory information.