EXCITATORY AND INHIBITORY RESPONSE ADAPTATION IN THE SUPERIOR OLIVE COMPLEX AFFECTS BINAURAL ACOUSTIC PROCESSING

Short-term adaptation was examined in single unit recordings from 113 superior olive neurons of anaesthetized 3- to 6-month-old Long-Evans r ats. Responses to an equal intensity BF probe tone presented 1 ms afte r an 'adapting' BF tone were adapted by 56.3 +/- 2.6% (mean +/- S.E.) compared to responses at a 512 ms delay. The rapid decrease in dischar ge rate during adapting tones often approximated exponential time cour ses with time constants of less than 20 ms. The recovery from adaptati on was exponential with time constants of 106 +/- 20.0 ms. The magnitu de of adaptation and time course of recovery following monaural stimul ation of binaurally excited (EE) neurons were not significantly differ ent in both input pathways. Additionally, in 60% of EE neurons; an 'ad apting' tone presented to one ear reduced subsequent responses to prob e tones presented to the opposite ear. Binaural stimulation resulted i n equal or greater adaptation of responses than monaural stimulation o f either ear. The recovery of binaural excitatory responses generally followed a time course between recovery functions for ipsilateral and contralateral monaural stimuli. Lateral Superior Olive (LSO) neurons e ncode sound source location through the interaction of ipsilateral exc itation and contralateral inhibition (IE). Ipsilaterally driven excita tory responses in LSO neurons exhibited the greatest magnitude of adap tation (68.5 +/- 21.1%). Adaptation of inhibition was observed in over half of IE neurons. Responses of LSO neurons to binaural BF probe sti muli were greatest immediately after a 200 ms BF 'inhibitory adapting' stimulus to the contralateral ear, and decreased with greater interst imulus delays. Responses to binaural stimulation were constant after p rior binaural adaptation, when the magnitude and recovery of adaptatio n to monaural stimuli were similar for excitation and inhibition (8/25 IE cells). The functional significance and possible sites of adaptati on processes are discussed.