Monday, June 2, 2008

Shalev, Mevorach & Humphreys (in press) in Neuropsychologia

The authors investigate the deficits of two patients with parietal lesions. They find that the patients have a selective deficit in encoding the order of letters in a string, with spared ability to identify letter identities. For example, the patients are much more likely to make false positive responses (in lexical decision) to nonwords formed by transposing letters of words than to nonwords formed by replacing letters. In contrast, a patient with a left occipitotemporal lesion did not show this pattern.

The authors conclude that letter identity and position are encoded separately. Suprisingly, in the ensuing discussion of models of orthographic encoding, they do not reference any of the recent developments in this area; the most recent reference is to 2001 paper on the dual-route model.

I would explain their results as follows. In my article on alexia, I propose that the serial letter representation is transformed into two different high-level orthographic representations - an open-bigram encoding on the ventral (occipitotemporal) route, and a graphosyllabic encoding on the dorsal (occipito-parieto-frontal) route. The latter would provide a more robust encoding of letter order, as open-bigrams introduce ambiguity. If the dorsal graphosyllabic encoding is abolished, the result should be a less robust encoding of letter order in lexical processing, leading to a decreased sensitivity to transpositions. This is exactly what is observed in these parietal patients.

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