Neurons sensitive to interaural phase disparity in gerbil superior olive: diverse monaural and temporal response properties

M. W. Spitzer and M. N. Semple Department of Anatomy and Neurobiology, University of California, Irvine 92717, USA. 1. We assessed mechanisms of binaural interaction underlying detection of interaural phase disparity (IPD) by recording single-unit responses in the superior olivary complex (SOC) of the anesthetized gerbil (Meriones unguiculatus). Binaural responses were obtained from 58 IPD-sensitive single units, 44 of which were histologically localized. Monaural responses were also obtained for 52 of 58 IPD-sensitive units. Additionally, responses were recorded from 16 units (best frequency < 2.4 kHz) in lateral SOC that were excited by ipsilateral stimulation and inhibited by contralateral stimulation (EI), none of which was IPD sensitive. Our results are consistent with a mechanism of binaural interaction involving detection of coincident excitatory inputs from the two ears. There was no compelling evidence of binaural sensitivity arising from IPD-dependent interactions of phase-locked excitatory and inhibitory inputs from the two ears. Despite the uniformity of binaural interactions, considerable diversity of temporal and monaural response properties was observed. 2. Monaural and binaural responses of 35 of 58 IPD-sensitive units were phase locked to the period of low-frequency (< 2.5 kHz) tones. Most phase-locking units were bilaterally excitable and, consistent with the coincidence-detection model, their IPD selectivity could be predicted from the difference between the mean phases of the monaural responses. The remaining units (23 of 58) did not phase lock in response to monaural or binaural tones. Most non-phase-locking units failed to respond to monaural stimulation of one or both ears (monaurally unresponsive units). 3. Some IPD-sensitive units were inhibited by monaural stimulation of the ipsilateral ear or both ears. A few units responded only at the onset of monaural and binaural tones. Phase locking was present in responses of some, but not all, of these monaurally inhibited and onset units. 4. Most IPD-sensitive neurons were encountered at sites within or immediately adjacent to the cell column of the medial superior olive (MSO). IPD-sensitive units were also recorded in the lateral superior olive (LSO), in the superior paraolivary nucleus (SPN), and within a region forming a medial-dorsal cap around MSO. Bilaterally excitable unites were concentrated around MSO, but were also encountered in SPN, the medial-dorsal region, and LSO. Some mona