Friday, July 28, 2006

Cracking the Language Code...

An interesting article by McNealy et al. in J. Neuroscience begins to look at the neural basis of detecting word boundaries in continuous speech using functional magnetic resonance imaging (fMRI). From their abstract: "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. "

Figure: left superior temporal gyrus activity associated with listening to the artificial language conditions (compared with the random syllables condition) during the speech stream exposure task was significantly correlated with participants’ ability to discriminate words that occurred in the artificial languages. Credit: J. Neuroscience

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.

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