Cortical substrates of perceptual confusion between pitch and timbre

Pitch and timbre, two fundamental perceptual attributes of sound, are critical for both speech and music. Although governed by different acoustic parameters, pitch and brightness (one of the primary dimensions of timbre) can be perceptually confused. Here we identify potential neural substrates of this well-known illusion by independently manipulating pitch and brightness while measuring fMRI responses. First, we show that the preferred pitch of individual voxels within auditory cortex decreases systematically as stimulus brightness increases, and vice versa, consistent with predictions from perceptual confusion. Second, we demonstrate that pitch and brightness share a common tuning gradient across auditory cortex, implying a shared trajectory of cortical activation for changes in each dimension. The pattern of results at both local and global cortical levels are consistent with the principle of efficient coding, with the brain exploiting correlations between pitch and brightness in natural sounds via a shared neural code.