The representation of order and randomness in the auditory cortex

Perception fundamentally relies on the brain’s ability to extract structure from its sensory environment. The auditory cortex possesses significant sensitivity to statistical regularities in tone sequences. To examine this phenomenon at the single-neuron level, we used multitone sequences and recorded responses from the auditory cortex of awake adult female Sprague Dawley rats. Tones were 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 phase sensitivity may emerge from a simple, biologically plausible computation. Neurons in the auditory cortex exhibit a continuum of preferences from order to randomness, and show sensitivity to the position of tones in abstract repeating patterns.