Neural correlates of individual differences in speech categorisation: evidence from subcortical, cortical, and behavioural measures

Categorisation is a fundamental cognitive ability to group different objects as the same. This ability is particularly indispensable for human speech perception, yet individual differences in speech categorisation are nonetheless ubiquitous. The present study investigates the neurophysiological mechanisms underlying the variability in categorisation of voice-onset time (VOT). Subcortical and cortical speech-evoked responses are recorded to investigate speech representations at two functional levels of auditory processing. Individual differences in psychometric functions correlate positively with how faithfully subcortical responses encode VOT differences. Moreover, individuals also differ in how strongly the subcortical and cortical representations correlate with each other. Listeners with gradient categorisation show higher correspondences between the two representations, indicating that acoustic information is relayed faithfully from the subcortical to the cortical level; listeners with discrete categorisation exhibit decreased similarity between the two representations, suggesting that the subcortical acoustic encoding is transformed at the cortical level to reflect phonetic category information.