Single-unit physiology of the cat anteroventral cochlear nucleus: A comparison of the marginal shell and the central core

The marginal shell of the anteroventral cochlear nucleus (AVCN), which comprises the granule-cell layer (GCL), small cell cap (SCC) and the cell-poor rind , is anatomically different from the central core of the AVCN. This study investigated single-unit response properties of the AVCN marginal shell of the unanesthetized, decerebrate cat and contrasted them with those obtained from the AVCN central core in the same animals. This study represents 38 marginal-shell units in 10 cats and 62 central-core units in 15 cats. The sites of all of the 38 shell units were localized using reconstructed electrode tracks. Two different groups of marginal shell units existed: one that did not or had very weak response to any acoustic stimuli (32%) and another that responded well to acoustic stimulus (68%). One unifying characteristic of both groups is that they mostly exhibited low spontaneous rates. Among the ones that are acoustically well driven, a large majority (68%) of them exhibited wide dynamic ranges $({\ge} 50$ dB) to tone, noise or both with some as wide as 89 dB making them suitable candidates for playing a role in encoding intensity over a wide range of intensities. In contrast, a large majority (80%) of the core units exhibited narrow dynamic ranges, ${\u3c} 50$ dB, often showing quickly-rising and saturating rate-level functions to tone and noise. The post-stimulus time histograms (PSTHs), a measure of temporal response, of the well-driven shell units included unusual (33%), pause-build (29%), and onset (24%) types with few primary-like and regular-firing types. The excitatory-inhibitory area (EIA), a measure of excitation/inhibition in a frequency-intensity plane, of the well-driven shell units exhibited both simple and complex types with some of the complex types having large areas of inhibition. In contrast, the core units mainly exhibited EIA types that were of the simpler types. The differences between the shell and core neural groups were statistically significant, for all of the physiological characteristics examined. The above findings suggest that the well-driven shell units may play a role in encoding acoustic stimulus intensity and some of the not-driven and weakly-driven shell units may play a role in multi-sensory processing.