Temporal Encoding of the Voice Onset Time Phonetic Parameter by Field Potentials Recorded Directly From Human Auditory Cortex

Abstract

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, whereas 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 ms. 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 ms evoked components time-locked to consonant release and voicing 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 morphology 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, there was a marked decrease in amplitude of the transient response to the second click in trains with interpulse intervals of 20-25 ms. Speech-evoked AEPs recorded from the posterior superior temporal gyrus lateral to Heschl's gyrus displayed comparable response features, whereas field potentials recorded from three locations in the planum temporale did not contain components time-locked to voicing onset. This study demonstrates that VOT at least partially is 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.

Auditory-evoked potentials to frequency increase and decrease of high- and low-frequency tones

“…The double-peaked N 100 that has been described in response to speech signals with long Voice Onset Times (VOT) is in line with this suggestion, even though the two peaks have been attributed to the release burst and to the onset of voicing (Sharma et al, 2000). A similar response pattern was obtained in intracranial recordings from the human cortical surface (Liegeois-Chauvel et al, 1999;Steinschneider et al, 1999) and from animals (Steinschneider et al, 1994).…”

Differential Ear Effects of Profound Unilateral Deafness on the Adult Human Central Auditory System

“…As noted by Näätänen and Picton (1987) and demonstrated by the intracranial recordings of a number of investigators (e.g., Liègeois-Chauvel et al 1991Steinschneider et al 1999), the N 1 potential as recorded from the scalp represents the sum of three or more generators. It is certainly the case that the regional dipole source model represents a first approximation and a center of gravity for this complex generator activity.…”

Efficient Encoding of Vocalizations in the Auditory Midbrain

“…This hypothesis is grounded in observations of phase-locked responses throughout the auditory system, originating in the auditory nerve (Galambos and Davis, 1943) and extending to auditory cortex (Steinschneider et al, 1999). Information theoretic analyses of real and simulated sensory neurons have quantified how variability in spike timing reduces the information content of the neural code (Bialek et al, 1991;Rieke et al, 1993Rieke et al, , 1995Rokem et al, 2006;Shamir et al, 2007).…”