A hierarchy of responses to auditory regularities in the macaque brain.

Can monkeys learn simple auditory sequences and detect when a new sequence deviates from the stored pattern? Here we tested the predictive-coding hypothesis, which postulates that cortical areas encode internal models of sensory sequences at multiple hierarchical levels, and use these predictive models to detect deviant stimuli. In humans, hierarchical predictive coding has been supported by studies of auditory sequence processing, but it is unclear whether internal hierarchical models of auditory sequences are also available to nonhuman animals. Using fMRI, we evaluated the encoding of auditory regularities in awake monkeys listening to first- and second-order sequence violations. We observed distinct fMRI responses to first-order violations in auditory cortex and to second-order violations in a frontoparietal network, a distinction only demonstrated in conscious humans so far. The results indicate that the capacity to represent and predict the structure of auditory sequences is shared by humans and nonhuman primates.