Modes of Lexical Access in the Callosotomized Brain

Abstract

Left hemisphere processing is typically characterized as analytic and serial whereas the right hemisphere is characterized as wholistic and parallel. Word recognition may be an exception to this dichotomy if the letter-by-letter alexia produced by left hemisphere damage reflects the reading abilities of the right hemisphere. We investigated this possibility by studying prelexical and lexical processes in the separated hemispheres of callosotomy patient J. W. A word superiority effect demonstrated in each visual field suggests that both hemispheres have access to a visual lexicon. Error patterns, letter recognition thresholds, and lexical decision performance as a function of word length suggest that the left hemisphere tends to utilize a parallel access mode, whereas the right hemisphere mode is less efficient and may be serial. Furthermore, only J. W.'s left hemisphere showed letter priming, an outcome consistent with observations in letter-by-letter alexia. These findings suggest that the right hemisphere may have an independent visual lexicon and may provide an alternative although less efficient route to reading. We suggest that a serial encoding strategy results because the global processing mode for which the right hemisphere is specialized is largely ineffective for word reading.

Hemispheric Independence in Word Recognition: Evidence from Unilateral and Bilateral Presentations

“…In their view, nonwords in either VF are recognized via the slow, serial process and thus show length effects. This view has been supported by an experiment with a split-brain patient (Reuter-Lorenz & Baynes, 1992). Other hemispheric length effects have been observed in nonpronounceable letter strings, suggesting that the LH is more efficient than the RH in processing local elements of a display (Eng & Hellige, 1994).…”

Reevaluating split-fovea processing in word recognition: Hemispheric dominance, retinal location, and the word-nonword effect

“…To investigate this claim, words and nonwords were presented to the left or right of fixation at eccentricities that placed them either entirely in foveal locations (with medial edges 0.25º from fixation) or entirely in extrafoveal locations (with medial edges 2.20º from fixation). As in some previous studies of hemispheric asymmetry, performance in each location was assessed using the Reicher-Wheeler task (after Reicher, 1969, andWheeler, 1970) to provide an assessment of the wordnonword effect and LH dominance for word recognition that would not be contaminated by perceptual asymmetry and guesswork (see, e.g., Jordan et al, 1998Jordan et al, , 2000Reuter-Lorenz & Baynes, 1992). 2 An eyetracking system linked to a computer-controlled, fixation-contingent display ensured accurate fixation when each word was presented.…”

“…Thus, when stimuli are presented briefly, observers are more likely to encode information from words than from nonwords, which, in turn, produces a processing advantage for word stimuli. Moreover, other studies using similar displays of words and nonwords but presented as extrafoveal lateralized displays (e.g., Jordan et al, 2000;Reuter-Lorenz & Baynes, 1992) have reported that word-nonword effects occur when stimuli are projected to the LH, but are absent (or greatly reduced) when stimuli are projected to the RH. Thus, when stimuli are presented in lateralized displays outside foveal vision, only stimuli projected to the LH produce a substantial word-nonword effect, because only these stimuli directly activate LH processes specialized for word recognition.…”

Picture the difference: electrophysiological investigations of picture processing in the two cerebral hemispheres

“…Studies of picture processing in the two hemispheres provide an interesting counterpart to studies of word processing for a number of reasons. First, the use of pictures circumvents the concerns raised by left hemisphere dominance for word recognition [47,57] and for language processing more generally. Levine and Banich [41] showed, for example, that while the typical left hemisphere advantage is observed for word reading, no lateralized differences are found for the naming of pictures of the same objects (see also Experiment 1 of Biederman and Cooper [4]).…”