Neurons in auditory cortex are sensitive to the probability of stimuli: responses to rare stimuli tend to be stronger than responses to common ones. Here, intra- and extracellular recordings from the auditory cortex of halothane-anesthetized rats revealed the existence of a finer sensitivity to the structure of sound sequences. Using oddball sequences in which the order of stimulus presentations is periodic, we found that tones in periodic sequences evoked smaller responses than the same tones in random sequences. Significant reduction in the responses to the common tones in periodic relative to random sequences occurred even when these tones consisted of 95% of the stimuli in the sequence. The reduction in responses paralleled the complexity of the sound sequences and could not be explained by short-term effects of clusters of deviants on succeeding standards. We conclude that neurons in auditory cortex are sensitive to the detailed structure of sound sequences over timescales of minutes.
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