Hemispheric asymmetries in the resolution of lexical ambiguity.

The linguistic phenomenon of lexical ambiguity has been intensively investigated as a means of gaining insight into general mechanisms of lexical access. It is now evident that both context and meaning frequency are significant factors in the determination of lexical outcomes. This suggests that hemispheric processes may be relevant to the resolution of lexical ambiguity, because both factors have been shown to have differential implications for the processing of language in the hemispheres. This study set out to examine the effects of context and meaning frequency on the resolution of ambiguous word meanings within the hemispheres. Sentences presented at the beginning of each trial embodied contexts which expressed either the dominant or subordinate meaning of a terminating homographic prime. Laterally presented target words reflected senses of the prime which were either consistent with, or inconsistent with, the context created by the preceding sentence. The most interesting results were observed at short prime-target intervals where it was found that although dominant meanings of the target did not give rise to visual field differences, subordinate meanings evoked facilitated responses only in the left visual field. This result suggests that the right hemisphere immediately and exhaustively activates the various meanings associated with a word, while in the left hemisphere initial access is selectively restricted to the dominant meaning. It is proposed that this reflects a model of language comprehension in which the right hemisphere plays a supportive role by making available a set of alternative and less probable word meanings, thus freeing the left hemisphere to focus cognitive resources upon the most probable meaning of a word in a given context.

Interhemispheric communication is via direct connections.

Two priming experiments, using normal university students as subjects, independently projected low imagery primes and concrete target words to the left or right visual fields (LVF or RVF) to examine the merits of three spreading activation models of interhemispheric communication: (i) callosal relay of a semantically encoded prime; (ii) transfer of products activated as a result of the spread of activation; and (iii) direct connections between the hemispheres. The first experiment temporally separated pairs by a stimulus onset asynchrony (SOA) of 250 ms and obtained strong support for the direct connections model. Priming effects were obtained only when the prime was projected to the RVF and the target to the LVF. The pattern of priming effects suggested that low imagery words projected to the left hemisphere can activate concrete associates in the right hemisphere via direct callosal connections between the two. In the second experiment, the SOA was increased to 450 ms. This time, RVF-RVF priming was obtained along with RVF-LVF priming. The findings are interpreted within a modification of Bleasdale's (1987) framework, where abstract/low imagery words and concrete/high imagery words are represented in separate subsystems in the left hemisphere lexicon. Support was also found for the view that the left hemisphere is comprised of a complex network of abstract and concrete words, while the right hemisphere operates as a subsidiary word processor, subserving linguistic processing with a limited, special purpose lexicon comprised of associative connections between concrete, imageable words (e.g., Zaidel, 1983a; Bradshaw, 1980). Interhemispheric communication in the priming procedure appears to occur at the semantic level, via direct connections between the hemispheres.

Hemispheric priming in a reading task.

Some theories of reading and of reading disorders assume that the right hemisphere plays an important role in reading. However, despite the evidence supporting the competence of the right hemisphere in recognizing isolate words, there is little direct evidence to support the claim that the right hemisphere is involved in the continuous reading of connected text. This study used a stationary window technique to present text passages in a continuous reading task. At intervals during the reading of the text, a lexical decision was required to a target projected to the left or right side of the visual field. On some trials, the target was primed by a semantic associate which appeared in the passage immediately prior to the presentation of the target. It was found that these associative primes facilitated responses to LVF and RVF targets to an equal degree. It was also found, in agreement with previous investigations, that overall RTs to LVF targets were longer than RTs to RVF targets. It is suggested that these results indicate that the right hemisphere is actively involved in the comprehension of text in normal reading. However, the right hemisphere may not acquire text information directly, but may instead receive most of its information following initial analysis and decoding by the left.

The effect of complexity upon hemispheric specialization for reading Chinese characters.

A number of reports in the literature have suggested that there is a right hemisphere advantage for the processing of single Chinese or Japanese characters. There are, nonetheless, many studies which have produced contradictory findings, suggesting that the factor or factors underlying the lateral asymmetry have not been clearly identified. The present study investigated the proposal that visual complexity of single Chinese characters, as measured by stroke number, was related to a right hemisphere advantage for processing this material. However, increasing the level of complexity of the characters was found to be related to the development of a left hemisphere advantage, thus clearly disconfirming the proposal. It is argued that the results are more satisfactorily interpreted in terms of the relationship between hemisphere specialization and the spatial frequency of the stimuli.

Semantic category priming in the left cerebral hemisphere.

The representation of semantic codes in the cerebral hemispheres and the interhemispheric communication of these codes, was investigated in two priming experiments where prime and target words were independently projected to the left or right visual fields (LVF or RVF). Nonassociated category exemplars were employed as related pairs in a lexical decision task and separated by a stimulus onset asynchrony of 250 msec in Experiment 1 and 450 msec in Experiment 2. Both experiments obtained priming effects when primes and targets were both projected to the RVF, but not the LVF. Semantic category primes projected to the RVF also facilitated responses to LVF targets, but no LVF-RVF priming was obtained. This suggests that semantic category information is relayed from left to right hemisphere, but not vice versa. The results are consistent with the view that semantic categories are represented in the left hemisphere.

Picture-name priming in the cerebral hemispheres.

In a lexical decision task, pictures and names of common objects were presented in succession to the left or right visual fields in an investigation of the relationship between visual and verbal representations within and between the hemispheres. On each trial, a picture was projected to the left or right side of the visual field, followed by a string of letters to the same or different side. The letter string could be the name of the object in the picture, an unrelated word, or a nonword. Although a strong priming effect was observed when the name of an object followed its picture, this did not depend on which visual fields had registered the stimuli. Activation of name codes was apparently independent of visual field of presentation of picture and word. This result was consistent with the view that representational systems for pictures and their names are not differentially specialized across the hemispheres. A further finding was that absolute response time to words was shorter when stimuli were divided between the visual fields, even at an SOA of 1000 msec. It is suggested that this result is due to the activation of picture-processing mechanisms within a hemisphere by the first (pictorial) stimulus, which causes a delay in processing the subsequent verbal stimulus in that hemisphere.

Associative priming in the hemispheres as a function of SOA.

The representation of associative codes in the cerebral hemisphere was investigated in two priming experiments where associated prime and target words were independently projected to the left or right visual fields. The first experiment, using a stimulus onset asynchrony of 250 msec, found priming in all visual field conditions, except that in which both prime and target were projected to the right hemisphere. A second experiment was conducted to determine whether this absence of right hemisphere priming was due to an inadequate interval of time between prime and target. In this experiment, a stimulus onset asynchrony of 450 msec revealed significant priming in all visual field conditions. The results suggest that lexical representations are activated more slowly in the right hemisphere than in the left.

Semantic and phonemic priming in the cerebral hemispheres.

Representation of semantic and phonemic codes in the cerebral hemispheres was investigated in two priming experiments where prime and target words were independently projected to the left or right visual fields. The first experiment, using phonemic primes, confirmed the view that phonological information is not accessible to the right hemisphere. Priming effects were obtained only when the prime and target were both projected to the right visual field. The second experiment, employing category exemplars as primes, again found the left hemisphere to be the principal locus of the priming effects. The right hemisphere was unable, by itself, to activate words related to the exemplar prime. However, projection of the prime to the right visual field significantly facilitated responses to left visual field targets. The present findings support the view advanced by Drews (Neuropsychologia 25, 419-427, 1987) and Levy and Trevarthen (J. exp. Psychol., Hum. Percep. Perform. 2,299-312, 1976) that the left lexicon is structured in accordance with an hierarchy of logical semantic relationships, while the right lexicon is organized on the basis of simple associations between concepts. It is suggested, furthermore, that the patterns of semantic, but not phonological, activation invoked by a prime may be relayed between the two lexicons.

The effect of retention interval upon hemispheric processes in recognition memory.

Simple concrete nouns were presented unilaterally in a continuous recognition memory procedure. Each word was presented twice in a session, and subjects were required to signal the second occurrence of a word. Retention interval was manipulated by varying the lag separating word presentations, and lags of 1, 4, 8, and 32 items were employed in the design. Words were projected, on first and second presentations respectively, to the following visual fields: LL, LR, RR, and RL. No visual field effects were observed at lag 1, but an advantage for RVF probes was evident from lag 4 onwards. At lag 8, a relative superiority emerged from uncrossed versus crossed presentation. The results support the idea that lateral asymmetries develop as processing engages deeper and more complex levels of representation. There was also evidence that memory representations are more salient following direct, rather than transcallosal, stimulation of a hemisphere.