Cortical map asymmetries in the context of transcallosal excitatory influences

There is long-standing disagreement among experimentalists about whether transcallosal interhemispheric influences are primarily excitatory or inhibitory. Past computational models exploring this issue have encountered a similar dilemma: inhibitory callosal influences best explain hemispheric functional asymmetries, but excitatory callosal influences best explain transcallosal diaschisis. We recently hypothesized that this dilemma might be resolved by assuming excitatory callosal influences and a subcortical mechanism for cross-midline inhibition. Here we explore the feasibility of this hypothesis by examining a model of map formation in corresponding left and right cortical regions. The results show for the first time that both map asymmetries and diaschisis-like changes can be produced in a single model, suggesting that subcortical inhibitory processes may contribute more to asymmetric cortical functionality than is generally recognized.

A computational model of lateralization and asymmetries in cortical maps

While recent experimental work has defined asymmetries and lateralization in left and right cortical maps, the mechanisms underlying these phenomena are currently not established. In order to explore some possible mechanisms in theory, we studied a neural model consisting of paired cerebral hemispheric regions interacting via a simulated corpus callosum. Starting with random synaptic strengths, unsupervised (Hebbian) synaptic modifications led to the emergence of a topographic map in one or both hemispheric regions. Because of uncertainties concerning the nature of hemispheric interactions, both excitatory and inhibitory callosal influences were examined independently. A sharp transition in model behavior was observed depending on callosal strength. For excitatory or weakly inhibitory callosal interactions, complete and symmetric mirror-image maps generally appeared in both hemispheric regions. In contrast, with stronger inhibitory callosal interactions, partial to complete map lateralization tended to occur, and the maps in each hemispheric region often became complementary. Lateralization occurred readily toward the side having a larger cortical region or higher excitability. Asymmetric synaptic plasticity, however, had only a transitory effect on lateralization. These results support the hypotheses that interhemispheric competition occurs, that multiple underlying asymmetries may lead to function lateralization, and that the effects of asymmetric synaptic plasticity may vary depending on whether supervised or unsupervised learning is involved. To our knowledge, this is the first computational model to demonstrate the emergence of topographic map lateralization and asymmetries.

Interhemispheric effects on map organization following simulated cortical lesions

During recent years there has been increasing use of neural models to investigate the implications of hypotheses about brain and cognitive disorders. Here we systematically study the effects of sudden simulated lesions on cortical maps in a neural model consisting of left and right hemispheric regions connected by a corpus callosum. The model identifies conditions under which damage to one hemispheric region leads to reorganization of the contralateral, intact hemispheric region. The intact hemisphere's participation in the recovery process is found to be a function of pre-existing map lateralization/symmetry and lesion size, indicating that interpretation of future experimental work should take these factors into account.

Metrics for cortical map organization and lateralization

Cerebral lateralization refers to the poorly understood fact that some functions are better controlled by one side of the brain than the other (e.g. handedness, language). Of particular concern here are the asymmetries apparent in cortical topographic maps that can be demonstrated electrophysiologically in mirror-image locations of the cerebral cortex. In spite of great interest in issues surrounding cerebral lateralization, methods for measuring the degree of organization and asymmetry in cortical maps are currently quite limited. In this paper, several measures are developed and used to assess the degree of organization, lateralization, and mirror symmetry in topographic map formation. These measures correct for large constant displacements as well as curving of maps. The behavior of the measures is tested on several topographic maps obtained by self-organization of an initially random artificial neural network model of a bihemispheric brain, and the results are compared with subjective assessments made by humans.

Long bone growth during prolonged intermittent corticosteroid treatment and subsequent rehabilitation

Immature A/J mice were treated for up to 7 weeks intermittent doses of triamcinolone hexacetonide and were thereafter allowed to recover for 7 weeks. Qualitative and quantitative morphological measurements were performed on the epiphyseal cartilage plate and diaphyseal bone of the humerus. By the third injection significant structural changes were noted in the cartilaginous tissue followed by a complete cessation of bone growth. The hormonal inhibitory effect on long bone growth lasted throughout the experimental period. However, at the end of the recovery period the length of the humerus was 96% of the normal. In contrast, the humeral width at midshaft and the width of its medullary cavity revealed slower recovery, achieving only 80% of the control values. Following rehabilitation, the growth of experimental epiphyseal plates exceeded that of nontreated animals as their width and the number of hypertrophic chondrocytes were 131% and 125% of their controls respectively. Thus, in A/J mice (a highly susceptible inbred strain of mice) intermittent (every four days) administration of a long-acting corticosteroid hormone arrested endochondral and periosteal bone formation; the former, however, underwent full recovery following the termination of the hormonal treatment.

Corticosteroid-induced mandibular growth retardation and palatal malformation in the ICR mouse fetus

Pregnant ICR mice were treated with triamcinolone hexacetonide at various stages of gestation. The mandibular ramus and its condylar cartilage were studied histologically in both viable and non-viable offspring. In addition, measurements were made of the overall height of the posterior vertical dimension of the mandible and of condylar height and width. Significant changes were noted in these parameters. Concomitantly, marked changes were observed in the various zones of the condylar cartilage. A very high incidence of cleft palate was noted in newborn and stillborn mice previously treated with triamcinolone. A possible correlation between mandibular growth retardation and palatal clefting is discussed.

Effects of corticosteroid hormone on the epiphyseal growth center of immature pregnant mice

Immature pregnant mice treated with glucocorticoid hormone (triamcinolone hexacetonide) were killed 1 week following delivery. The epiphyseal growth plates of the humerus and their cellular constituents were analyzed quantitatively. It became evident that pregnancy per se evokes a marked suppressive effect upon young cartilage cells, thus interfering with the normal process of endochondral bone formation. Pregnant mice treated with glucocorticoid hormone experienced an even higher degree of chondrocytic derangement, however, to a lesser extent than chondroxytes of nonpregnant mice treated with the hormone. It therefore appears that pregnancy-induced elevation of systemic estrogens and progesterone provides some kind of a protection against the antianabolic activity of glucocorticoids upon chondrocytes.