Primary auditory cortex represents the location of sound sources in a cue-invariant manner

Abstract

Auditory cortex is required for sound localisation, but how neural firing in auditory cortex underlies our perception of sources in space remains unknown. We measured spatial receptive fields in animals actively attending to spatial location while they performed a relative localisation task using stimuli that varied in the spatial cues that they provided. Manipulating the availability of binaural and spectral localisation cues had mild effects on the ferrets performance and little impact on the spatial tuning of neurons in primary auditory cortex (A1). Consistent with a representation of space, a subpopulation of neurons encoded spatial position across localisation cue types. Spatial receptive fields measured in the presence of a competing sound source were sharper than those measured in a single-source configuration. Together these observations suggest that A1 encodes the location of auditory objects as opposed to spatial cue values. We compared our data to predictions generated from two theories about how space is represented in auditory cortex: The two-channel model, where location is encoded by the relative activity in each hemisphere, and the labelled-line model where location is represented by the activity pattern of individual cells. The representation of sound location in A1 was mainly contralateral but peak firing rates were distributed across the hemifield consistent with a labelled line model in each hemisphere representing contralateral space. Comparing reconstructions of sound location from neural activity, we found that a labelled line architecture far outperformed two channel systems. Reconstruction ability increased with increasing channel number, saturating at around 20 channels.\n\nSignificance statementOur perception of a sound scene is one of distinct sound sources each of which can be localised, yet auditory space must be computed from sound location cues that arise principally by comparing the sound at the two ears. Here we ask: (1) do individual neurons in auditory cortex represent space, or sound localisation cues? (2) How is neural activity read out for spatial perception? We recorded from auditory cortex in ferrets performing a localisation task and describe a subpopulation of neurons that represent space across localisation cues. Our data are consistent with auditory space being read out using the pattern of activity across neurons (a labelled line) rather than by averaging activity within each hemisphere (a two-channel model).