Sex differences in brain asymmetry of the rodent

The beauty of diversity in cognitive neuroscience: The case of sex-related effects in language production networks

Over the past few decades, several studies have focused on potential sex-related differences in the trajectories of language development and functioning. From a behavioral point of view, the available literature shows controversial results: differences between males and females in language production tasks may not always be detectable and, even when they are, are potentially biased by sociological and educational confounding factors. The problem regarding potential sex-related differences in language production has also been investigated at the neural level, again with controversial results. The current minireview focuses on studies assessing sex-related differences in the neural networks of language production. After providing a theoretical framework of language production, it is shown that the few available investigations have provided mixed results. The major reasons for discrepant findings are discussed with theoretical and methodological implications for future studies.

Anticlockwise or clockwise? A dynamic Perception-Action-Laterality model for directionality bias in visuospatial functioning

Orientation bias and directionality bias are two fundamental functional characteristics of the visual system. Reviewing the relevant literature in visual psychophysics and visual neuroscience we propose here a three-stage model of directionality bias in visuospatial functioning. We call this model the 'Perception-Action-Laterality' (PAL) hypothesis. We analyzed the research findings for a wide range of visuospatial tasks, showing that there are two major directionality trends in perceptual preference: clockwise versus anticlockwise. It appears these preferences are combinatorial, such that a majority of people fall in the first category demonstrating a preference for stimuli/objects arranged from left-to-right rather than from right-to-left, while people in the second category show an opposite trend. These perceptual biases can guide sensorimotor integration and action, creating two corresponding turner groups in the population. In support of PAL, we propose another model explaining the origins of the biases - how the neurogenetic factors and the cultural factors interact in a biased competition framework to determine the direction and extent of biases. This dynamic model can explain not only the two major categories of biases in terms of direction and strength, but also the unbiased, unreliably biased or mildly biased cases in visuosptial functioning.

Variation in lateralization: Selected samples do not a population make

The two major points of Denenberg's article are (1) that animals have lateralized brains, and (2) that the pattern of cerebral lateralization is consistent across species (i.e., “the left hemisphere will be primarily involved in communicative functions,” the right hemisphere with processing “spatial and affective information.” In addition, there is an unstated assumption that the pattern of lateralization is consistent within species. The evidence reviewed by Denenberg leaves little doubt that nonhuman animals have asymmetrically organized brains. However, there are problems with the suggestion that there is a consistent pattern of cerebral lateralization within or across different populations of species.

On asymmetries exhibiting a near-equiprobable distribution of directions

In his target article as well as in other writings, Denenberg presents a view of lateralization with which I fundamentally disagree: namely, that an affirmation of lateralization in a population is to be based primarily, if not exclusively, on observing a nonequiprobable distribution of asymmetric forms in that population.

An asymmetric view of brain laterality

The enigma of hemispheric specialization of the human brain continues to attract the attention of BBS readers. Although the lateralization of language is obviously specific to man, some scientists find the idea of human uniqueness unacceptable. Corballis and Morgan (1978) presented hemispheric dominance in man as a special case of a left-right maturational gradient, examples of which can be found throughout the animal kingdom. According to Denenberg, brain laterality can be induced in animals by nonlateralized environmental factors such as handling. Since nonlateralized influences can only unmask latent asymmetries, Denenberg's position is essentially similar to the views espoused by Corballis and Morgan (1978) and can, therefore, be criticized on the same grounds.

Hemispheric laterality in animals and the effects of early experience

A review of research with chicks, songbirds, rodents, and nonhuman primates indicates that the brain is lateralized for a number of behavioral functions. These findings can be understood in terms of three hypothetical brain processes derived from a brain model based on general systems theory: hemispheric activation, interhemispheric inhibition, and interhemispheric coupling.

Left-hemisphere activation occurs in songbirds and nonhuman primates in response to salient auditory or visual input, or when a communicative output is required. The right hemisphere is activated in rats when spatial performance is required, and in chicks when they are placed in an emotion-provoking situation. In rats and chicks interhemispheric activation and inhibition occur when there is an affective component in the environment (novelty, aversive conditioning) or when an emotional response is emitted (copulation, attack, killing). An interhemispheric coupling (correlation) found in rats and rabbits implies that the hemispheres are two major components in a control system with a negative feedback loop. Early-experience variables in rats can induce laterality in a symmetric brain or facilitate its development in an already biased brain.

It is predicted that functional lateralization, when present, will be similar across species: the left hemisphere will tend to be involved in communicative functions while the right hemisphere will respond to spatial and affective information; both hemispheres will often interact via activation-inhibition mechanisms when affective or emotional processes are involved. Homologous brain areas and their connecting callosal fibers must be intact at birth and must remain intact throughout development for lateralization to reach its maximum level. Injury to any portion of this unit will result in hemispheric redundancy rather than specialization. One major function of early experience is to provide stimulation during development, which acts to enhance the growth and development of the corpus callosum, thereby giving rise to a more specialized brain.

Turning bias in humans is influenced by phase of the menstrual cycle

Previous studies have provided evidence that humans demonstrate subtle, but measurable, turning biases when tested in the absence of environmental constraints. Preferences for leftward or rightward rotation have been repeatedly demonstrated in rodents and appear to be modulated to a significant degree by ovarian hormones, particularly estrogen. In the present study, we examined the turning biases of adult women at the midluteal and menstrual phases of the menstrual cycle, associated with high and low levels of estradiol and progesterone, respectively. Saliva samples were collected during each test session, and salivary concentrations of estradiol and progesterone were measured using radioimmunoassays. Overall, a rightward-turning bias was evident; however, a minority of the women displayed consistent leftward biases. Among right-turning subjects, turning biases were significantly weaker at the midluteal phase than at the menstrual phase. These results suggest that the mechanisms underlying human turning biases are subject to modulation by ovarian hormones.

A sex difference in turning bias in humans

A large body of literature has documented the existence of individual preferences in turning direction among rodents which appear to be dependent on striatal dopaminergic mechanisms. Recent work has indicated that humans also demonstrate individual turning preferences, and that these preferences may also be related to the nigrostriatal dopamine system. We describe here a new method for measuring turning preferences in humans and report a sex difference in the magnitude of the directional preference. While both males and females tended to turn towards the right, this tendency was significantly stronger among females. Analyses of test-retest reliability across two sessions (1-2 weeks apart) indicated that, in general, the rotation task elicited consistent turning biases. However, the turning biases of males and of females using oral contraceptives were significantly more consistent than those of regularly cycling females. These results are compatible with the animal literature and provide indirect evidence that ovarian hormones may modulate the mechanism(s) underlying this motor asymmetry.

Right-, left-dominance and ambidexterity in grasp reflex in human newborn: importance of left brain in cerebral lateralization

The grasp reflex was studied in human newborn without familial sinistrality. Of 60 females, 26 (43.3%) were right-handed and 34 (56.7%) ambidextrous. Of 62 males, 20 (32.3%) were right-handed, 39 (62.9%) ambidextrous, and 3 (4.8%) left-handed. There was a nonsignificant preponderance of right-dominance in females and a significant preponderance of nonright-handedness in males. In right-handers, the mean right minus left (R-L) grasp-reflex showed a positive linear correlation with the grasp-reflex from the right and left hands, with a higher correlation for the right hand. In ambidexters, the R-L grasp reflex did not show any significant correlation with the grasp reflex from the right and left hands. The mean grasp-reflex from right and left were found to be significantly smaller in ambidextrous males and females then right-handed males and females, with a much higher significance for the right hand. It was concluded that females tended to have a more pronounced reflex lateralization than males. The results also indicated that the left brain may be more important than the right brain for the development of a spinocerebral motor lateralization in humans.

Paw preference in cats: distribution and sex differences

Paw preference assessed by a food-reaching test was studied in male and female cats. Of the total sample (N = 66), 34 (51.5%) were right-preferent, 24 (36.4%) left-preferent, and 8 (12.1%) ambilateral. In the total sample, there was evidence for an overall paw preference, general paw preference, right-, and left-paw preference. The distribution of the right- minus left-paw reaches was neither normal, nor U or J shaped. Of the males (N = 24), 10 (41.7%) were right-pawed, 12 (50.5%) left-pawed, and 2 (8.3%) ambilateral. In males, there was evidence for an overall, general, and right-, left-paw preference relative to no preference. The right- minus left-paw reaches fitted to guassian data with two prominent peaks due to the right- and left-preferents. In females (N = 42), 22 (52.4%) were right-preferent, 14 (33.3%) left-preferent, and 6 (14.3%) ambilateral. There was an overall, general, and right-preference but not a left-preference relative to no preference. The distribution of the right- minus left-paw reaches was neither normal nor U or J shaped. The female right-preferents showed a right-bias compared to males. The left-preferent males were more left-preferent than the right-preferent males are right preferent. The mean right- minus left- paw reaches for the female right-preferents were significantly higher than those for the male right-preferents. There was no significant difference between the right- minus left-paw reaches of the male and female left-preferents. The paw preferences exhibited consistency over time; no learning tendencies were established during testing periods for at least 10 days. Considering the mean right-paw reaches for each successive day (N = 10), the mean right-paw uses in the right-preferents was higher in females than males. The mean left-paw uses in left-preferents was about the same for males and females. In males, the mean left-paw uses for the left-pawed males were higher than the right-paw reaches for the male right-preferents. In females, there was no difference between the right paw reaches of the right-preferents and the left-paw reaches of the left-preferents. It was concluded that there is a right-bias in paw preference of cats, which is caused by the female right-preferents under the influence of a biological factor.(ABSTRACT TRUNCATED AT 400 WORDS)

Corpus callosum: region-specific effects of sex, early experience and age

In infancy, rats were provided handling stimulation and compared at 110 and 215 days of age with non-handled controls. Measurements were made of corpus callosum area, perimeter and length; and width measures were taken at 7 points along the longitudinal axis of the callosum. Callosal size was larger in males than in females, even when adjusted for the larger brain weight of the male. At 110 days handling stimulation increased callosal parameters and resulted in a more regular callosum in males, but this effect was no longer apparent by 215 days. Within the callosum, region-specific effects were found, suggesting that certain callosal fiber populations were involved. Handled males have previously been shown to be more lateralized than non-handled males; thus at least in this experimental system, increased callosal size and regularity is associated with greater hemispheric specialization.

Heritable determinants of left-right bias in the rat

The offspring of matings of rats having opposite or same-sided turning biases were tested for turning biases as adults and the degree of similarity to the parents' biases assessed. There were significant and equivalent tendencies for the male offspring to have the same bias as the male parent and the opposite bias as the female parent. Although, overall, female offspring were distributed randomly with respect to the parents' biases, a significant tendency for female offspring to have biases opposite those of the female parent was apparent in litters having more males than females. Based on reports indicating a relationship between the sex ratio of a litter and levels of testosterone in female fetuses, it was suggested that in utero exposure to testosterone reverses the coding of a heritable female influence and induces a tendency for the offspring to have biases opposite those of the female parent. The origins of sidedness in the rat appear to involve a complex interaction between heredity and hormones.

Sexually dimorphic cerebral asymmetries in 2-deoxy-D-glucose uptake during postnatal development of the rat: correlations with age and relative brain activity

Using a modification of the 2-deoxy-D-glucose technique we have studied developmental changes in local cerebral glucose incorporation. The data were analyzed in terms of per cent uptake, relative brain activity and left-right asymmetry for 7 brain regions. We have shown previously that left-right asymmetries exist at birth and now report: (a) that left-right asymmetries change with age, there being both left-to-right and right-to-left maturational gradients in different structures; and (b) that in most brain regions, the more active a structure is relative to the rest of the brain, the more likely that structure is right-biased and vice-versa. Some of these relationships manifest sexual dimorphism. Collectively, these data indicate that there are dynamic relationships between brain activity, brain asymmetry and brain development in the rat.

On the evolution and growth of lateralization

This is a welcome attempt to seek common biological principles underlying laterality in animals and humans. I do not think it is wholly convincing, but this is partly because many of the results from nonhuman species are not yet firmly established or understood. I suspect that the author's characterization of lateralization, with the left hemisphere supposedly specialized for communicative functions and the right for spatial and affective functions, will require modification. For instance, it now seems fairly clear that the left hemisphere in humans plays a general role in the production and perception of sequences not restricted to communicative acts (Craig 1980; Kimura 1979), and indeed some of the examples of left-hemispheric specialization listed in Table 2 are not obviously communicative. Even so, there are some fairly striking parallels between humans and nonhumans with respect to the pattern of lateralization, and one suspects that common principles are operating. At the same time, in the enthusiastic search for functional asymmetries, we should not overlook the striking degree of bilateral symmetry that characterizes the brains of all animals, including humans.

Cerebral predominance in the monkey?

The issues raised by Dr. Denenberg are complex, not least because the apparent communalities of hemispheric specialization among birds, rodents, monkeys, and human beings are also associated with negative instances (e.g., for parrots, Nottebohm 1976; for Macaco, other than fuscata, Petersen et al. 1978). To extend the available evidence I would like to refer to the preliminary findings of Garcha et al. (1980) on the monkey.

Hemispheric laterality and an evolutionary perspective

Denenberg rightly stresses the importance of studying ethologically meaningful species-specific behavior in animals, and makes the interesting distinction between lateralization at an individual and at a population level. However, in the case of man, I believe Denenberg is wrong in arguing that lateralization in the individual increases with maturation. The overall evidence nowadays tends very much to the contrary. Moreover, with respect to a population, why should it become lateralized? If there is indeed an advantage for the individual in hemispheric specialization, why should the direction of such specialization be so consistent across a majority of individuals, whether human or, as Denenberg points out, other members of the phylum? Is there an evolutionary advantage in most animals' sharing the same direction, or is it a necessary consequence of some other preexisting, more fundamental anatomical, biochemical, or physical property of the organism and its constituents? If the former, why are not all members of the species, rather than just a majority, lateralized in the same direction? (Or, to put it another way, what is the evolutionary advantage to the species or individual of dimorphism, of retaining a minority who polarize in the opposite direction?) If the latter - i.e., if lateralization is a necessary consequence of some prior state - then there should not be any dimorphism, exceptions, or minority members, unless they are somehow disadvantaged in consequence. Indeed, there is some evidence of a cognitive deficit in sinistrals, though it is disputed (see Bradshaw 1980 for review), and others have even suggested that the species as a whole may benefit in some way from such an uneven dimorphism (Levy 1974), but what evidence is there for such propositions with respect to rats, apes, monkeys, or chicks? This is an issue that should be addressed in any general model that includes laterality in animals. [See Corhallis & Morgan: “On the Biological Basis of Human Laterality” BBS 1(2) 1978.]