ROLE OF SYNAPTIC INHIBITION IN PROCESSING OF DYNAMIC BINAURAL LEVEL STIMULI

We have recently discovered a paradoxical aftereffect associated with inhibition in the gerbil auditory midbrain. Single neurons in the infe rior colliculus (IC) were assessed for sensitivity to a virtual motion stimulus produced by modulating the interaural level difference (ILD) , a major cue for sound localization. The class of neuron studied was predominantly excited by contralateral stimulation and inhibited by ip silateral stimulation. Sound pressure level was modulated trapezoidall y at the ipsilateral ''inhibitory'' ear, whereas the contralateral ''e xcitatory'' level remained constant. When the inhibitory stimulus was decreased within a range of sound levels that maintained suppression u nder static conditions, an unexpected discharge was often elicited, ap parently because of an aftereffect of synaptic inhibition. In contrast , when the inhibitory stimulus was increased within a range of sound l evels that produced only modest suppression under static conditions, n euronal discharge was often profoundly suppressed. In many cases the ' 'conditioned enhancement'' or ''conditioned suppression'' persisted fo r several seconds after the modulation of ILD, and such conditioned re sponses were influenced by the modulation depth and rate. To test the effect of inhibition in the IC directly, glycine and GABA were pulsed from a glass recording pipette during a constant monaural excitatory s timulus. The acoustically elicited discharge rate was potentiated mark edly if preceded immediately by the brief (0.5-10 sec) application of inhibitory transmitter. Collectively, these results revealed unusually long-lasting effects of inhibition that may establish a new range of acoustic cues to which the neuron responds best. This may have broad i mplications for processing ensuing auditory stimuli.