FIRING OF INFERIOR COLLICULUS NEURONS IN RESPONSE TO LOW-FREQUENCY SOUND STIMULATION DURING SLEEP AND WAKING

In vivo extracellular recordings of 102 units in the central nucleus o f the inferior colliculus (IC), were made in chronically implanted gui nea-pigs during the sleep/ wake cycle. During wakefulness, the units w ere classified according to their response characteristics. Most neuro ns (63%) recorded showed changes, increasing or decreasing in the numb er of evoked discharges during the animal's transitions between wakefu lness and slow-wave sleep. In the paradoxical sleep phase, the result was similar; changes were observed in most neurons, while only 11% of units did not shift their discharge pattern during ipsilateral sound s timulation. The post-stimulus time histogram of the overall evoked pat tern of discharge showed sleep/wake dependency, i.e. changed in 35% of the units recorded during the 50 ms of sound stimulation. Fifty-five percent of auditory neurons did not show any change in the spontaneous firing rate during slow-wave sleep as compared to the previous waking period, while 22% exhibited a discharge increase and 23% decreased th eir firing. During paradoxical sleep, 14 out of 17 cells increased the ir spontaneous firing rate. The IC auditory neurons send descending co nnections to regions such as the dorsal pontine nuclei, known to media te sleep processes. Thus, for constant auditory input, the firing rate or number of discharge variations are due to functional shifts in the sleeping brain. Auditory processing is present during sleep and diffe rs from that observed during wakefulness. Differences were observed in the evoked firing number and/or spontaneous rate, as well as in the p attern of discharge. The ultimate reason for auditory unit shifts duri ng sleep remains yet unexplained.