Debra L. Kigar

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 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.

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.

Individual Differences in the Anatomy of the Corpus Callosum: Sex, Hand Preference, Schizophrenia and Hemisphere Specialization

Recent studies of the gross anatomy of the corpus callosum show that there is marked variation in its size and shape, but also considerable consistency in these variations across very diverse studies. One study to date has reported a larger callosum, particularly in the mid and anterior regions, in mixed and left handers compared to consistent right banders. Several reports have examined possible sex differences in callosal anatomy and have produced apparently inconsistent results. The evidence clearly does not support a larger posterior splenial region in absolute size in females. However, a minority of the studies suggest that the posterior region, proportional to the size of the total callosum, may be larger in females than in males. Further clarification is needed. The early studies of callosal anatomy in schizophrenia suggested a thicker callosum in schizophrenics. Subsequent studies do not support this finding and may be confounded by variables such as chronological age, body size, brain size, and type of control group. Any anatomical differences between schizophrenic and normal individuals may involve some interaction of callosal region, sex and hand preference. These results are discussed in relation to individual differences in hemisphere specialization and brain function.

Neuroanatomical Aspects of Hemisphere Specialization in Humans

Anatomical asymmetries between the right and left hemispheres have been documented in the human brain since the last century. Much of the early work focussed on the Sylvian (lateral) fissure and the observation that this fissure was longer on the left more often than on the right side (e.g., Cunningham, 1892; Eberstaller, 1890). More recent anatomists pursued this morphological difference and looked within the Sylvian fossa in order to measure the horizontal surface of the temporal lobe which lies posterior to the first transverse gyrus (Heschl’s gyrus, the primary auditory receiving area) (e.g., Pfeiffer, 1936; von Economo & Horn, 1930). Early on, the area of this region, called the planum temporale, was found to be larger on the left side (see Figure 1). Much of the current interest in neuroanatomical asymmetry was rekindled by the report of Geschwind and Levitsky (1968) who observed that the linear extent of the lateral edge of the planum temporale was greater on the left side in 65 percent of 100 brain specimens. Other more recent studies (e.g., Wada, Clarke & Hamm, 1975; Witelson & Pallie, 1973) have confirmed this finding. Several reviews of asymmetry in the planum temporale in adults as well as in children are available (see e.g., Witelson, 1977; 1983).

A computer-assisted direct-imaging system to obtain numerical densities of neurons in human cortex

Studies of the numerical density of microscopic items in brain tissue is a time-consuming endeavor. However, such information is important for numerous issues such as the relationship between structure and function in the normal brain, individual differences, and studies of brains of neuropsychiatric patients. A computer-assisted imaging system specifically devised to obtain estimates of numerical densities in human cortex is described here. Its main advantage is that the microscopist can analyze the original image directly under the microscope, and most aspects of data acquisition and quantitative analysis are accomplished by the computer. The key features of the system are a Microvid (an electronic camera lucida) and the use of X, Y, and Z stage encoders in conjunction with three-dimensional computer software. The complete system is relatively inexpensive and is simple to set up and use. The reliability and validity of the numerical densities obtained using this system are documented.