Marginal shell of the anteroventral cochlear nucleus: single-unit response properties in the unanesthetized decerebrate cat.

The marginal shell of the anteroventral cochlear nucleus (AVCN) is anatomically different from its central core. We investigated 38 single units in the shells of 10 cats and contrasted them with 62 single units in the cores of 15 cats. The sites of all shell units were localized with the use of reconstructed electrode tracks. The shell units were divided into acoustically well-driven (68%) and weakly/not-driven (32%) subgroups. The shell units mostly exhibited low spontaneous rates (SRs). Among the well-driven shell units, a large majority (68%) exhibited wide dynamic ranges (> or = 50 dB) to tones, noise, or both, with some ranges as wide as 89 dB. In contrast, a large majority (80%) of the core units exhibited narrow dynamic ranges (< 50 dB) to tones and noise. The poststimulus time histograms (PSTHs) of the well-driven shell units included pause-build (29%), onset (24%), and unusual (33%) types, whereas those of the core units included mainly primary-like (47%) and chopper (29%) types. The excitatory-inhibitory areas (EIAs) of the well-driven shell units included types I/III (47%), III (22%), IV (13%), and II (9%), whereas those of the core units included mainly types III (52%) and I/III (32%). On the basis of Fisher's exact tests, we conclude that the shell and core neural groups of the AVCN are significantly different regarding all of the following physiological characteristics: SR, maximum driven rate, threshold and dynamic range to tones and noise, frequency response area, PSTH type, latency, and EIA type. Wide dynamic ranges of the well-driven shell units suggest that they may play a role in encoding absolute intensity of acoustic stimulus.