SGER: MEG Studies of Speech Perception

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MARANTZ This study employs MEG imaging techniques to investigate how the brain represents and uses linguistic categories. Ultimately this research will explore all levels of linguistic analysis from the phonetic to the semantic. The current project starts with phonetic and phonological categories - distinctive features of speech sounds - and how they are realized during speech perception at auditory cortex. The primary goal of this preliminary research is to compare the brain's reaction to the acoustic as opposed to the linguistic properties of speech. For example, voiced and voiceless stops (in English) are distinguished acoustically by voicing onset time (VOT), a continuous physical dimension. Perception of speech sounds, however, isn't continuous along the VOT dimension. Along this dimension, sounds are heard categorically, as belonging to either the voiced or voiceless categories. Relatively large differences in VOT within a category are difficult to perceive while relatively small differences that cross the boundary between categories are easy to perceive. It is clear that the auditory system and brain do not map the physically continuous dimension of VOT linearly, or log-linearly as pitch and loudness are mapped. Possible neurophysiological bases of categorical perception have been suggested from the results of experiments with animals. This project asks to what extent the human response resembles the animal models and what constitutes linguistic category perception over and above the physical response common to the mammalian auditory system.