Sally P. Springer

Left Brain, Right Brain: Perspectives from Cognitive Neuroscience

Introduction - left brain, right brain and the new sciences of the mind the discovery of asymmetry - clues from the clinic exploring asymmetry in the normal brain handedness, sex, and the brain from the clinic to the laboratory - intergrating neuropsychology and neuroscience the evolution and development of asymmetry pathology and asymmetry hypotheses and speculation - beyond the data postscript - cerebral asymmetry as a continuing theme in the study of the brain.

Psychologic and neurologic consequences of partial and complete cerebral commissurotomy

Preliminary psychologic testing was carried out on four patients who had undergone surgical sectioning of a portion or all of the corpus callosum and anterior commissure as a treatment for uncontrollable seizures. Results confirm earlier findings indicating the importance of the forebrain commissures in the interhemispheric exchange of a variety of sensory and motor information, and demonstrate that particular portions of the commissural system are responsible for transferring the information of specific sensory modalities. The patients also showed surprising abilities in performing complex tasks assumed to require integration of information from both hemispheres.

The anterior commissure in man: functional variation in a multisensory system.

Abstract The anterior commissure, which has been presumed to play a minor role in interhemispheric communication, was tested for the transfer of visual, auditory, and olfactory information in patients with complete sections of the corpus callosum. Four of five patients tested with presumed intact anterior commissures demonstrated interhemispheric transfer of verbal and pictorial stimuli presented visually to a single hemisphere. Evidence was also obtained for auditory and olfactory transfer, although successful interhemispheric communication in all three modalities was not found for any one patient. The data suggest that the human anterior commissure is capable of mediating multisensory, interhemispheric messages of a complex nature and provide evidence of functional plasticity in a phylogenetically early cerebral structure.

The ontogeny of hemispheric specialization: Evidence from dichotic listening in twins

Abstract To determine if variation in the direction and/or degree of hemispheric asymmetry for speech perception among right handers has a heritable component, 53 monozygotic (MZ) and 35 dizygotic (DZ) right handed, same sex twin pairs were tested with a 240 trial consonant-vowel dichotic listening test. Intraclass correlations for MZ and DZ pairs for handedness and various measures of ear asymmetry, including absolute values, were not significantly different, while MZ pairs showed a significantly higher correlation for overall total correct, a measure unrelated to the laterality scores. Concordance for direction of ear asymmetry alone was not significantly greater among MZ than DZ pairs. These results support the hypothesis that variation in direction and degree of lateralization for speech processing in right handers is non-genetic in origin. A comparison of MZ, DZ, and singleton groups showed all three to be equivalent in terms of mean scores on the laterality and total correct measures, indicating that these twins are not atypical of the general population on these measures. Sex differences as well as effects of familial sinistrality were observed in the twin sample.

Interrelationships among subject variables believed to predict cerebral organization

Abstract A variety of subject variables have been proposed as having predictive value for determining cerebral organization. Subject variables proposed as likely candidates include strength of handedness, familial sinistrality, writing posture, and sex. The present study examined the interrelationships among these variables by means of a questionnaire given to 847 undergraduates.

Left ear performance in dichotic listening following commissurotomy

Abstract Contribution of the left ear stimulus to dichotic listening performance following commissurotomy was studied in five patients. In two tasks, subjects were asked to identify in writing both members of a pair of competing stimuli, either digits or CV syllables. A third task required subjects to integrate high and low frequency components of a single word presented dichotically. Left ear performance was at chance level for CV syllables but exceeded 80% for four out of five patients on the digit stimuli. All patients showed evidence of being able to utilize left ear information in the dichotic fusion task. Results indicate that apparent supression of left ear material in the dichotic task is a function of spectral-temporal overlap between competing stimuli.

Hemispheric specialization for speech in language-disordered children.

Abstract The present experiment addressed the hypothesis that children with language disorders may have atypical patterns of hemispheric asymmetry of function. Ten children with various language disorders and 10 normal controls were given a 480 trial consonant-vowel dichotic listening test to obtain a measure of speech laterization. The data were analyzed in terms of performance as a function of whether the two dichotic inputs shared a phonetic feature (voice or place), in addition to percent correct for each ear. Group analysis of the data revealed no significant differences between the experimental children and the controls with respect to the magnitude of the right ear advantage or the effect of shared phonetic features, although the experimental children reported significantly fewer syllables overall. An analysis of the performance of individual children, however, showed that the size of the ear asymmetry was inversely related to severity of linguistic impairment.

Laterality in twins: The relationship between handedness and hemispheric asymmetry for speech

The present study examined the relationship between dichotic listening performance and handedness in twins. The 53 monozygotic (MZ) and 35 dizygotic (DZ) pairs concordant for right-handedness displayed ear asymmetries and total correct scores comparable to those found in right-handed singletons. The left- and right-handed members of the 19 MZ and 8 DZ pairs discordant for handedness were also similar to left- and right-handed singletons, respectively, with regard to ear asymmetry and overall performance. These data demonstrate that the relationship between handedness and brain organization observed in both MZ and DZ twins is similar to that found in the singleton population. The pattern of intraclass correlations obtained suggests that handedness discordance may be associated with greater differences in ear asymmetry within MZ pairs than within DZ pairs, possibly reflecting differences in the etiology of discordance for handedness in the two groups. In contrast, MZ cotwins were more similar than DZ cotwins, regardless of handedness, when total correct performance was measured.

The relationship between auditory sensitivity and ear asymmetry on a dichotic listening task

Using an adaptive forced-choice procedure, the average absolute sensitivity of the right ears of a group of right-handed males was found to be slightly greater than that of their left ears. There was no ear difference in performance on a monaural syllable-identification task using easily detectable stimuli, however. The magnitude of the ear difference in absolute sensitivity was significantly correlated with performance on a dichotic-listening task. In a second experiment, the adaptive forced-choice procedure was used to assess differences in absolute sensitivity in a group of left-handed males. The right ears of those left-handers showing a right-ear advantage on the dichotic-listening task were slightly more sensitive than their left ears. The left ears of those left-handers showing a dichotic left-ear advantage were slightly, but not significantly, more sensitive than their right ears. The correlation of the ear differences in absolute sensitivity with performance on the dichotic-listening task for the left-handers was not significantly different from zero. Results of both experiments are discussed in terms of their implications for the nature of hemispheric asymmetry of function, and the interpretation of dichotic and monaural asymmetries.

Performance asymmetries in dichotic listening: The role of structural and attentional mechanisms

Abstract The present study was designed to test a prediction derived from the attentional model of the ear asymmetry phenomenon observed in dichotic listening. Pairs of consonant-vowel syllables were presented through headphones and through loudspeakers to three commissurotomy and 20 neurologically normal subjects. Proponents of the attentional model have suggested that both groups of subjects should show an asymmetry in favor of the stimulus delivered from the right that is independent of whether headphones or loudspeakers are used. Results showed that the right side advantage obtained in the headphone modality decreased but did not disappear for both subject groups when stimuli were presented through loudspeakers. Implications for the role of structural and attentional mechanisms in dichotic listening performance are considered.