Sexual variation in cortical localization of naming as determined by stimulation mapping

Cognitive and Emotional Mapping With SEEG

Mapping of cortical functions is critical for the best clinical care of patients undergoing epilepsy and tumor surgery, but also to better understand human brain function and connectivity. The purpose of this review is to explore existing and potential means of mapping higher cortical functions, including stimulation mapping, passive mapping, and connectivity analyses. We examine the history of mapping, differences between subdural and stereoelectroencephalographic approaches, and some risks and safety aspects, before examining different types of functional mapping. Much of this review explores the prospects for new mapping approaches to better understand other components of language, memory, spatial skills, executive, and socio-emotional functions. We also touch on brain-machine interfaces, philosophical aspects of aligning tasks to brain circuits, and the study of consciousness. We end by discussing multi-modal testing and virtual reality approaches to mapping higher cortical functions.

Language organization and reorganization in epilepsy

The vast majority of healthy individuals are left hemisphere dominant for language; however, individuals with left hemisphere epilepsy have a higher likelihood of atypical language organization. The cerebral organization of language in epilepsy has been studied with invasive procedures such as Wada testing and electrical cortical stimulation mapping (ESM), and more recently, with noninvasive neuroimaging techniques such as functional magnetic resonance imaging (fMRI). Investigators have used these techniques to explore the influence of unique clinical features inherent in epilepsy that might contribute to the reorganization of language, such as location of seizure onset, age of seizure onset, and extent of interictal epileptiform activity. In this paper, we review the contribution of these and other clinical variables to the lateralization and localization of language in epilepsy, and how these patient-related variables affect the results from these three different, yet complementary methodologies. Unlike the abrupt language changes that occur following acute brain injury with disruption of established language circuits, converging evidence suggests that the chronic nature of epileptic activity can result in a developmental shift of language from the left to the right hemisphere or re-routing of language pathways from traditional to non-traditional areas within the dominant left hemisphere. Clinical variables have been shown to contribute to cerebral language reorganization in the setting of chronic seizure disorders, yet such factors have not been reliable predictors of altered language networks in individual patients, underscoring the need for language lateralization and localization procedures when definitive identification of language cortex is necessary for clinical care.

Brain organization for language from the perspective of electrical stimulation mapping

A model for the organization of language in the adult humans brain is derived from electrical stimulation mapping of several language-related functions: naming, reading, short-term verbal memory, mimicry of orofacial movements, and phoneme identification during neurosurgical operations under local anesthesia. A common peri-Sylvian cortex for motor and language functions is identified in the language dominant hemisphere, including sites common to sequencing of movements and identification of phonemes that may represent an anatomic substrate for the “motor theory of speech perception.” This is surrounded by sites related to short-term verbal memory, with sites specialized for such language functions as naming or syntax at the interface between these motor and memory areas. Language functions are discretely and differentially localized in association cortex, including some differential localization of the same function, naming, in multiple languages. There is substantial individual variability in the exact location of sites related to a particular function, a variability which can be partly related to the patient's sex and overall language ability and which may depend on prior brain injury and, perhaps subtly, on prior experience. A common “specific alerting response” mechanism for motor and language functions is identified in the lateral thalamus of the language–dominant hemisphere, a mechanism that may select the cortical areas appropriate for a particular language function.

Sex differences in mathematical reasoning ability in intellectually talented preadolescents: Their nature, effects, and possible causes

Several hundred thousand intellectually talented 12-to 13-year-olds have been tested nationwide over the past 16 years with the mathematics and verbal sections of the Scholastic Aptitude Test (SAT). Although no sex differences in verbal ability have been found, there have been consistent sex differences favoring males in mathematical reasoning ability, as measured by the mathematics section of the SAT (SAT-M). These differences are most pronounced at the highest levels of mathematical reasoning, they are stable over time, and they are observed in other countries as well. The sex difference in mathematical reasoning ability can predict subsequent sex differences in achievement in mathematics and science and is therefore of practical importance. To date a primarily environmental explanation for the difference in ability has not received support from the numerous studies conducted over many years by the staff of Study of Mathematically Precocious Youth (SMPY) and others. We have studied some of the classical environmental hypotheses: attitudes toward mathematics, perceived usefulness of mathematics, confidence, expectations/ encouragement from parents and others, sex-typing, and differential course-taking. In addition, several physiological correlates of extremely high mathematical reasoning ability have been identified (left-handedness, allergies, myopia, and perhaps bilateral representation of cognitive functions and prenatal hormonal exposure). It is therefore proposed that the sex difference in SAT-M scores among intellectually talented students, which may be related to greater male variability, results from both environmental and biological factors.

Cortical language mapping in epilepsy: a critical review

One challenge in dominant hemisphere epilepsy surgery is to remove sufficient epileptogenic tissue to achieve seizure freedom without compromising postoperative language function. Electrical stimulation mapping (ESM) of language was developed specifically to identify essential language cortex in pharmacologically intractable epilepsy patients undergoing left hemisphere resection of epileptogenic cortex. Surprisingly, the procedure remains unstandardized, and limited data support its clinical validity. Nevertheless, ESM for language mapping has likely minimized postoperative language decline in numerous patients, and has generated a wealth of data elucidating brain-language relations. This article reviews the literature on topographical patterns of language organization inferred from ESM, and the influence of patient characteristics on these patterns, including baseline ability level, age, gender, pathology, degree of language lateralization and bilingualism. Questions regarding clinical validity and limitations of ESM are discussed. Finally, recommendations for clinical practice are presented, and theoretical questions regarding ESM and the findings it has generated are considered.

A female advantage in the imitation of gestures by preschool children

Adult research has suggested a sex difference in the neuroanatomical organization and efficiency of the limb praxis system. The present study used a videotape analysis of movement errors to investigate whether a sex difference in limb praxis can be identified in childhood. Preschool girls (n = 33) committed fewer "praxic" errors than boys (n = 31) (i.e., errors that resemble those seen in limb apraxia) when imitating non-representational gestures and when imitating common, everyday gestures with representational content. In none of the conditions did the female advantage generalize to "nonpraxic" error categories. The findings were not explained by a female advantage in cognitive development. The current study supports the hypothesis of a sex difference in limb praxis that is present from an early age.

A female advantage in the serial production of non-representational learned gestures

Clinical research has demonstrated a sex difference in the neuroanatomical organization of the limb praxis system. To test for a corresponding sex difference in the functioning of this system, we compared healthy men and women on a gesture production task modeled after those used in apraxia research. In two separate studies, participants were taught to perform nine non-representational gestures in response to computer-generated color cues. After extensive practice with the gestures, the color cues were placed on a timer and presented in randomized sequences at progressively faster speeds. A detailed videotape analysis revealed that women in both studies committed significantly fewer 'praxic' errors than men (i.e., errors that resembled those seen in limb apraxia). This was true during both the untimed practice trials and the speeded trials of the task, despite equivalent numbers of errors between the sexes in the 'non-praxic' (i.e., executory) error categories. Women in both studies also performed the task at significantly faster speeds than men. This finding was not accounted for by a female advantage in extraneous elements of the task, i.e., speed of color processing, associative retrieval, or motor execution. Together, the two studies provide convergent support for a female advantage in the efficiency of forelimb gesture production. They are consistent with emerging evidence of a sex difference in the anatomical organization of the praxis system.

Sex differences in EEG coherence during a verbal memory task in normal adults

Coherence analysis was applied to the EEG of 15 female and 15 male subjects who had to memorise dichotically presented lists of concrete nouns. The EEG was recorded from 16 scalp electrodes placed in accordance with the 10/20 system. The results show significant gender-related differences in total coherence reactivity due to a greater increase of rest to task coherence in female than in male subjects. Women differed by showing higher coherence reactivity in the right hemisphere for all analysed (theta 1, theta 2, alpha 1, and alpha 2) frequency bands. They also had more extensive task-induced increases of interhemispheric coherence in theta bands in comparison with single changes in male subjects. In women these changes were mainly between the frontal electrodes of the left hemisphere paired with posterior electrodes of the other hemisphere. These findings indicate sex-related differences in functional cortical organisation during verbal memory tasks.

Gender differences in the human cerebral cortex: more neurons in males; more processes in females

This study's objective was to investigate morphometric gender differences of the cerebral cortex in six males and five females, 12 to 24 years old. Though human brains lack sexual dimorphism on routine neuropathologic examinations, gender-specific brain weight, functional, and morphologic differences exist, suggesting that cortical differences may be found. Yet the cerebral cortex may be exempt from gender differences, as demonstrated by the fact that normal males and females perform comparably on intelligence tests. Stereologic morphometry on standardized histologic sections from 30 bilateral cortical loci determined cortical thickness, neuronal density, and derived neuronal number estimates. The mean +/- SD cortical thickness of the 60 loci examined was similar in males and females with right and left hemispheric gender ratios being balanced. In contrast, the average neuronal density of the same 60 loci was significantly higher in the male group than in the female group, and the corresponding mean male-to-female ratios were 1.18 in the right and 1.13 in the left hemisphere, which differ significantly from each other and from the balanced cortical thickness ratios. Estimates of neuronal numbers -- the product of neuronal thickness times density -- were 13% higher in males than in females, with mean male-to-female ratios of 1.13 in both hemispheres. The data provide morphologic evidence of considerable cerebral cortical dimorphism with the demonstration of significantly higher neuronal densities and neuronal number estimates in males, though with similar mean cortical thickness, implying a reciprocal increase in neuropil/neuronal processes in the female cortex.

Cognitive Bias, Functional Cortical Geometry, and the Frontal Lobes: Laterality, Sex, and Handedness

Performance of patients with quadrant lesions on the inherently ambiguous Cognitive Bias Task (CBT) suggests sexual dimorphism in the fundamental aspects of functional cortical geometry, by emphasizing different cerebral axes. In right-handed males, extreme context-dependent and context-independent response selection biases are reciprocally linked to left vs. right frontal systems. In right-handed females, these complementary biases appear to be reciprocally linked to posterior vs. frontal cortices. Frontal lobe functions are more lateralized in males than females due to sexual dimorphism of the left frontal systems. Both in males and females, patterns of CBT scores in non-right-handers with quadrant lesions are opposite to those found in right-handers. This suggests the existence of two functionally and neurally distinct cognitive selection mechanisms. Both mechanisms involve the frontal lobes, but their exact neuroanatomy depends on sex and handedness.

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.

Intrahemispheric sex differences in the functional representation of language and praxic functions in normal individuals

Kimura (1980, The Behavioral and Brain Sciences, 3, 240-241; 1983, Canadian Journal of Psychology, 37, 19-35; 1987, Canadian Psychology, 28, 133-147) recently proposed that there are intrahemispheric sex differences in the organization of particular language and praxic functions such that in females these functions are more focally represented. This hypothesis, as well as supporting data, was derived from research with brain-injured subjects. The purpose of the present study was to test the utility of using dual-task methodology for investigating sex-related variation in intrahemispheric functional organization in normal individuals. Women were found to show greater interference than men on concurrent tasks of language and right-hand finger tapping. In support of Kimura's model, the present findings yielded evidence suggestive of intrahemispheric sex differences in the "functional distance" between language and praxic functions within the left hemisphere of normal individuals.

The relationship between cerebral lateralization and cognitive ability: suggested criteria for empirical tests

Many recent investigations in cognitive neuropsychology have been devoted to a fundamental question: are individual differences in cerebral lateralization reliably associated with differences in cognitive ability? The results have been highly inconsistent and inconclusive, with some studies showing a relationship, others not. We propose that research on this question has been hampered by inadequate clarification of the criteria required for a proper test. We suggest what these criteria should be, and, for purposes of illustration, we evaluate representative tests of the lateralization--cognition hypothesis in light of these criteria. We conclude that studies that meet our criteria lend at least a modest measure of support for the hypothesis.