Wednesday, June 25, 2008

Work from Eran Zaidel's lab

Zaidel's lab has done a series of interesting experiments on hemifield lexical decision. The following findings seem particularly intruiging. (1) In English, LVF performance is worse than RVF performance for acceptance of words, but performance for rejection of pseudowords is equivalent across VFs. In Hebrew, this interaction is not present. (2) Reading ability (vocabulary and comprehension) is correlated with LVF/RH measures in English, but not Hebrew. To explain these phenomena, the authors suggest that lexical processing is more left-lateralized in Hebrew. I'd like to suggest an alternative account of these findings.

Consistent with imaging evidence for left lateralization of the VWFA, I assume that LVF/RH stimuli are projected to the LH pre-lexically, and that VF differences therefore arise at a pre-lexical, orthographic level. Recall that the SERIOL model posits a monotonically-decreasing activation gradient. In left-to-right languages, formation of the activation gradient requires learned visual processing in the RH in order to invert the acuity gradient into the activation gradient. In right-to-left languages, this learned processing would occur in the LH. The process of inverting the acuity gradient is especially costly in left-to-right languages because it is coupled with callosal transfer to the LH.

For words presented to the LVF/RH in English, incomplete inversion of the acuity gradient would lead to a non-optimal encoding of letter order, especially for longer words. This would tend to make words look unfamiliar, thus there would be an impact for accuracy on words, but not pseudowords, creating the observed interaction. More frequent readers would gain reading expertise in vocabulary, comprehension, and learned string-specific processing in RH visual areas. Thus the correlation between reading ability and LVF measures. In Hebrew, acuity-gradient inversion is not required in the LVF/RH, so LVF measures are not correlated with reading ability. Similarly, the word/pseudoword interaction is not present. (You don't see the opposite VF pattern because acuity-gradient inversion in the LH is more efficient than in the RH because it is not combined with callosal transfer.)

So I suggest that their findings indicate that specialized visual processing of strings is more right-lateralized in English and more left-lateralized in Hebrew. This idea of RH specialization for early visual string processing in English is consistent with the results of an EEG study, which showed that a length effect was right-lateralized initiallly (at 90 ms) and then became left-lateralized (at 200 ms) (Hauk, Davis, Ford, Pulvermuller & Marslen-Wilson, 2006).

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