Parallel letter processing in the left and right hemispheres : What is the difference?

Abstract

Numerous studies have demonstrated a left-hemisphere (LH) advantage over the right hemisphere (RH) in reading. The presence of a word length effect (i.e. increasing reading times with the number of letters) in the RH but none in the LH has been frequently replicated. The most widely accepted account of this contrast proposes that the LH is able to process letters in parallel while the RH is unable to do so, and thus has to rely on sequential letter processing (Lavidor et al., 2002). Here, we rather propose that the RH is less able than the LH at discriminating a target letter from other visually similar letters of the alphabet and that this difficulty increases with the number of letters to be processed in parallel. To assess this hypothesis, two letter-matching tasks were conducted in normal subjects while using lateralised stimulus displays. In Exp. 1, the pair of letters to be matched were presented simultaneously whereas they were presented sequentially in Exp. 2 (the first letter localized at the fixation point and the second to the right or left of fixation). The results show a stronger confusability effect (longer reaction time with visually similar than with visually dissimilar letters) with RH than LH displays only when both letters are presented simultaneously. These results suggest that hemispheric differences in reading are based on a difficulty of the RH (relative to the LH) at discriminating visually similar letters which, when encountered, trigger a sequential processing of letters aimed at facilitating perceptual disambiguation.