LOW-FREQUENCY NEURONS IN THE LATERAL SUPERIOR OLIVE EXHIBIT PHASE-SENSITIVE BINAURAL INHIBITION

Responses of low characteristic frequency (CF) neurons in the lateral limb of the lateral superior olive (LSO) of chinchilla and rat to binaural stimuli at various interaural phase and intensity differences were examined and compared to responses from previous studies of high CF neurons. Ninety-six LSO neurons from chinchillas and 10 LSO neurons from rats with CFs <1,200 Hz were characterized. The majority of these neurons displayed phase-locked tone-evoked temporal discharge patterns to ipsilateral CF stimuli. Similar to high-CF LSO neurons, low-CF LSO neurons were excited by ipsilateral stimuli and inhibited by contralateral stimuli, with discharge rate sensitive to interaural intensity differences (IID). Discharge rate increased as ipsilateral intensity was increased and decreased as contralateral stimulus intensity was increased. Binaural inhibition, inhibition of ipsilaterally evoked activity by contralateral stimuli, was dependent on interaural phase differences (IPD) in the majority of low-CF LSO neurons. Responses of phase-sensitive neurons to binaural stimuli often varied with 90 or 180° changes in IPD from total inhibition to a facilitated response when compared to responses to control ipsilateral stimuli alone. In summary, like high-CF LSO neurons, LSO neurons with low CFs (<1,200 Hz) were ipsilaterally excited and contralaterally inhibited (EI) and were sensitive to IID. Unlike most high-CF EI LSO neurons, which are not responsive when the azimuth of the stimulus is directly in front of or directly behind the animal, many low-CF LSO neurons are responsive to these stimuli. Localization of sounds may involve coprocessing of information by the LSO, medial superior olive (MSO), and the lateral limb of the LSO with one structure being more important depending on the location, frequency, and/or duration of the sound.