Ž . Cognitive Brain Research 6 1998 285–294 Research report Magnetic fields elicited by a tone onset time continuum in humans P.G. Simos ) , J.I. Breier, G. Zouridakis, A.C. Papanicolaou Department of Neurosurgery, The UniÕersity of Texas, Houston Medical School, Houston, TX, USA Accepted 30 December 1997 Abstract Ž . Event-related Fields ERFs were recorded in eleven normal volunteers in response to four two-tone stimuli selected from a tone onset Ž . time TOT continuum. The latter has been used extensively in the past as an analog of voice onset-time, an important cue for making Ž < < < <. Ž voicing distinctions e.g., pa versus ba . TOT values ranged from 0 to 60 ms in 20-ms steps. Three components of the ERF P50m, . N1m, and P2m in the hemisphere contralateral to the stimulated ear were analyzed. The most significant finding was an abrupt reduction Ž . in the peak field amplitude of N1m bilaterally as TOT values increased from q20 to q40 ms. No systematic differences were noted between the 0 and q20 ms stimuli or between the q40 and q60 ms tokens. On the other hand, peak component latencies rose steadily with increasing TOT values. The discontinuity in the change of peak RMS as a function of TOT is in close agreement with behavioral evidence of categorical perception of similar non-speech as well as speech stimuli. q 1998 Elsevier Science B.V. Keywords: Magnetoencephalography; Human; Tone onset time; Voice onset time; Speech; Auditory cortex 1. Introduction Ž . In speech perception, voice-onset time VOT is consid- ered an important acoustic cue for voicing contrasts in- w x volving syllable–initial stop consonants 16,18 . VOT is defined as the temporal delay between consonant release and the first glottal pulse, i.e., the separation of the articu- lators resulting in the release of a burst of air from the Ž vocal tract. Perception of VOT contrasts i.e., consonant– . vowel syllables sampled from a continuum of VOT values is categorical in nature, a phenomenon that is operationally defined by two different, but related, paradigms. First, Ž . when consonant–vowel CV syllables, in which the con- sonant sound varies systematically in equal steps of VOT values, are randomly presented to adult listeners for identi- fication, they are typically classified into two distinct categories. For instance, when listeners are exposed to a bilabial voicing continuum with VOT values ranging from Ž << << . 0 to 60 ms in 20 ms steps i.e., a b y p continuum , they identify consonants in the 0- and in the q20-ms tokens as Ž <<. voiced i.e., as b and the consonants in the q40-, and in ) Corresponding author. Department of Neurosurgery, University of Texas Medical School, 6431 Fannin Suite 7.148, Houston, TX 77030. Fax: q1-713-500-7787; E-mail: asimos@heart.med.uth.tmc.edu Ž <<. the q60-ms tokens as voiceless i.e., as p . Second, listeners reliably discriminate tokens from different pho- Ž netic categories e.g., the 0- and q20-ms VOT stimuli . from the q40- and the q60-ms VOT tokens , while discrimination is at chance levels for tokens in the same Ž category e.g., for the 0- vs. 20-ms tokens, and for the 40- . w x vs. 60-ms tokens 1,18 . By pooling data from the two tasks, researchers place the phonetic boundary for voicing distinctions involving bilabial consonants at approximately Ž . 30 ms i.e., between the q20- and the q40-ms stimuli w x 1,18 . Behaviorally identifiable phonetic category boundaries w x are present soon after birth in humans 2,5,14 . Moreover, categorical-like discrimination of VOT contrasts has been reported in several animal species, including the rhesus w x w x monkey 13 and the chinchilla 12 . These findings lend strong support to the hypothesis that at least some aspects of categorical perception of VOT contrasts are based on general properties of the mammalian auditory system, and are not unique to humans or to language. This hypothesis predicts that a detailed examination of the speech stimulus should reveal acoustic properties that can serve as cues to the mammalian auditory system Ž . including that of humans , and reliably predict psychoa- coustic measurements. With respect to voicing contrasts, two psychoacoustic phenomena that have drawn much 0926-6410r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.