Monaural and binaural response properties of neurons in the inferior colliculus of the rabbit: effects of sodium pentobarbital.

1. We studied the effects of sodium pentobarbital on 22 neurons in the inferior colliculus (IC) of the rabbit. We recorded changes in the sensitivity of these neurons to monaural stimulation and to ongoing interaural time differences (ITDs). Monaural stimuli were tone bursts at or near the neuron's best frequency. The ITD was varied by delivering tones that differed by 1 Hz to the two ears, resulting in a 1-Hz binaural beat. 2. We assessed a neuron's ITD sensitivity by calculating three measures from the responses to binaural beats: composite delay, characteristic delay (CD), and characteristic phase (CP). To obtain the composite delay, we first derived period histograms by averaging, showing the response at each stimulating frequency over one period of the beat frequency. Second, the period histograms were replotted as a function of their equivalent interaural delay and then averaged together to yield the composite delay curve. Last, we calculated the composite peak or trough delay by fitting a parabola to the peak or trough of this composite curve. The composite delay curve represents the average response to all frequencies within the neuron's responsive range, and the peak reflects the interaural delay that produces the maximum response. The CD and CP were estimated from a weighted fit of a regression line to the plot of the mean interaural phase of the response versus the stimulating frequency. The slope and phase intercept of this regression line yielded estimates of CD and CP, respectively. These two quantities are thought to reflect the mechanism of ITD sensitivity, which involves the convergence of phase-locked inputs on a binaural cell. The CD estimates the difference in the time required for the two inputs to travel from either ear to this cell, whereas the CP reflects the interaural phase difference of the inputs at this cell. 3. Injections of sodium pentobarbital at subsurgical dosages (less than 25 mg/kg) almost invariably altered the neuron's response rate, response latency, response pattern, and spontaneous activity. Most of these changes were predictable and consistent with an enhancement of inhibitory influences. For example, if the earliest response was inhibitory, later excitation was usually reduced and latency increased. If the earliest response was excitatory, the level of this excitation was unaltered or slightly enhanced, and changes in latency were minimal. 4. The neuron's response pattern also changed in a predictable way. For example, a response with an inhibitory pause could either change to a response with a longer pause or to a response with an onset only.(ABSTRACT TRUNCATED AT 400 WORDS)