Single-cell activity in human STG during perception of phonemes is organized according to manner of articulation

A long-standing controversy persists in psycholinguistic research regarding the way phonemes are coded in human auditory cortex during speech perception. Whereas the motor theory of speech perception suggests that phonemes are organized in terms of common articulatory gestures that generate them, auditory theories argue that phonetic processing is organized based on common spectro-temporal patterns in phoneme waveforms. Here, we recorded spiking activity in the superior temporal gyrus (STG) from six neurosurgical patients who performed a listening task with phoneme stimuli. Using a Naïve-Bayes model, we show that single-cell responses to phonemes are governed by articulatory features that have acoustic correlates (manner-of-articulation) and organized according to sonority, with two main clusters for sonorants and obstruents. We further find that ‘neural similarity’ (i.e. the similarity of evoked spiking activity between pairs of phonemes), is comparable to the ‘perceptual similarity’ (i.e. how much the pair of phonemes sound similar) based on perceptual confusion assessed behaviorally in healthy subjects. Thus phonemes that were perceptually similar, also had similar neural responses. Our findings establish that phonemes are encoded according to manner-of-articulation, supporting the auditory theories of perception, and that the perceptual representation of phonemes can be reflected by the activity of single neurons in STG.