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Cortical dynamics of language function and dysfunction

Riitta Salmelin

Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, 02015 HUT, Espoo, Finland

MEG is an ideal tool for elucidating language function in the human brain: it allows repeated measurements on the same subjects in different paradigms and provides information about both the active areas and their relative timing. This combination is clearly essential for deducing the possible roles of the various brain areas in linguistic tasks, which necessarily involve several levels of processing from sensory input to memory.

Disorders of language perception and production provide important insights into the neural organization of language. For example, to characterize the neurophysiology of reading, it is extremely useful to compare dyslexic and normal readers.

We showed isolated words and non-words to adult dyslexic and control subjects. The response patterns differed both in spatial organization and in time behavior: A sequence of an early (180 ms) response in the left inferior occipitotemporal cortex, followed by activation in the left superior temporal lobe in the fluent readers, was not paralleled in the dyslexics. In particular, the early left posterior response was missing or delayed [1]. In another set of ex-periments, we created a context for the words, by making use of the N400 paradigm [2]. In both groups, the left middle superior temporal cortex was most consistently sensitive to semantic congruity. However, the onset of the semantic activation, at about 200 ms in the fluent readers, was delayed by approximately 100 ms in the dyslexic subjects, again pointing to abnormalities within the first 200 ms after word onset, in pre-semantic processing.

To study in detail the early stages of visual word recognition, which thus appear to be dysfunctional in dyslexia, we recorded responses to letters, syllables, words, and symbol strings of equal length, embedded in variably noisy backgrounds: The left inferior occipitotemporal cortex seemed to be specifically involved in processing words 140 - 190 ms after stimulus onset, and the amplitude/latency of this response correlated with the speed of reading the words aloud. Two cortical areas are thus strongly involved in fluent reading: The left inferior temporo-occipital cortex responds to letter strings within the first 200 ms and may well be the gateway from visual to linguistic processing. Then, 200 - 600 ms after word onset, activation of the left middle superior temporal cortex reflects lexical/semantic analysis.

  1. Salmelin, R., Service, E., Kiesilä, P., Uutela, K., and Salonen, O. Ann. Neurol. 40: 157-162, 1996
  2. Helenius, P., Salmelin, R., Service, E., and Connolly, J. Brain 121: 1133-1142, 1998

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Temporal Aspects of Human Cortical Information Processing
Proceedings of the Finnish Japanese Symposium, Otaniemi, June 14 - 17, 1998
Edited by O.V. Lounasmaa
Internet page created Fri, Sep 18, 1998 at 07:28:46 with Frontier. Peter Berglund, peter@neuro.hut.fi