Temporal encoding of the voice onset time phonetic parameter by field potentials recorded directly from human auditory cortex

Temporal encoding of the voice onset time phonetic parameter by field poten tials recorded directly from human auditory cortex. J. Neurophysiol. 82: 23 46-2357, 1999. Voice onset time NOT) is an important parameter of speech th at denotes the time interval between consonant onset and the onset of low-f requency periodicity generated by rhythmic vocal cord vibration. Voiced sto p consonants (/b/, /g/, and /d/) in syllable initial position are character ized by short VOTs, whereas unvoiced stop consonants (/p/, /k/, and t/) con tain prolonged VOTs. As the VOT is increased in incremental steps, percepti on rapidly changes from a voiced stop consonant to an unvoiced consonant at an interval of 20-40 ms. This abrupt change in consonant identification is an example of categorical speech perception and is a central feature of ph onetic discrimination. This study tested the hypothesis that VOT is represe nted within auditory cortex by transient responses time-locked to consonant and voicing onset. Auditory evoked potentials (AEPs) elicited by stop cons onant-vowel (CV) syllables were recorded directly from Heschl's gyrus, the planum temporale, and the superior temporal gyrus in three patients undergo ing evaluation for surgical remediation of medically intractable epilepsy. Voiced CV syllables elicited a triphasic sequence of field potentials withi n Heschl's gyrus. AEPs evoked by unvoiced CV syllables contained additional response components time-locked to voicing onset. Syllables with a VOT of 40, 60, or 80 ms evoked components time-locked to consonant release and voi cing onset. In contrast, the syllable with a VOT of 20 ms evoked a markedly diminished response to voicing onset and elicited an AEP very similar in m orphology to that evoked by the syllable with a 0-ms VOT. Similar response features were observed in the AEPs evoked by click trains. In this case, th ere was a marked decrease in amplitude of the transient response to the sec ond click in trains with interpulse intervals of 20-25 ms. Speech-evoked AE Ps recorded from the posterior superior temporal gyrus lateral to Heschl's gyrus displayed comparable response features, whereas field potentials reco rded from three locations in the planum temporale did not contain component s time-locked to voicing onset. This study demonstrates that VOT at least p artially is represented in primary and specific secondary auditory cortical fields by synchronized activity time-locked to consonant release and voici ng onset. Furthermore, AEPs exhibit features that may facilitate categorica l perception of stop consonants, and these response patterns appear to be b ased on temporal processing limitations within auditory cortex. Demonstrati ons of similar speech-evoked response patterns in animals support a role fo r these experimental models in clarifying selected features of speech encod ing.