Righting The Balance

by Christine Chiarello

Right hemisphere and verbal communication. By Joanette, Y.,Goulet, P., &
Hannequin, D. 1990. New York: Springer-Verlag. Pp. 312 ISBN 0 387-97101-7.

The link between the left cerebral hemisphere and the processing of language may be the most well-established structure-function correlation in human neuropsychology. For most of us, a substantial left hemisphere injury would produce a serious disruption of our ability to converse, to write, and to properly interpret linguistic messages ranging from banal advertising slogans to the most intricate literary inventions. Yet a
comparable right hemisphere injury would produce no readily apparent deficit in these verbal abilities. So impressive is this dichotomy that the terms nonverbal and right are often used interchangeably when discussing the cerebral hemispheres. However, a surprising amount of data has accumulated in the past two decades to challenge the assumption
that left hemisphere language dominance necessarily implies no contribution from the
right hemisphere in language use.

The volume authored by Joanette, Goulet, and Hannequin is a welcome
compendium of research on the role of the right hemisphere in verbal communication. The authors wisely do not limit their review to studies examining traditional linguistic domains such as phonology, morphology, and syntax. Rather, they employ a broader criterion in order to
survey those skills relevant to the use of language in context. lt is no coincidence that evidence for a right hemisphere role in language processing began to appear as researchers turned their attention from language as an arbitrary system of formal rules to language
as a tool in the service of human communication. As this book documents, a unique contribution of the right hemisphere is evident primarily in the semantic and pragmatic aspects of language use.

The authors begin by describing the historical context from which questions regarding left and right hemisphere communicative processes emerged: early, pretheoretical studies documenting differential effects of left vs right unilateral brain injury. This is followed by an important chapter discussing the limitations inherent in studying each of the three populations from which we can gain insight about right hemisphere function:
right brain injured patients, commissurotomy (i.e., split-brain) patients, and neurologically normal persons tested by presenting auditory or visual stimuli to a single hemisphere. Because only indirect means are available for investigating human brain-behaviour relations, definitive experiments are not possible. We must rely on converging evidence from very
different populations each of which can provide only a narrow perspective on the issues. Therefore, it is essential to acknowledge the methodological limitations of each at the outset, to avoid the over interpretations which have sometime beset laterality research in the past. Joanette, et al. provide a useful discussion of these issues in this early chapter, and continue to offer appropriate qualifications in subsequent, empirical
chapters.

The remainder of the book consists of five chapters, each of which critically surveys the evidence for right hemisphere participation in verbal communication in a different domain. Each of these chapters can be thought of as examining a particular hypothesis about right hemisphere verbal processing. While this organization has the virtue of focussing on those issues which have engendered the most research interest, it may do so at the expense of some significant studies (e.g., Schneiderman & Saddy, 1988) that don't fit into this particular schema.

One chapter considers the hypothesis that the right hemisphere is involved in the language recovery seen by some patients with left hemisphere lesions. Despite the intuitive appeal of this notion, a close examination of the data by the authors revealed little strong support for this view. There are a few case reports of initially aphasic patients with left hemisphere lesions whose language function deteriorated after a second lesion of the right hemisphere (either naturally occurring or induced by temporary chemical anesthetization). However, this data is difficult to evaluate in the absence of incidence figures on the proportion of patients whose language does not worsen after a second right hemisphere insult. In addition, Joanette, et al. conclude that the shift from right ear to left ear dominance in dichotic listening among left hemisphere injured aphasic patients is not indicative of transfer of language function to the intact right hemisphere. One is left with the interpretation that if the right hemisphere does compensate for left hemisphere language dysfunction, this has yet to be demonstrated conclusively. However, it is important to point out that this does not preclude some right hemisphere participation in normal language processing. Rather, it would suggest that there are some language functions exclusive to the left hemisphere that the right hemisphere cannot assume.

The longest chapter reviews evidence for a right hemisphere role in lexical semantic processing, primarily the comprehension of word meanings. Data from split-brain and intact subjects demonstrates that the right hemisphere has the potential to interpret word meanings and semantic relationships, although the extent to which this potential is utilized in normal language processing is still unsettled. Right hemisphere injured
patients are deficient in judgments about the figurative aspects of word meanings (metaphor,connotation), as well as in some semantic word retrieval tasks.

This section of the volume reveals some of the weaknesses of attempting to account for neuropsychological data without an overall theoretical model of the phenomenon under scrutiny. While there are extant models of the lexicon and semantic memory (e.g. Neely, 1991; Schwanenflugel, in press), these are not appealed to in the consideration of right hemisphere lexical semantic processing. Thus it is sometimes difficult for the reader to appreciate the significance of the numerous experiments reviewed in this
chapter. For example, experiments involving semantic priming, Stroop interference, and word meaning judgments are meticulously described, but it is not clear how the various findings relate to each other nor how each can contribute to an overview of right hemisphere lexical semantic processing. lt is also disappointing that a critical appraisal by the authors, which is evident in other chapters, is lacking here, particularly with respect to the data from neurologically normal subjects. Without an overall theoretical framework, there is a tendency to fall back on the interpretations offered by the original investigators. This makes for a rather confusing and inconclusive chapter, despite the fact that some of the strongest evidence for right hemisphere language has been uncovered in the processing of word meanings.

The authors next examine the hypothesis that in one type of acquired reading disorder, deep dyslexia, some of the symptoms reflect right hemisphere lexical semantic processing: semantic errors in reading words aloud, inability to read pronounceable nonwords, and relatively poor reading of abstract and/or function words. After a careful analysis of the relevant studies, the authors conclude that, while this hypothesis remains plausible, there is little conclusive evidence to date. This, like the aphasia recovery data, suggests that it may not be appropriate to view left hemisphere lesions as uncovering latent right hemisphere verbal abilities. One could just as well conjecture that such lesions could mask some normal right hemisphere processes via pathological alterations in interhemispheric communication.

A potential role for the right hemisphere in the interpretation and/or production of speech prosody is considered in the following chapter. The authors review a great deal of evidence that the right hemisphere is critically involved in the comprehension of prosodically conveyed information, whether emotional or linguistic. They argue that the right hemisphere's superiority in this domain should be attributed to its proficiency in the perceptual decoding of suprasegmental phonetic information, and not to an advantage for processing the types of information (i.e., affective) which are usually conveyed prosodically.

Finally, the authors consider pragmatics. Patients who have sustained right hemisphere lesions have documented deficits in discourse, sentence interpretation tasks, humor appreciation, and in the interpretation of speech acts and metaphor. These impairments occur in the context of normal performance on standard linguistic tests which reveal deficits after left hemisphere injury. It is interesting that left hemisphere
lesioned aphasic patients perform normally on many tests of pragmatic abilities. Thus, the right hemisphere may be more involved in how language is used to convey communicative intent than in the more elementary linguistic processes at which the left hemisphere excels.

Taken as a whole, the studies reviewed by Joanette, et al., demonstrate that the right cerebral hemisphere is involved in several important aspects of verbal communication. Substantial evidence now exists to posit a significant role for the right hemisphere in lexical semantics, prosody, and pragmatics. In each of these areas, right hemisphere competence is more prominent for comprehension than for production. Furthermore, many
of the communicative processes attributed to the right hemisphere are not shared by the more verbally competent left hemisphere. Thus it is probably not appropriate to view the right hemisphere verbal system as a subset of that available to the left hemisphere. The right hemisphere does not contain a miniature or degraded version of the left's linguistic system, but rather may subserve communicative processes which complement those of the left hemisphere.

The authors have done an excellent job of summarizing this research, and this volume will be a helpful resource for both students and researchers. However, this book has a decidedly descriptive slant. The three page concluding chapter falls to address any of the wider issues raised by this body of research. One is left wondering what this work might tell us about the structure of the human mind, and why cognitive scientists should care which hemisphere mediates a given verbal process. To some extent, this is probably an accurate reflection of the state-of-the-art among researchers of right hemisphere language. Because the view that the left hemisphere is the linguistic system was so entrenched in neuropsychology, simply demonstrating any right hemisphere verbal competence was deemed worthy of note. However, the evidence amassed in the current volume can leave little doubt that the right hemisphere does play a role in verbal communication. It is now appropriate for researchers to consider the significance of this information for theories of hemispheric specialization and language representation in the brain.

One question raised by the data reviewed here concerns the convergence of lexical semantics, prosody, and pragmatics. Is it just a coincidence that the right hemisphere can subserve these three aspects of language, or is there some common right hemisphere mechanism that is recruited in each case? Why are these, and not other linguistic processes available to the right hemisphere? Perhaps the authors evade this issue in
order to avoid the simplistic left-right dichotomizing that has plagued laterality research in the past. However, shunning nonexplanatory dichotomies should not prevent us from propositing generalizations that may provide insights about the brain and behaviour, as long as these yield testable predictions.

As computational models of the mind develop and become increasingly sophisticated, the need for neurologically plausible constraints on possible architectures will become more acute. Neuropsychological evidence, such as that reviewed in the current volume, can be an important source of such constraints. For example, does a complex function such as the use of language in context require the use of two partially independent, but interacting systems? This might be the case if the computations underlying the
communicative functions subserved by each hemisphere were fundamentally incompatible. Questions such as these will require a much deeper analysis of the neuropsychological data than is typically offered in the literature, but the evidence reviewed by Joanette, et al. is obviously relevant to much broader theoretical issues in cognitive science.

In sum, the present volume is a useful addition to the literature on the neuropsychology of language. Future investigations can build on the evidence reviewed here to construct theoretical models of language and communication which take into account what is known about right, as well as left, hemisphere processing. For the moment, we are left with some tantalizing pieces of information which invite further scrutiny.

References

Neely, James H. (1991) "Semantic priming effects in visual word recognition: A selective review of current findings and theories." In Basic Processes in Reading: Visual Word Recognition. Derek Besner and Glyn Humphreys (eds.). Hillsdale: Lawrence Eribaum Associates.

Schnelderman, Eta I. and Saddy, J. Douglas (1988) "A linguistic deficit
resulting from right hemisphere damage." Brain and Language 34.1 :38-53.

Schwanenflugel, Paula (ed.) (In press) The Psychology of Word Meanings.
Hillsdale:
Lawrence Eribaum Associates.

Christine Chlarello teaches in the Cognitive Neuroscience Program at
Syracuse University. She has written extensively on aphasia in the prelingually deaf and on hemispheric dominance.