Readers construct at least two interrelated representations when they comprehend a text: (a) a propositional representation containing the individual ideas that are derived from each sentence and the relations among them and (b) a discourse model, a representation of the context or situation to which the text refers. We used a paradigm called “item priming in recognition” to examine how these representations are stored in the cerebral hemispheres. In Experiment 1, the priming paradigm was used in combination with a lateralized visual field (VF) procedure. We found evidence that readers' representations were structured according to propositional relations, but only in the left hemisphere. Item recognition was facilitated when a concept was preceded by another concept from the same proposition when targets were presented to the left, but not to the right, hemisphere. We found priming in both hemispheres, however, when targets were context-appropriate senses of ambiguous words or topics of passages. In Experiment 2, we replicated the priming effects in three callosotomy patients. We argue that the distinction between a propositional representation and a discourse model is important with respect to how discourse is represented in the brain. The propositional representation appears to reside in the left hemisphere, whereas aspects of the discourse model appear to be represented in both hemispheres.

Three patients with complete resection of the corpus callosum were tested in a series of memory tasks to determine the effects of callosotomy on the encoding and retrieval of information in memory. Verbal and pictorial conjunction tests were administered to measure patients' ability to consolidate the elements of a stimulus into an accurate composite memory. Patients were also tested in a paired-associate learning task to determine the consequences of callosotomy on the encoding and retrieval of associations between stimuli. Although callosotomy patients were unimpaired in the verbal conjunction task, results from both the pictorial conjunction task and the paired-associate learning task suggest that the absence of callosal cross-talk impairs encoding in these patients. In addition, the pattern of results in the paired-associate learning task suggests that callosotomy impairs retrieval processes. The role of the callosum in the formation of memory traces for nonverbal material and associations between verbal stimuli is discussed.

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.

The literature pertaining to the representation of language in the right hemisphere of hemispherectomy and callosotomy patients is reviewed to ascertain whether it provides an empirical basis for the assertions that (1) the right hemisphere participates in the recovery of language in aphasia, (2) the right hemisphere mediates the reading errors of deep dyslexic and pure alexic patients, and (3) the right hemisphere plays a fixed role in normal reading processes. At present, there appears to be some support for the first assertion, limited support for the second (if individual variation in representation can be accepted), and disconfirming evidence for the third in the data from these populations.