MEG correlates of categorical perception of a voice onset time continuum in humans.

Event-related magnetic fields (ERFs) were recorded from the left hemisphere in nine normal volunteers in response to four consonant-vowel (CV) syllables varying in voice-onset time (VOT). CVs with VOT values of 0 and +20 ms were perceived as /ga/ and those with VOT values of +40 and +60 ms as /ka/. Results showed: (1) a displacement of the N1m peak equivalent current dipole toward more medial locations; and (2) an abrupt reduction in peak magnetic flux strength, as VOT values increased from +20 to +40 ms. No systematic differences were noted between the 0 and +20 ms stimuli or between the +40 and +60 ms CVs. The findings are in agreement with the results of multiunit invasive recordings in non-human primates regarding the spatial and temporal pattern of neuronal population responses in the human auditory cortex which could serve as neural cues for the perception of voicing contrasts.