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PubMed2002

Midline corpus callosum is a neuroanatomical focus of fetal alcohol damage.

Prenatal exposure to high levels of alcohol often induces birth defects that combine morphological stigmata with neurological or neuropsychological deficits. But it has proved problematic to diagnose these syndromes in adolescents and adults, in whom the morphological signs are absent or attenuated, the behavioral deficits nonspecific, and the exposure history often difficult to reconstruct. Localizing the associated brain abnormalities might circumvent most of these difficulties. To this end, three-dimensional (3D) locations were recorded for 67 homologous points on or near the corpus callosum in magnetic resonance (MR) brain images from 60 adolescents and adults who were normal, 60 diagnosed with fetal alcohol syndrome, and 60 diagnosed with fetal alcohol effects. We combined the standard statistical approach to this type of geometric data, Procrustes analysis, with a multivariate strategy focusing on differences in variability. In this data set, the shape of the corpus callosum and its vicinity proves systematically much more variable in the alcohol-affected brains than in those of the normal subjects. From this excess variability follows a promising classification rule, having both high sensitivity (100 out of 117) and high specificity (49 out of 60) in this sample. The discrimination uses four landmark points and two summary scores of callosal outline shape. The information from the corpus callosum and vicinity, as viewed in MR brain images of full-grown subjects, may serve as a permanent record of the prenatal effects of alcohol, even in patients who are first suspected of these syndromes relatively late in life or who lack the facial signs of prenatal alcohol damage. The statistical pattern underlying the callosal diagnosis also leads to speculations on mechanisms of the prenatal damage.

Adolescent↗
PubMed2003

Some glial progenitors in the neonatal subventricular zone migrate through the corpus callosum to the contralateral cerebral hemisphere.

The great majority of glial cells of the mammalian forebrain are generated in the perinatal period from progenitors in the subventricular zone (SVZ). We investigated the migration of progenitors from the neonatal (postnatal day 0, P0) rat forebrain SVZ by labeling them in vivo with a green fluorescence protein (GFP) retrovirus and monitoring their movements by time-lapse video microscopy in P3 slices. We identified a small number of progenitors that migrated tangentially within the corpus callosum (CC) and crossed the midline. These cells retained a relatively uniform morphology: the leading process was extended toward the contralateral side but showed no process branching or turning away from the migratory direction. Net migration requires the elongation of the leading process and nuclear translocation, and the migrating cells in the CC showed both modes. We confirmed the presence of unmyelinated axon bundles within the P3 CC, but failed to detect any radially directed glial processes (vimentin- or GLAST-immunolabeled fibers) spanning through the CC. Confocal images showed a close proximity between neurofilament-immunolabeled axons and the leading process of the GFP-expressing progenitors in the CC. The destination of the callosal fibers was examined by applying DiI to the right cingulum; the labeled fibers ran throughout the CC and reached the left cingulate and motor areas. The distribution and final fates of the retrovirus-labeled cells were examined in P28 brains. A small proportion of the labeled cells were found in the contralateral hemisphere, where, as oligodendrocytes and astrocytes, they colonized predominantly the cortex and the underlying white matter of the cingulate and secondary motor areas. The distribution pattern appears to coincide well with the projection direction of the callosal fibers. Thus, glial progenitors migrate across the CC, presumably in conjunction with unmyelinated axons, to colonize the contralateral hemisphere.

Animals↗
PubMed2004

Bilateral effects of spinal overhemisections on the development of the somatosensory system in rats.

Connections of the forepaw regions of somatosensory cortex (S1) were determined in rats reared to maturity after spinal cord overhemisections at cervical level C3 on postnatal day 3. Overhemisections cut all ascending and descending pathways and intervening gray on one side of the spinal cord and the pathways of the dorsal funiculus contralaterally. Bilateral lesions of the dorsal columns reduced the size of the brainstem nuclei by 41%, and the ventroposterior lateral subnucleus (VPL) of the thalamus by 20%. Bilateral lesions also prevented the emergence of the normal cytochrome oxidase barrel pattern in forepaw and hindpaw regions of S1. Injections of wheat germ agglutinin conjugated to horseradish peroxidase were placed in the forepaw region of granular S1 and surrounding dysgranular S1 contralateral to the hemisection. The VPL nucleus was densely labeled, whereas the adjoining ventroposterior medial subnucleus, VPM, representing the head, was unlabeled. Thus, there was no evidence of abnormal connections of VPM to forepaw cortex. Foci of transported label in the ipsilateral hemisphere appeared to be in normal locations and of normal extents, but connections in the opposite hemisphere were broadly and nearly uniformly distributed in sensorimotor cortex in a pattern similar to that in postnatal rats. Rats with incomplete lesions that spared the dorsal column pathway on the left side but not the right demonstrated surprisingly normal distributions of callosal connections in the nondeprived right hemisphere, even though the injected left hemisphere was deprived. Thus, the development of the normal pattern of callosal connections depends on dorsal column input and not on normal interhemsipheric interactions.

Afferent Pathways↗
PubMed1977

Abnormal interhemispheric connections in the visual system of Boston Siamese cats: a physiological study.

In the normal cat most visual fibers in the splenium of the corpus callosum are concerned with a representation of the vertical meridian of the visual field. These fibers for the most part originate from cortical regions forming the border between adjacent architectonic areas, such as the anatomical 17-18 border. Due to a genetic mutation in Boston Siamese cats, the anatomical 17-18 border now represents a region in the ipsilateral visual field roughly 20 degrees away from the vertical meridian, and the representation of the vertical midline is displaced from the border to regions within areas 17 and 18 proper. Do visual fibers in the Boston Siamese cat corpus callosum originate, as in normal cats, from the 17-18 border even though the vertical meridian is no longer represented there? The present paper deals with this question from a physiological standpoint. Single visual fibers in the corpus callosum of 11 Boston Siamese and two normal cats were recorded using extracellular microelectrodes. Receptive fields were mapped and their positions used to infer the likely cortical site of origin for each fiber. In confirmation of earlier findings, most callosal receptive fields in the normal cat were closely apposed to the vertical meridian, as would be expected if only those visual cortical regions which represent the vertical meridian, such as the border between areas 17 and 18, contribute to the corpus callosum. In Boston Siamese cats, however, an abnormally large amount of visual field was represented in the corpus callosum, with some receptive fields located as far as 25 degrees away from the vertical meridian. The representation of the vertical meridian was nevertheless substantial. The present findings suggest, therefore, that in Boston Siamese cats callosal fibers do not originate solely from the boundary regions between adjacent cortical areas, such as the anatomical 17-18 border, but originate also from other cortical regions, especially the vertical meridian representation, regardless of their location with respect to such boundaries.

Animals↗
PubMed1984

Cortical connections of the middle temporal visual area (MT) and the superior temporal cortex in owl monkeys.

The connections of the middle temporal visual area (MT) and an adjoining superior temporal visual region (ST) were investigated with injections of 3H-proline and horseradish peroxidase (HRP) in adult owl monkeys. Injections in MT revealed an extensive pattern of intrinsic connections that appeared to be organized in a series of bands across MT. MT was found to project ipsilaterally to an array of visual areas including the first (V I) and second (V II) visual areas, the dorsolateral visual area (DL), and the dorso-medial visual area (DM). When injection sites and projection zones were related to previously described retinotopic maps, it was apparent that the projections of MT to these fields were largely homotopic. Other projections were to posterior parietal cortex, the dorsointermediate visual area (DI), the ventral visual area (V), and possibly the medial visual area (M). In addition, dense projections to cortex on the rostral border of MT were used to define a new subdivision of visual cortex, the superior temporal visual region (ST), as the major projection zone of MT. Callosal connections of MT were mainly to MT, with some terminations in DL and ST. Injections of 3H-proline into ST revealed a diffuse projection to largely layer I of MT, and dense projections to posterior parietal cortex, cortex in more rostral parts of the superior temporal sulcus, ventromedial inferior temporal cortex, and the region of the frontal eye fields. Callosal projections were largely to ST cortex of the other cerebral hemisphere. The results place MT and V II in distinctly different processing chains. While both MT and V II receive the major cortical outputs of V I, V II relays to DL and DL relays to subdivisions of inferior temporal cortex. In contrast, the most significant cortical target of MT appears to be to ST, which relays to posterior parietal cortex and other targets.

Animals↗
PubMed1985

Interhemispheric pathways of the hippocampal formation, presubiculum, and entorhinal and posterior parahippocampal cortices in the rhesus monkey: the structure and organization of the hippocampal commissures.

The interhemispheric pathways originating in the hippocampal formation, presubiculum, and entorhinal and posterior parahippocampal cortices and coursing through the fornix system were investigated by autoradiographic tracing in 29 rhesus monkeys (Macaca mulatta). The results revealed that crossing fibers are segregated into three contiguous systems. A ventral hippocampal commissure lies at the transition between the body and anterior columns of the fornix in the vicinity of the subfornical organ and the interventricular foramina of Monro; it is formed by axons arising in the most anterior (uncal and genual) subdivisions of the hippocampal formation. A dorsal hippocampal commissure lies inferior to the posterior end of the body of the corpus callosum; it is formed by axons arising in the presubiculum and entorhinal cortex of the anterior parahippocampal gyrus and the proisocortical and neocortical subdivisions of the posterior parahippocampal gyrus but not in the hippocampal formation. A hippocampal decussation lies between the ventral hippocampal commissure and dorsal hippocampal commissure; it is formed by axons arising in the body of the hippocampal formation. In contrast to the fibers of the ventral hippocampal commissure and dorsal hippocampal commissure, which terminate in contralateral cortical areas, these decussating fibers terminate in the contralateral septum. Thus, the ventral hippocampal commissure and dorsal hippocampal commissure of the rhesus monkey appear to be homologous to similarly designated structures in other mammals. To the extent that these observations also apply to the interhemispheric fibers of the human hippocampal formation and parahippocampal areas, their possible preservation must be considered when interpreting the effect of callosal transection on seizures and the results of "split-brain" studies, since callosal transection may fail to sever the hippocampal commissures in their entirety.

Animals↗
PubMed1986

Changing role of forebrain astrocytes during development, regenerative failure, and induced regeneration upon transplantation.

When the cerebral midline is lesioned in the embryo or neonate, the would-be callosal axons form neuromas. We have shown that an untreated Millipore implant inserted between the neuromas in young acallosal animals can support the migration of immature astrocytes that, in turn, support the de novo growth of commissural axons between the hemispheres. Since callosal neuromas persist into adulthood, we asked whether a critical period exists after which reactive glia no longer promote axon growth. We found that a critical period does exist and have documented a variety of changes in reactive gliosis that, in part, may lead to the axon growth-refractory state. In acallosal mouse postnates given untreated implants on or prior to day 8, glial fibrillary acidic protein (GFAP)+, stellate-shaped astrocytes migrated and attached to the implant by inserting foot processes into the pores of the filter. This form of gliotic response established an axon growth-promoting substratum within 24-48 hours after implantation. During this critical stage there was no evidence of scar formation or necrosis at or around the implant surface. However, when acallosal mice were implanted on or later than postnatal day 14, extensive tissue degeneration occurred, and a mixed population of astrocytes and fibroblasts invaded the surface of the filter, producing a dense scar. Reactive cells within the scar did not promote axonal outgrowth. To determine whether glia from neonates can influence the host environment and/or induce axonal regeneration in acallosal animals after the critical period, we harvested immature astrocytes on Millipore from critical-period mouse forebrains and transplanted the glia-coated prostheses into the brains of post-critical-period acallosal animals. Such transplants reduced glial scarring in the host, inhibited extensive bleeding and secondary necrosis, and promoted axonal regeneration. Our studies suggest that when controlled with a prosthesis, gliosis during the critical period is a beneficial process that can promote the reconstruction of malformed axon pathways; that in older animals a variety of changes in reactive glia and the extracellular matrix may work together to hinder axon regeneration after the critical period; and that axonal regeneration in the postcritical CNS may be stimulated by reintroducing an immature glial environment at the lesion site.

Animals↗
PubMed1988

The development of the corpus callosum in cats: a light- and electron-microscopic study.

Changes in the size and shape of the corpus callosum (CC)--and in number, size, and structure of callosal axons--between embryonic day 38 (E38) and postnatal day 150 (P150) were studied by light and electron microscope in 25 kittens. The development of the CC was divided into three phases: 1. Embryonic development (E38, 53, 58): At E38, only part of the body of the CC was formed. At E53 and E58, the CC was still very short, but its different parts (genu, body, and splenium) had formed. The cross-sectional callosal area (CCA) was 5.4 mm2 at E53 and 5.6 mm2 at E58. The CC contained 46.3 and 56.4 million axons at E53 and E58 respectively. Mean axon diameters were 0.26 micron at E53 and 0.27 micron at E58. 2. Early postnatal development (P4, 9, 15, 18, 21, 26): The CC at P4 was much longer than at E58 and still slightly elongated during this phase; CCA reached 8.55 mm2 at P4 and 8.88 mm2 at P26. There was a substantial axonal loss (66.8 million at P4 and 52.6 million at P26). From P15 onward, premyelinated and myelinated axons were seen. Mean axon diameter increased from 0.30 micron at P4 to 0.33 micron at P26. 3. Late postnatal development (P39, 57, 92, 107, 150). The CC grew dramatically in both length and thickness, the latter especially in the genu. CCA was 10.1 mm2 at P39 and 15.3 mm2 at P150. The number of axons still decreased (46.5 million at P39 and 31.9 million at P150). The growth of the CCA paralleled the increase of myelinated axons (0.5% at P26 and 29.6% at P150 and in the mean axon diameters (0.34 micron at P39 and 0.42 micron at P150). A number of axonal ultrastructural peculiarities (electron-dense bodies, large vacuoles, lamellated bodies, etc., including those mentioned below) were noticed; their frequency at different ages was estimated as the percent of total axons. Interestingly, accumulations of vesicles inside axons increased from 4.1% at E53 to 8.9% at P26, dropped to 0.2% at P39, and remained below 1% thereafter. Swollen mitochondria increased from 0.2% at E53 to 0.9% at P26 and dropped to 0.06% (on the average) from P39 onward. Accumulations of vesicles and swollen mitochondria increased during the phase of rapid axonal elimination; thus, they may indicate axonal retraction and/or degeneration. Microglia-gitter cells and astrocytes showing signs of phagocytosis were found during the embryonic and early postnatal development and may be involved in axon elimination.

Aging↗
PubMed1989

Forms and measures of adult and developing human corpus callosum: is there sexual dimorphism?

The sexual dimorphism of the human corpus callosum (CC) is currently controversial, possibly because of difficulties in morphometric analysis. We have reinvestigated the issue by using morphometric techniques specially designed to yield objective measurements of CC size and shape. The development of the CC was studied with similar techniques in order to investigate whether its final shape and size might be influenced by axonal elimination, as could be expected from previous animal studies. We have measured the CCs of 32 men and 26 women; 27 male and 19 female CCs were from brain tissue, the others were from magnetic resonance imaging graphs. Women tended to have 1) a smaller cross-sectional callosal area (CCA); 2) a larger fraction of CCA in the posterior fifth of the CC; 3) more slender CCs; and 4) more bulbous splenia. These differences could not be detected by simple inspection but were demonstrated by measurement and statistical analysis. However, CCA was correlated with the other sexually dimorphic parameters, and the sex-related differences in the latter became nonsignificant when variations in CCA were factored out or when male and female populations with similar CCA were compared. In addition, we analyzed CCs of 16 male and 16 female fetuses and of 13 male and 15 female infants and children. This sample ranged in age between 20 weeks of gestation and 14 years but covered in detail the period up to 14 months after birth. CCA increased throughout the latter period but decreased slightly between about 33 weeks of gestation and the beginning of the second postnatal mouth. This decrease coincided with thinning of the CC and a marked increase in bulbosity of the splenium. No sexual dimorphism could be demonstrated until the beginning of the postnatal period. In the age group between birth (at term) and the 14th month, CCA was, as in the adult, larger in males. Unlike in the adults, the CC was longer in males and the bulbosity index was the same in the two sexes. Axonal elimination may play a role in the perinatal pause in CCA growth and in the concomitant changes in callosal shape.

Adolescent↗
PubMed1990

Ventral temporal cortex in the rat: connections of secondary auditory areas Te2 and Te3.

The present study in the rat deals with the hodological organization of two cytoarchitectonically distinct areas lying caudoventrally (Te2) or ventrally (Te3) to the primary auditory area (Te1). The afferent and efferent systems of connections were identified by using the properties of retrograde and anterograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP). Large tracer deposits in the ventral temporal cortex involving Te2, Te3, and the dorsal bank of the perirhinal cortex induced a dense retrograde and anterograde pattern of labeling in the following nuclei of the medial geniculate (MG) complex: caudodorsal (MGCD), dorsal (MGD), medial (MGM), suprageniculate (SG), and peripeduncular area (PPA). The ventral nucleus (MGV) was only slightly labeled in its caudal division. Several extrageniculate structures were also labeled. Retrograde cell labeling occurred in centers giving rise to ascending systems of diffuse projections: locus coeruleus (LC), dorsal raphe nucleus (DR), and basal magnocellular nucleus (B). Slight anterograde labeling was present in the dorsal and external cortices of the inferior colliculus (IC), central gray, deep layers of the superior colliculus (SC), reticular thalamic nucleus (RT), and caudate putamen (CPU). Callosal connections were also noted with the contralateral homotopic cortex. In the cases in which there was a notable extension of the zone of diffusion of the tracer into the dorsal bank of the perirhinal cortex, a characteristic pattern of labeling in the subparafascicular, reuniens and paraventricular thalamic nuclei, mammillary complex, lateral and dorsal hypothalamic nuclei, amygdaloid complex, laterodorsal tegmental nucleus, subiculum, and retrosplenial cortex was displayed. Tracer deposits restricted to Te2 induced a dense labeling of the caudal, ventrolateral MGD, lateral PPA and, to a lesser extent, MGCD. The MGM and SG were only slightly labeled. Extrageniculate afferents essentially consist of sparse projections from LC, DR, and B, whereas efferent fibers are directed to the dorsal cortex of the IC, central gray, deep SC layers, and CPU. Callosal connections were also identified. Following tracer deposits restricted to Te3, dense labeling occurred in the MGD, mostly in its medial division, in the caudal MGM, and in the PPA. The MGCD, SG, and MGV were only sparsely labeled. Extrageniculate afferents arise from LC, DR, and B, and efferents are directed to the RT and dorsal cortex of the IC. Contralateral connections with the homotopic cortical area were also noted. Te2 and Te3 share some degree of similitude in their pattern of connections with the MG complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Prenatal formation of the normal mouse corpus callosum: a quantitative study with carbocyanine dyes.

Judgment of abnormalities in fetal cortical axon development is more sensitive when a good standard of normal ontogeny is established. The recent availability of postmortem tract tracing methods has greatly improved the observation of axon extension and growth cone morphology in mouse fetuses, which allows much stronger statements about the timing of crucial steps in the formation of the corpus callosum in particular. The first outgrowth and crossing of midplane by axons of the corpus callosum (CC) were examined in 153 normal mouse embryos and fetuses of the hybrid cross B6D2F2/J with carbocyanine dyes applied to brains fixed by perfusion. In most brains a crystal of DiI was inserted into either frontal, parietal, temporal, or occipital cortex in one hemisphere, and a crystal of DiA was placed into a different site in the opposite hemisphere. Although dye diffusion obscured the emergence of axons, linear regression analysis revealed that the first callosal axons emerged from their cortical cells of origin at about 0.4 g body weight or 15.5 days after conception for all four sites. Subsequent axon growth rate was substantially faster for those from frontal cortex (3.2 mm/day) than occipital cortex (1.8 mm/day). Axons from frontal cortex crossed the cerebral midplane first (0.69 g, E16.3), followed by those from parietal (0.74 g), temporal (0.77 g) and occipital cortex (0.92 g, E16.9). Prior to crossing midplane, the pioneering CC axons were usually 200 microns or less in advance of the main bundle, but when they crossed midplane and encountered CC axons growing from homotopic sites in the opposite hemisphere, the pioneering axons were often 0.5 to 2.5 mm ahead of the main bundle. Growth cones were usually large and complex until they had crossed midplane and were thereafter smaller with simple and flat morphologies. The topography of axons in the CC at midplane was organized according to cortical region of origin from the very beginning, when the CC was only a small cap over the hippocampal commissure and dorsal septum. The quantitative results provide a convenient standard for normal callosal development in mice and should facilitate comparative studies.

Animals↗
PubMed2006

The corpus callosum as an evolutionary innovation.

The corpus callosum (CC) is the major interhemispheric fibre bundle in the eutherian brain and has been described as a true evolutionary innovation. This paper reviews the current literature with regard to functional, developmental and genetic concepts that may help elucidate the evolutionary origin of this structure. It has been suggested that the CC arose in the eutherian brain as a more direct and, therefore, more effective system for the interhemispheric integration of topographically organized sensory cortices than the anterior commissure (AC) and hippocampal commissure (HC) already present in nonplacental mammals. It can also be argued, however, that the ability of the CC to integrate the newly evolving motor cortices of placental mammals may have played a role in the evolutionary fixation of this structure. Investigations into the developmental mechanism involved in the formation of the CC and their underlying patterns of gene expression make it possible to formulate a tentative hypothesis about the evolutionary origin of this commissure. This paper suggests that changes in the developmental patterns of the expression of certain regulatory genes may have allowed a first group of callosal pioneering axons to cross the cortical midline. These pioneering fibres may have used the axons of the HC to find their way across the midline. Additional callosal fibres may then have fasciculated with these pioneers. Once the CC had formed in this way, more complex systems of axonal guidance may have evolved over time, thus enabling a gradual increase in the size and complexity of the CC.

Animals↗
PubMed2002

GABA-B-related activity in processing of transcallosal response in cat motor cortex.

GABA-ergic characteristics of transcallosal (TC) responses were studied with specific antagonists of both GABA-A and GABA-B receptor subtypes. We used a paired-pulse paradigm to get insight into the role of GABA in interhemispheric interactions between motor cortices. 3-Amino-2-(4-chlorophenyl)-propylphosphonic acid (phaclofen) and 3-aminopropyl-diethoxymethyl-phosphinic acid (CGP 35348) were used as GABA-B antagonists and bicuculline methiodide (BMI) was used to block the GABA-A receptor. Although both GABA-A and GABA-B antagonists increased spike discharge upon transcallosal stimulation, in both pyramidal tract and non-pyramidal tract neurons, they had different effects on the responses to the first and second stimuli of paired-pulse stimulation (200 msec interval). Although the inhibition seen with the second stimulus was greatly attenuated by antagonists of the GABA-B receptor, it was maintained in the presence of the GABA-A antagonist. These finding support a presynaptic regulation of callosal transmission by GABA-B receptors in the callosal synapse of the cat motor cortex.

Animals↗
PubMed2002

Ivemark syndrome with agenesis of the corpus callosum: a case report with a review of the literature.

Asplenia associated with situs ambiguus, symmetric liver, bilateral trilobulated lungs, and a complex heart defect was diagnosed on autopsy in a 14-day-old infant. Furthermore, examination of the brain displayed agenesis of the corpus callosum (ACC) with pachygyria and hydrocephalus. The characteristic association of asplenia with visceroatrial heterotaxia is traditionally named after the Swedish pediatrician, Ivemark. Although exceptional, association of Ivemark syndrome with callosal agenesis has been reported recently. The concept of 'developmental fields' describes morphogenetically reactive units of the embryo determining and controlling the development of complex structures in a hierarchical manner. Lateralization defects such as situs inversus, asplenia or polysplenia due to defective left-right axis development, as well as decussation defects such as ACC, are considered as defects of the primary developmental field. Therefore, additional callosal agenesis in Ivemark syndrome may be a coherent and synchronic defect in the primary developmental field rather than a causally independent malformation.

Abnormalities, Multiple↗
PubMed2001

Geometric morphometrics of corpus callosum and subcortical structures in the fetal-alcohol-affected brain.

BACKGROUND: Although experienced clinicians have been diagnosing fetal alcohol syndrome (FAS) for nearly 30 years, the rest of the spectrum of fetal alcohol damage is not being classified effectively. This article describes a quantification of neuroanatomical structure that may supply a useful discriminator of prenatal brain damage from alcohol. It is demonstrated in a data set of adults of both sexes. METHODS: Ninety adults (45 males) were examined by magnetic resonance imaging (MRI). These subjects were group-matched for age and ethnicity across three diagnoses: FAS, fetal alcohol effects (FAE), and normals. All FAS and FAE were heavily alcohol-exposed in utero; normals were not. From T(1)-weighted MR brain images, we extracted 3D morphometric representations of shape for 33-landmark point configurations and 40-point outlines of the corpus callosum along its midline (a slightly nonplanar structure). RESULTS: There are striking differences between exposed and unexposed in the statistical distributions of these two shapes. The differences are better characterized by excess variance in the exposed group than by any change in average landmark or outline shape. For each sex, combining the callosal outline data with the landmark data leads to a powerful quadratic discriminator of exposed from unexposed. The discriminating features include the relationship of brain stem to diencephalon, and localized variabilities of callosal outline shape, but not diagnosis (FAS vs. FAE). CONCLUSIONS: Statistical analysis of brain shape is a powerful new source of information relevant to fetal alcohol spectrum nosology and etiology. Patients with FAS and FAE do not differ in these brain shape features, but both differ from the unexposed. The aspects of brain shape that are especially variable may be entailed in the underlying neuroteratogenetic mechanisms.

Adolescent↗
PubMed1994

Zaidel's model of interhemispheric dynamics: empirical tests, a critical appraisal, and a proposed revision.

Zaidel has proposed a model of intra- and interhemispheric dynamics which attempts to explain field and hand effects in visual reaction time experiments. The model includes prototypes of two extreme cases: a "callosal relay" pattern, in which interhemispheric transfer is inferred, and a "direct access" pattern, in which each hemisphere is inferred to execute the task independently. Five visual reaction time experiments were carried out to test six predictions which Zaidel has made within the framework of his model. Some of these predictions were supported but others were rejected. These findings, in conjunction with findings of other researchers, were critically analyzed in view of determining the extent to which Zaidel's model can be revised in order to fit the ensemble of relevant findings. It was concluded that the model can indeed be revised but that this requires a redefinition of conditions indicative of presence or absence of interhemispheric relay. A new model is proposed assuming exclusively excitatory callosal relay and focussing on magnitude and valence of field effects obtained at each hand, considered in their interrelation.

Adult↗
PubMed1998

Interhemispheric communication is via direct connections.

Two priming experiments, using normal university students as subjects, independently projected low imagery primes and concrete target words to the left or right visual fields (LVF or RVF) to examine the merits of three spreading activation models of interhemispheric communication: (i) callosal relay of a semantically encoded prime; (ii) transfer of products activated as a result of the spread of activation; and (iii) direct connections between the hemispheres. The first experiment temporally separated pairs by a stimulus onset asynchrony (SOA) of 250 ms and obtained strong support for the direct connections model. Priming effects were obtained only when the prime was projected to the RVF and the target to the LVF. The pattern of priming effects suggested that low imagery words projected to the left hemisphere can activate concrete associates in the right hemisphere via direct callosal connections between the two. In the second experiment, the SOA was increased to 450 ms. This time, RVF-RVF priming was obtained along with RVF-LVF priming. The findings are interpreted within a modification of Bleasdale's (1987) framework, where abstract/low imagery words and concrete/high imagery words are represented in separate subsystems in the left hemisphere lexicon. Support was also found for the view that the left hemisphere is comprised of a complex network of abstract and concrete words, while the right hemisphere operates as a subsidiary word processor, subserving linguistic processing with a limited, special purpose lexicon comprised of associative connections between concrete, imageable words (e.g., Zaidel, 1983a; Bradshaw, 1980). Interhemispheric communication in the priming procedure appears to occur at the semantic level, via direct connections between the hemispheres.

Adult↗
PubMed2002

Microstructural but not macrostructural disruption of white matter in women with chronic alcoholism.

The results of in vivo neuroimaging studies assessing whether and where brain white matter damage occurs in alcoholic women is controversial. To address this controversy, we examined regional white matter macrostructure and microstructure, the latter of which may be more sensitive to the detection of subtle fiber disruption than gross measures of size. Accordingly, we used conventional magnetic resonance imaging (MRI) to quantify regional callosal size and diffusion tensor imaging (DTI) to examine intravoxel coherence (fractional anisotropy, FA) and intervoxel coherence (C) of white matter of the genu and splenium of the corpus callosum and of the centrum semiovale in 12 detoxified alcoholic women and 18 control women. Additional analyses examined sex differences in FA and C in alcoholic women compared with alcoholic men. Despite absence of group differences in regional areas of callosal macrostructure, the alcoholic women had lower FA and C in genu and centrum semiovale than the control group of women. These measures also correlated with total lifetime consumption of alcohol and performance on a test of visual search in the alcoholic women. Sex comparisons revealed similar extents of FA abnormality in the genu and centrum semiovale in alcoholic men and women and differential effects in other DTI measures, with abnormalities present in splenium FA and C in the men and abnormalities present in centrum C in the women. These results provide in vivo evidence for disruption of white matter microstructure in alcoholic women not necessarily detectable with coarser measures of white matter mass and perhaps antedating its appearance.

Adult↗