Induced neural beta oscillations predict categorical speech perception abilities

Neural oscillations have been linked to various perceptual and cognitive brain operations. Here, we examined the role of these induced brain responses in categorical speech perception (CP), a phenomenon in which similar features are mapped to discrete, common identities despite their equidistant/continuous physical spacing. We recorded neuroelectric activity while participants rapidly classified sounds along a vowel continuum (/u/ to /a/). Time-frequency analyses applied to the EEG revealed distinct temporal dynamics in induced (non-phase locked) oscillations; increased β (15-30 Hz) coded prototypical vowel sounds carrying well-defined phonetic categories whereas increased γ (50-70 Hz) accompanied ambiguous tokens near the categorical boundary. Notably, changes in β activity were strongly correlated with the slope of listeners' psychometric identification functions, a measure of the “steepness” of their categorical percept. Our findings demonstrate that in addition to previously observed evoked (phase-locked) correlates of CP, induced brain activity in the β-band codes the ambiguity and strength of categorical speech percepts.