Rethinking hierarchy: the auditory system as an integrated cortical-subcortical network.

Mammalian sensory systems are traditionally viewed as hierarchical pathways in which subcortical nuclei relay signals from peripheral receptors to the cortex, where sensory information is contextualized for perception and behaviour. However, the auditory pathway contains an unusually large number of anatomically complex, recurrently connected subcortical nuclei that transmit heavily pre-processed information to the cortex. Emerging evidence shows that the auditory system functions as an integrated cortical-subcortical network that is heavily influenced by non-auditory inputs and extensive bidirectional connectivity at nearly all levels. Subcortical structures are not passive relays but active participants in computations traditionally attributed to cortex, including adaptive coding of sound statistics, multisensory integration, encoding of behavioural relevance and action, and learning. Although certain transformations occur hierarchically - such as brainstem spatial processing - sensitivity to most sound features forms a continuum across the auditory pathway, with modulation by other sensory, motor and cognitive systems at every stage. The degree of subcortical pre-processing may explain why emergent cortical properties are harder to identify in audition than in vision, in which cortical and subcortical receptive field properties are more distinct. This distributed circuit organization opens new avenues for understanding how the brain constructs perception and guides behaviour from the fusion of sensory evidence and contextual knowledge.