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The Journal of Neuroscience, July 19, 2006, 26(29):7629-7639; doi:10.1523/JNEUROSCI.5501-05.2006

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Behavioral/Systems/Cognitive
Cracking the Language Code: Neural Mechanisms Underlying Speech Parsing

Kristin McNealy,^1,5,6 John C. Mazziotta,^1,3,4,5 and Mirella Dapretto^1,2,3,5,6

¹Ahmanson-Lovelace Brain Mapping Center, Semel Institute for Neuroscience and Human Behavior, ²Department of Psychiatry and Biobehavioral Sciences, ³The Brain Research Institute, ⁴Departments of Neurology, Pharmacology, and Radiological Sciences in the David Geffen School of Medicine, ⁵Neuroscience Interdepartmental Program, and ⁶Center for Culture, Brain, and Development, University of California, Los Angeles, Los Angeles, California 90095

Correspondence should be addressed to Mirella Dapretto, Ahmanson-Lovelace Brain Mapping Center, 660 Charles E. Young Drive, Los Angeles, CA 90095. Email: mirella{at}loni.ucla.edu

Word segmentation, detecting word boundaries in continuous speech, is a critical aspect of language learning. Previous research in infants and adults demonstrated that a stream of speech can be readily segmented based solely on the statistical and speech cues afforded by the input. Using functional magnetic resonance imaging (fMRI), the neural substrate of word segmentation was examined on-line as participants listened to three streams of concatenated syllables, containing either statistical regularities alone, statistical regularities and speech cues, or no cues. Despite the participants’ inability to explicitly detect differences between the speech streams, neural activity differed significantly across conditions, with left-lateralized signal increases in temporal cortices observed only when participants listened to streams containing statistical regularities, particularly the stream containing speech cues. In a second fMRI study, designed to verify that word segmentation had implicitly taken place, participants listened to trisyllabic combinations that occurred with different frequencies in the streams of speech they just heard ("words," 45 times; "partwords," 15 times; "nonwords," once). Reliably greater activity in left inferior and middle frontal gyri was observed when comparing words with partwords and, to a lesser extent, when comparing partwords with nonwords. Activity in these regions, taken to index the implicit detection of word boundaries, was positively correlated with participants’ rapid auditory processing skills. These findings provide a neural signature of on-line word segmentation in the mature brain and an initial model with which to study developmental changes in the neural architecture involved in processing speech cues during language learning.

Key words: fMRI; language; speech perception; word segmentation; statistical learning; auditory cortex; inferior frontal gyrus

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Received Dec. 22, 2005; revised June 7, 2006; accepted June 7, 2006.

Correspondence should be addressed to Mirella Dapretto, Ahmanson-Lovelace Brain Mapping Center, 660 Charles E. Young Drive, Los Angeles, CA 90095. Email: mirella{at}loni.ucla.edu

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