Oscillation may play a role in time domain central auditory processing

To study how sound intensity altered the temporal response pattern of a unit, we recorded from 92 single neurons in the inferior colliculus (IC) of the little brown bat and investigated their firing patterns in response to brief tone pulses (2 msec duration) at the characteristic frequency of the unit over a wide dynamic range (10-90 dB sound pressure level). We found two unusual response characteristics at high sound levels in approximately one-third of the IC neurons investigated. For 16 IC neurons (17%), an increase in sound level not only elicited a shorter response latency and an increase in spike count but also transformed the firing pattern of the unit from phasic to periodic; this pattern was more pronounced at higher sound levels. The firing periodicity was unit specific, ranging from 1.3 to 6.7 msec. Twenty-seven IC neurons (29%) exhibited a longer response latency at higher sound levels compared with lower sound levels [i.e., paradoxical latency shift (PLS)]. The majority of this population showed a one or more quantum increase in latency when sound level was elevated. The quantum shift was also unit specific, ranging from 1.2 to 8.2 msec. We further investigated the firing patterns of 14 IC neurons showing PLS before, during, and after iontophoretic application of bicuculline. For 12 of these neurons, drug application abolished the PLS and transformed the firing patterns of the unit at high sound levels from phasic into sustained periodic discharges. Our results suggest that neural oscillation in combination with ordinary inhibition may be responsible for the creation of PLSs shown previously to be important for temporal information processing.