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Saturday stream 2 Session 11.35 - 12.50 Length 25 minutes
Piers Cornelissen
University of Newcastle, Newcastle upon Tyne, UK P.L.Cornelissen@newcastle.ac.uk
Abstract
In the last 5 years, MEG (magentoencephalographic) studies have revealed much about the nature, location and timing of letter-string processing in the brain. In non-dyslexic people, the following distinct patterns of cortical activity are observed during single word reading: 1) Earliest responses occur in the occipital midline (90-130ms latency) and are related to non-specific processing of all visual stimuli. 2) At ~150ms latency, non-dyslexic subjects show bilateral inferior occipito-temporal activity. The left hemisphere (LH) component responds preferentially to letters, compared to equivalent length symbol strings, for example. The right hemisphere (RH) component responds equivalently to letters and symbol strings. However, unlike the LH component, the RH response does appear to be sensitive to the position of items in a string. Of particular interest is the finding that, in samples of Finnish dyslexics at least, the LH occipito-temporal response to letter-strings is either absent or diminished. 3) Lexicality effects (i.e. significant differences between random consonant strings and words) are absent before ~200ms, but are found in the sustained responses of predominantly left perisylvian cortex 300ms after stimulus onset. Behavioural and neuroimaging data suggest a dissociation between left and right hemispheric processing of the global and local attributes of complex visual stimuli. As a working hypothesis, we suggest that early processing of letter-strings in the brain may involve a combined identity-space code that is computed in parallel. Accordingly, the left-hemisphere may handle the local attribute of letter-identity while the right-hemisphere may deal with the global attribute of spatial-position.
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