Temporal Encoding Of The Voice Onset Time (VOT) Phonetic Parameter By Field Potentials Recorded Directly From Human Auditory Cortex. Mitchell Steinschneider1, Igor O. Volkov2, M. Daniel Noh2, P. Charles Garell2, and Matthew A. Howard III2. 1Departments of Neurology and Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461 and 2Department of Surgery (Division of Neurosurgery), University of Iowa College of Medicine, Iowa City, Iowa 52242.
APStracts 6:0382N, 1999.
Voice onset time (VOT) is an important parameter of speech that denotes the time interval between consonant onset and the onset of low frequency periodicity generated by rhythmic vocal cord vibration. Voiced stop consonants (/b/, /g/, and /d/) in syllable initial position are characterized by short VOTs, while unvoiced stop consonants (/p/, /k/, and /t/) contain prolonged VOTs. As the VOT is increased in incremental steps, perception rapidly changes from a voiced stop consonant to an unvoiced consonant at an interval of 20-40 msec. This abrupt change in consonant identification is an example of categorical speech perception, and is a central feature of phonetic discrimination. This study tested the hypothesis that VOT is represented within auditory cortex by transient responses time-locked to consonant and voicing onset. Auditory evoked potentials (AEPs) elicited by stop consonant-vowel (CV) syllables were recorded directly from Heschl's gyrus, the planum temporale and the superior temporal gyrus in three patients undergoing evaluation for surgical remediation of medically intractable epilepsy. Voiced CV syllables elicited a triphasic sequence of field potentials within 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 msec evoked components time-locked to consonant release and voicing onset. In contrast, the syllable with a VOT of 20 msec evoked a markedly diminished response to voicing onset, and elicited an AEP very similar in morphology to that evoked by the syllable with a 0 msec VOT. Similar response features were observed in the AEPs evoked by click trains. In this case, there was a marked decrease in amplitude of the transient response to the second click in trains with inter-pulse-intervals of 20-25 msec. Speech-evoked AEPs recorded from the posterior superior temporal gyrus lateral to Heschl's gyrus displayed comparable response features, while field potentials recorded from 3 locations in the planum temporale did not contain components time-locked to voicing onset. This study demonstrates that VOT is at least partially represented in primary and specific secondary auditory cortical fields by synchronized activity time-locked to consonant release and voicing onset. Furthermore, AEPs exhibit features that may facilitate categorical perception of stop consonants, and these response patterns appear to be based on temporal processing limitations within auditory cortex. Demonstrations of similar speech-evoked response patterns in animals support a role for these experimental models in clarifying selected features of speech encoding. 

Received 26 January 1999; accepted in final form 5 August 1999.
APS Manuscript Number J064-9.
Article publication pending Journal of Neurophysiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 13 August 1999