The representation of regularity and randomness in auditory cortex of awake rats

Auditory cortex possesses a remarkable ability to discriminate between tone sequences with different levels of statistical regularities. To examine this phenomenon at the level of single neurons, we recorded responses from auditory cortex of awake rats. The rats passively listened to multitone sequences composed of 4, 5 or 6 tones. The tones could be presented in periodic sequences with a fixed, repeated cycle; in a fully random condition; or in an intermediate condition in which cycles were maintained but tones were randomly permuted within each cycle (random cycle condition). Units showed a continuum of preferences between order-preferring neurons and randomness-preferring neurons. Unexpectedly, we found sensitivity to the position of sounds in the cycle (‘phase modulation’). The strongest and most consistent phase modulations were in the responses to the random cycle condition. Finally, we show how such sensitivities may emerge from a simple, biologically- plausible computation.