Responses to contralaterally presented tone bursts were recorded from single auditory neurons in the rat's dorsal cortex of the inferior colliculus (ICd). Most of these neurons either had low level of firing or generated no action potentials in the absence of sound stimulation. In response to tone bursts, about half of the ICd neurons fired at the onset while about 40% of the neurons fired over the entire duration of the sounds. The remaining ICd neurons fired transiently at the offset or both the onset and the offset of the sounds. In comparison with neurons in the central nucleus of the inferior colliculus, neurons in the ICd had much longer first spike latencies and much larger magnitudes of jitter in the first spike latency. Many neurons in the ICd displayed complex frequency tuning properties. No strong correlation was revealed between the characteristic frequency of a neuron and its location along an axis oriented from a rostrodorsolateral to a caudoventromedial part of the ICd. There are neurons in the ICd showing stimulus-specific adaptation. For these neurons, presentations of a tone burst elicited stronger responses when these presentations were embedded in a train of tone bursts with various different frequencies than clustered in a block. Neurons with stimulus-specific adaptation displayed phasic firing at the onset of a tone burst. Our results suggest that the ICd is important for detecting novel sounds in the acoustic environment.
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