A Neuropsychological Investigation of Cerebral Dominance and Dyslexia

Abstract

Numerous investigators have sought a connection between dyslexia, or difficulty in learning to read, and some identifiable abnormality of central nervous system structure or function. Building on evidence accumulated in previous studies, one experimenter, Wltelson (1974, 1976, 1977), developed an explanatory theory of dyslexla based upon the concept of incomplete cerebral domlnance. Briefly, Witelson's theory hypothesized that dyslexia occurs in those individuals whose braln functlons are not sufficiently laterally specialized in the usual manner, that is, with language or verbal processing specialized in the left cerebral hemisphere, and spatial processing in the right. According to her theory, in dyslexics, spatial processing is bilateral, located in both hemispheres, which interferes with the linguistic, sequential processing in the left hemisphere. The presence of this spatial processing activity and the lack of specialization which it implies can be experimentally measured with procedures designed to deliver sensory inputs to only one side of the brain initially. Visually, stimuli can be presented tachistoscopically to either the right or left visual hemifields, and tactually, stimuli can be presented to either the right or left hand. Since sensory inputs have been demonstrated to go directly from one hand or visual hemifield to the contralateral hemisphere, it can be determined which hemisphere is doing the processing. Witelson employed this type of procedure with spatial stimuli to test her hypothesis that spatial processing is bilateral in dyslexics, using subjects referred from various clinics. Her results were confirmatory for tactual stimuli but were less clear for stimuli presented visually. The present study attempted to replicate Witelson's results with a school population of elementary age males and employed her tactual task, a modified visual task, and a task that was thought to be a purer measure of spatial processing. It was expected that on all three tasks non-dyslexic subjects would perform better when the stimuli were presented to the left hand or visual fleld, and for the dyslexic subjects the difference between performance on right-sided and 1eft-sided tasks would not be significant. Thirty-five dyslexic subjects and thirty-five controls, reading at or above grade level, all aged 8-12, were selected from three area elementary schools. All subjects were administered the same three perfo~mance tasks. Task I was a visual-spatlal task employlng tachistoscopic presentation of data to one hemifield at a time. Task II was a tactual-spatial task employing nonsense shapes presented to both hands out of view of the subject. Task III was a tactual map-reading task requiring each subject to orient himself in space using information presented to only one hand. For Task I, results were non-confirmatory, with both groups of subjects performing better when the left hemifield was stimulated. On Task II, both groups displayed superiority of right hand performance, again an unexpected result. Task III results were also non-confirmatory, as the trend was in the expected direction, but not statistically significant. The generally non-confirmatory findings may be interpreted in several ways. Witelson's hypothesis of bilateral spatial processing in dyslexics may simply be in error, as some other investigators have also failed to replicate her results. Or the findings may be a function of problems in defining dyslexia, which is probably not a unitary syndrome, leadlng to problems in subject selection. An additional consideration is that of task validity; that is, the tasks used may need further refinement to ensure they are able to differentiate between right and left hemisphere processing.