Sandra F. Witelson

Hemispheric Specialization for Linguistic and Nonlinguistic Tactual Perception Using a Dichotomous Stimulation Technique

Summary The present study was undertaken to assess the relative participation of the left and right hemispheres in processing nonlinguistic and linguistic stimuli presented in the tactual modality in neurologically intact individuals. Two tests involving dichotomous tactile stimulation, one using nonsense shapes and the other letters, were given to 47 normal right-handed boys between the ages of 6 and 14 years. It was found that nonlinguistic tactile information was more efficiently processed in the right (nonspeech) hemisphere in neurologically intact individuals as had been previously inferred on the basis of the study of individuals with unilateral brain damage. The right hemisphere specialization for nonlinguistic tactual perception was found to be present as early as six years of age. It was also found, contrary to expectation, that simple tactile linguistic stimuli such as letters were not processed more efficiently by the left hemisphere. The results were interpreted as indicating that linguistic stimuli presented tactually must be analyzed first in a spatial code and then translated into a linguistic code. This suggests that there is no direct link between input and linguistic analysis in the tactual modality as there appears to be in the visual and auditory modalities. The role of environmental factors, both present and past, such as response mode and previous similar experience, in priming particular central processes and consequently modifying left-right perceptual asymmetries was also noted.

Women have greater density of neurons in posterior temporal cortex

Cytoarchitectonic area TA1 (von Economo) in the cortex of the planum temporale within the Sylvian fissure, which is auditory association cortex and documented to be part of the neural substrate of language functions, was studied quantitatively in the brain specimens of five women and four men (mean age of 50 year). All cases were documented to be medically and cognitively normal, and consistently right-handed. We investigated the possibility that the difference in brain size between men and women is reflected in differences in the numerical density of neurons in area TA1, an area associated with morphologic and psychological sex differences. Neuron counts were made directly through cell differentiation under the microscope from Nissl-stained sections. Cortical depth, the number of neurons through the depth of cortex under 1 mm2 of cortical surface (Nc), and the number of neurons per unit volume (Nv) were obtained for the total cortex and for each of the six layers in each hemisphere. For total cortex in both hemispheres, depth and Nc were similar, but Nv was greater by 11% in women, with no overlap of scores between the sexes. The sex difference in Nv was attributable to layers II and IV; in contrast, Nv did not differ between the sexes in layers III, V, and VI. This is the first report of such a sex difference in human cortex. The results suggest that the cortical functional unit has a different ratio of input and output components in men and women which could have implications for the sex differences in cognition and behavior. Due to the small sample size and the homogeneity of the cases studied, generalizability of the results requires replication by other studies. In addition, cytoarchitectonic mapping indicated that area TA1 also occurs in the vertical posterior wall of the Sylvian fissure, providing evidence that anatomical definition of the planum temporale should include the posterior vertical wall of the superior temporal gyrus.

THE RELATIONSHIP OF HAND PREFERENCE TO ANATOMY OF THE CORPUS CALLOSUM IN MEN

Area of the midsagittal section of the corpus callosum, particularly in the region of the isthmus, was found previously to be greater in non-consistent-right-handed than consistent-right-handed men in a sample of 15 postmortem cases. Seven cases were obtained subsequent to this analysis. The new cases showed the same association previously observed between hand preference and area of the corpus callosum and its isthmus. In addition, a high negative correlation was found between isthmal area and a quantitative score of the direction and magnitude of hand preference. In the new cases, handedness was predicted better than chance using statistical functions of callosal anatomy derived from the previous group of 15 cases. These results support a relationship between variation in callosal anatomy and handedness in men and the hypothesis of a relationship between callosal morphology and functional asymmetry. The lack of such a relationship among women suggests that the developmental mechanisms leading to callosal anatomical variation in relation to lateralization are influenced by sex hormones. Methodological issues in the use of magnetic resonance imaging for the quantitative study of callosal anatomy are discussed.

Neural sexual mosaicism : sexual differentiation of the human temporo-parietal region for functional asymmetry

Sex differences in human brain organization and behavior are documented by several converging lines of evidence based on patterns of functional asymmetry and cognitive abilities in normal adults and children, in patients with unilateral brain damage, and in clinical groups having atypical levels of sex hormones. Sex differences also exist in the structure of the human brain, and these are reviewed in detail herein. In addition, dichotomous differences, rather than just differences along a continuum, are noted in anatomical-functional correlations between men and women. Many of the anatomical differences cluster in the temporo-parietal regions of the brain, which subserve the asymmetric representation of some linguistic, motoric and spatial functions. The hypothesis is presented that development of the temporo-parietal region of the human brain is an anatomic network dependent on the organizing effects of sex hormones during embryonic or perinatal sexual differentiation, and that in each sex the pattern of functional asymmetries and cognitive attributes is differentially influenced by early sex hormone exposure. It is further suggested that the naturally occurring regressive events of cell death and axon elimination in early brain development contribute to the variation in the structure of the temporo-parietal region, and that this mechanism is differentially influenced by sex hormones in each sex. The specific, directional hypothesis put forward is that in early development of the male brain, lower levels of androgenic hormones or receptors lead to less regressive events in some brain regions, such as the temporo-parietal region, resulting in a larger isthmus of the corpus callosum, less cerebral functional asymmetry, and some cognitive correlates. Some supporting evidence for this hypothesis from neuropsychological studies of clinical groups and homosexual individuals is presented. The neuroanatomical correlate of functional asymmetry in posterior brain regions in women is not evident. The neural regressive events which occur in each sex may be related differently to lateralization. The concept of sexual mosaicism in the human brain is discussed.

Neurobiological Aspects of Language in Children.

This article discusses the relevance of the study of the neurobiology of cognitive development, both for an understanding of the neural bases of cognition and of the nature of cognition itself. A key issue is the age at which hemisphere specialization first appears and whether it changes over time. The neuropsychological literature concerning language in both normal and brain-damaged children is reviewed. The usefulness of studying cognition in other clinical disorders and variation in normal cognition is indicated. The various methods used in the research are described and the methodological and interpretational difficulties arising from the diversity of groups studied and the methods used are discussed. The model is advanced that hemisphere specialization exists from birth onward and does not undergo further change in either its nature or degree. The apparent increase in the extent of hemisphere specialization during childhood is interpreted as an epiphenomenon of the increasing cognitive and behavioral repertoire of the child. Neural plasticity, assumed to underlie recovery of function, is seen as being coexistent with, but independent of, hemisphere specialization.

The exceptional brain of Albert Einstein.

The case of Albert Einstein Resolving the neurobiological substrate of intelligence may be facilitated by the comparison of extreme cases with control groups within the framework of specific hypotheses. Albert Einstein is one of the intellectual giants of recorded history, and the preservation of his brain provides the possibility of an important case study. Since Einstein’s death, there has been no report of the gross anatomy of his brain. Here we present the first such study. Our investigation of Einstein’s brain was guided theoretically on the basis of current information of cortical localisation of cognitive functions. The generation and manipulation of three-dimensional spatial images and the mathematical representation of concepts would appear to be essential cognitive processes in the development of Einstein’s theory of r e l a t i v i t y . Einstein’s own description of his scientific thinking was that “. . . words do not seem to play any role”, but there is “associative play” of “more or less clear images” of a “visual and muscular type”. 0 Visuospatial cognition, 1 , 1 2 mathematical ideation, 1 a n d imagery of movement 3 are mediated predominantly by right and left posterior parietal regions. We hypothesised that the parietal lobes in particular might show anatomical differences between Einstein’s brain and the brains of controls.

A cognitive profile of homosexual men compared to heterosexual men and women

Matched groups of homosexual men, heterosexual men, and heterosexual women (n = 38 per group) were tested on three measures of spatial ability and two measures of fluency that typically reveal sex differences. For the three spatial tests and one of the fluency tests, the mean performance of homosexual men fell between those of the heterosexual men and women. The pattern of cognitive skills of homosexual men was different from that of heterosexual men: homosexual men had lower spatial ability relative to fluency. The cognitive pattern of homosexual men was not significantly different from that of heterosexual women. In addition, the results suggest that homosexual men classified on the basis of hand preference may form two subgroups that differ in cognitive pattern. These findings are compatible with the hypothesis that there is a neurobiological factor related to sexual differentiation in the etiology of homosexuality.

ANATOMIC ASYMMETRY IN THE TEMPORAL LOBES: ITS DOCUMENTATION, PHYLOGENESIS, AND RELATIONSHIP TO FUNCTIONAL ASYMMETRY*

There has been a recent surge in the number of studies indicating anatomic asymmetry in the posterior regions of the temporal lobes of the brain. This neural asymmetry was first observed for human adults, then for human infants and, most recently, for nonhuman primates. This evidence will be reviewed. The implications of the existence of such neuroanatomic asymmetry in nonlinguistic animals will be discussed in terms of the possible role the evolution of anatomic asymmetry may have for the phylogenetic development of cognition and the light it may shed on the nature of functional asymmetry of the hemispheres in man. The precautions for considering the anatomic asymmetry as a substrate for functional asymmetry will be indicated.

Left-handedness in homosexual men and women: Neuroendocrine implications

Although numerous researchers have hypothesized a biological factor in the etiology of homosexuality, there is a lack of empirical evidence. Previous investigations did not focus on behavioral functions of the brain. Using neuropsychological testing, we found an increased incidence of left-hand preference (defined as non-consistent right-hand preference) in a group of 32 homosexual women. A trend in the same direction was found in a group of 38 homosexual men. These results suggest that homosexual orientation has a neurobiological component possibly related to hemispheric functional asymmetry. The results are consistent with previous reports that (1) prenatal neuroendocrine events are a factor in the development of human sexual orientation and functional brain asymmetries, and (2) the mechanisms associated with homosexual orientation and related neuropsychological characteristics are different between the sexes, i.e. elevated levels of prenatal sex hormones in women and decreased levels in men.

Size of the human corpus callosum is genetically determined: an MRI study in mono and dizygotic twins

The factors determining the large variation seen in human corpus callosum (CC) morphology are as yet unknown. In this study heritability of CC size was assessed by comparing the concordance of CC midsagittal area in 14 monozygotic and 12 dizygotic twin pairs with a mean age of 27 years, using magnetic resonance imaging and various methods of calculating trait heritability. Heritability was high regardless of method of assessment. The application of a structural equation model resulted in the estimate that 94% of the variance in CC midsagittal size is attributable to the genome. This indicates that under normal conditions and before the effects of normal aging, there is very modest influence of the environment on CC morphology. The results suggest that correlates of CC size, such as the pattern of cerebral lateralization, cognitive abilities and neuropsychiatric dysfunction may be associated with the genetic determinants of CC morphology.