SENSITIVITIES OF CELLS IN ANTEROVENTRAL COCHLEAR NUCLEUS OF CAT TO SPATIOTEMPORAL DISCHARGE PATTERNS ACROSS PRIMARY AFFERENTS

This study tested the hypothesis that a cell in the anteroventral cochlear nucleus (AVCN) that receives convergent input from auditory nerve (AN) fibers can be sensitive to the temporal pattern of discharges on the set of AN fibers providing its input. The temporal discharge pattern across the population of low-frequency AN fibers was manipulated by varying the phase spectra of complex stimuli that had fixed, flat magnitude spectra. By introducing a phase shift with variable slope at a particular frequency, the relative times of discharge of phase-locked neurons with different characteristic frequencies (CFs) could be varied. In this manner the overall spatiotemporal discharge pattern across the array of AN fibers was systematically manipulated. Some low-frequency cells in the AVCN were sensitive to changes in the slope of the phase transition of the complex stimulus. The cells that were sensitive came from several cell types in the AVCN. Their responses were consistent with the hypothesis that these cells were sensitive to the temporal relationships between discharges on their primary inputs and that they received inputs with different CFs, because the phase shifts introduced relative time differences between different frequencies. Other cells were not sensitive to the degree of phase shift of the stimulus. This insensitivity implied either that these cells received inputs of the same, or nearly the same. CF, or that they were not sensitive to the time differences introduced by these changes in the phase spectra, or both. The cells that were sensitive to the manipulations of the phase spectrum were located in the posterior region of anterior AVCN and in the posterior region of and thus were presumably either globular bushy, small spherical bushy, or stellate cells. No sensitive cells were located in the most anterior region of the AVCN, where large spherical bushy cells are located. Temporal discharge patterns across the AN popultion in response to complex stimuli change as a function of level. Accordingly, the sensitivity of neurons to changes in the phase transitions of the complex stimuli used in this study was often affected by the levels of the stimulus. The sensitivity to changes in the phase spectrum was a frequency-specific effect. That is, a cell was most sensitive to changes made in phase that were centered near its CF and less sensitive to changes in phase that were introduced at frequencies below or above CF. The type of sensitivity to changes in phase spectra reported here is consistent with a coincidence mechanism operating on the converging AN inputs to these AVCN cells. For some neurons in the AVCN coincidence detection, or equivalently cross-correlation of the primary inputs, may represent a neural mechanism relevant for the processing of temporal information associated with complex stimuli.