Opposite Cerebral Hemisphere Research Articles

Accuracy of Visual Fields in Localizing MRI Lesions in Posterior Cerebral Artery Infarction.

The representation of the visual field in visual cortex was established over a century ago by correlating perimetric defects with the estimated location of war wounds. The availability of high-definition MRI offers the possibility of more precise correlation. Homonymous hemianopias disclosed on automated visual fields (HVFs) were drawn from an electronic medical record search from 2009 to 2020 at the Michigan Medicine, a tertiary care academic medical center. The patterns of the visual field defects (VFDs) were interpreted by a consensus of 2 authors. The VFDs were correlated with the location of MRI lesions in 92 patients with posterior cerebral artery (PCA) domain ischemic strokes, as determined by the neuroradiologist author, who was masked as to the VFDs. Among the 77 VFDs confined to 1 hemifield, 74 (96%) correctly predicted the side of the visual cortex lesion. In 3 cases, the MRI lesion in the opposite cerebral hemisphere was not foretold. Among the 15 VFDs present in both hemifields, 5 (33.3%) overestimated the MRI lesions, which were evident in only 1 hemisphere. Among the 30 VFDs confined to 1 quadrant, 29 (97%) correctly predicted the lesioned visual cortex quadrant. However, 14 VFDs failed to predict MRI lesions present in both superior and inferior visual cortex quadrants on the same side. Those unpredicted lesions mostly had subtle or indistinct signal abnormalities or were confined to anterior visual cortex, an area that is inaccessible with the HVF test protocol used in this study. In this study of PCA ischemic stroke, VFDs limited to 1 hemifield were accurate in locating the side and quadrant of the MRI visual cortex lesions. However, the quadrantic VFDs sometimes failed to predict that the lesions involved both the superior and inferior quadrants on the same side, largely because those lesions had subtle imaging features that defied accurate radiologic assessment or were out of the reach of the visual field test protocol.

The interactive functional biases of manual, language and attention systems

Hemispheric lateralisation is a fundamental principle of functional brain organisation. We studied two core cognitive functions—language and visuospatial attention—that typically lateralise in opposite cerebral hemispheres. In this work, we tested both left- and right-handed participants on lexical decision-making as well as on symmetry detection by means of a visual half-field paradigm with various target–distractor combinations simultaneously presented to opposite visual fields. Laterality indexes were analysed using a behavioural metrics in single individuals as well as between individuals. We observed that lateralisation of language and visuospatial attention as well as their relationship generally followed a left–right profile, albeit with differences as a function of handedness and target–distractor combination. In particular, right-handed individuals tended towards a typical pattern whereas left-handed individuals demonstrated increased individual variation and atypical organisation. That the atypical variants varied as a function of target–distractor combination and thus interhemispheric communication underlines its dynamic role in characterising lateralisation properties. The data further revealed distinctive relationships between right-handedness and left-hemispheric dominance for language together with right-hemispheric dominance for visuospatial processing. Overall, these findings illustrate the role of broader mechanisms in supporting hemispheric lateralisation of cognition and behaviour, relying on common principles but controlled by internal and external factors.

Comparative Evaluation of Apparent Diffusion Coefficient Values of White Matter Surrounding the Heterotopia in Children With Unilateral Subependymal Heterotopia.

Introduction: To compare the apparent diffusion coefficient (ADC) values of the white matter around heterotopia in children with unilateral subependymal heterotopia with those of the symmetrical normal cerebral hemisphere and control group.Methods: Between January 2011 and September 2021, 15 pediatric patients with unilateral focal subependymal heterotopia among 47 patients with heterotopia detected in brain magnetic resonance imaging (MRI) in our hospital were included in the study. The control group consisted of 15 age- and sex-matched children with normal neurological examination and normal brain MRI. In brain MRIs, ADC value was measured from the white matter around the heterotopia area and from the opposite cerebral hemisphere matched to the location, and from the bilateral location-matched white matter of the control group. The area of heterotopia was measured on axial T1-weighted MRI. The data were evaluated statistically.Results: There were eight girls and seven boys in the heterotopia group. The median age was 5.00 (min: 3, max: 14). There was no statistically significant difference between the ADC values of the heterotopia side and contralateral white matter of the heterotopia group. In addition, no statistically significant difference was found between the heterotopia side and opposite sides of the heterotopia and control groups ADC values.Conclusion: According to the findings of this study, no difference was found in the ADC values of the white matter around the lesion in children with subependymal heterotopia compared to the opposite cerebral hemisphere and control groups.

Genetic influences on cortical myelination in the human brain.

Cortical myelination, which is essential for interneuronal communication and neurodevelopment, has been reported to be under genetic control. However, the degree to which genes contribute to the variability of myelination, the pattern of genetic control, and how genes influence the organization of myelination are largely unknown. To answer these questions, the present study calculated heritability estimates for myelination of the cortical regions using the high quality structural magnetic resonance imaging (MRI) scans from the Human Connectome Project pedigree cohort (n = 873, 383/490 M/F, 22-36 years of age). Then, we used transcriptional profiles to evaluate the contribution of myelination-related genes (data from the Allen Human Brain Atlas) to explain interregional variations in cortical myelination. Our results showed that all the cortical areas were modestly to moderately influenced by genetic factors (h2 = 29%-66%, all Ps < 0.05 after Bonferroni correction). The genetic control of cortical myelination showed bilateral symmetry and an anterior-to-posterior gradation. A bivariate model indicated that the regions are strongly genetically correlated with their homologs in the opposite cerebral hemisphere. A cross-modal analysis did not find a correlation between cortical myelination and the expression levels of myelination-related genes. This could have been due to the small number of samples with expression data in each cortical region. Overall, our findings suggest that cortical myelination is shaped by genetic factors and may be useful to bridge the underlying genetic variants and the cognitive functioning and related neuropsychiatric disorders.

Human Cortical Thickness Organized into Genetically-determined Communities across Spatial Resolutions.

The cerebral cortex may be organized into anatomical genetic modules, communities of brain regions with shared genetic influences via pleiotropy. Such modules could represent novel phenotypes amenable to large-scale gene discovery. This modular structure was investigated with network analysis of in vivo MRI of extended pedigrees, revealing a "multiscale" structure where smaller and larger modules exist simultaneously and in partially overlapping fashion across spatial scales, in contrast to prior work suggesting a specific number of cortical thickness modules. Inter-regional genetic correlations, gene co-expression patterns and computational models indicate that two simple organizational principles account for a large proportion of the apparent complexity in the network of genetic correlations. First, regions are strongly genetically correlated with their homologs in the opposite cerebral hemisphere. Second, regions are strongly genetically correlated with nearby regions in the same hemisphere, with an initial steep decrease in genetic correlation with anatomical distance, followed by a more gradual decline. Understanding underlying organizational principles of genetic influence is a critical step towards a mechanistic model of how specific genes influence brain anatomy and mediate neuropsychiatric risk.

Glioblastoma Multiforme Mimetizando Encefalite Límbica

Selective and bilateral involvement of the limbic system by gliomas is rarely reported. The authors report the case of a 58 years-male complaining of short-term memory loss in the preceding three weeks. The initial MRI revealed areas of hyperintensity on T2 and FLAIR sequences involving both hippocampus, amygdala, parahippocampus gyrus and the fornix, with contrast enhancement in the right mesial temporal region. Limbic encephalitis diagnosis was assumed and steroid therapy was applied. All other evaluation work-up was negative and follow-up imaging was compatible with tumor. The neuropathologic study of the right temporal lesion revealed glioblastoma multiforme. Glioblastoma can crossover through the commissural white matter to the opposite cerebral hemisphere, most often through the corpus callosum. However, selective and bilateral invasion of the limbic system is rare and when it occurs it may mimic limbic encephalitis.

Posed versus spontaneous facial expressions are modulated by opposite cerebral hemispheres

Clinical research has indicated that the left face is more expressive than the right face, suggesting that modulation of facial expressions is lateralized to the right hemisphere. The findings, however, are controversial because the results explain, on average, approximately 4% of the data variance. Using high-speed videography, we sought to determine if movement-onset asymmetry was a more powerful research paradigm than terminal movement asymmetry. The results were very robust, explaining up to 70% of the data variance. Posed expressions began overwhelmingly on the right face whereas spontaneous expressions began overwhelmingly on the left face. This dichotomy was most robust for upper facial expressions. In addition, movement-onset asymmetries did not predict terminal movement asymmetries, which were not significantly lateralized. The results support recent neuroanatomic observations that upper versus lower facial movements have different forebrain motor representations and recent behavioral constructs that posed versus spontaneous facial expressions are modulated preferentially by opposite cerebral hemispheres and that spontaneous facial expressions are graded rather than non-graded movements.

The effects of chewing-side preference on human brain activity during tooth clenching: an fMRI study

Chewing-side preference (CSP) may be associated with dominant cerebral hemispheric organisation. However, little information exists regarding whether CSP is reflected by preferential activity in the opposite (to the CSP) cerebral hemisphere. The aim of this study was to investigate the effects of CSP on cerebral cortex response to bilateral tooth clenching. Sixteen right-handed participants with left (two men: 29·0±8·4 years old, six women: 32·3±4·8 years old) or right (four men: 31·0±6·1 years old, four women: 30·8±4·7 years old) CSP were scanned by functional magnetic resonance imaging during moderate levels of voluntary tooth clenching. The on-off sequence of scanning was 30 s of clenching (on) and 30 s of rest (off) a total of five times. The results showed that blood oxygen level-dependent signals in the contralateral (to the CSP) primary sensorimotor cortex increased more than in the ipsilateral primary sensorimotor cortex in participants with both left and right CSP (P≤0·001). The supplementary motor area was activated in participants with left (P≤0·001) but not right CSP. Activation of the inferior frontal gyrus and inferior parietal lobule was greater in participants with right versus left CSP (P≤0·001). Significant (P≤0·001) activation was observed in the parahippocampal gyrus in five of eight participants with left CSP, whereas no activation was observed in those with right CSP. These findings suggest a relationship between hemispheric dominance and CSP in the primary sensorimotor cortex responsible for bilateral tooth clenching.

Using repetitive transcranial magnetic stimulation for the treatment of tinnitus

Transcranial magnetic stimulation (TMS) has recently emerged as a potential treatment option for tinnitus.1–6 TMS induces electrical stimulation of cortical neurons by creating a brief, focused magnetic field over the surface of the brain.7 When magnetic pulses are delivered repetitively and rhythmically, the process is called repetitive TMS (rTMS). The magnetic field induced by rTMS is brief (microseconds), relatively weak (except directly under the coil, where 1–3 cm of tissue is stimulated, depending on coil configuration),8 and declines rapidly with distance from the coil (falling off sharply after 2 cm).7,9 Pulse trains can be delivered at low (≤1 Hz) or high (>1 Hz) frequencies, which tend to decrease or increase neural activity beneath the coil, respectively. Standard TMS coils are only able to directly stimulate the superficial cortex, but deeper brain structures and structures in the opposite cerebral hemisphere may be affected by TMS via connecting neural pathways.10,11 Most studies using rTMS for the treatment of tinnitus have used low-frequency magnetic pulses applied to one temporal lobe for 30 minutes, once daily, for 3–10 consecutive days. Some have used functional imaging with neuronavigation to select the treatment site based on asymmetrical cortical activation, while others have targeted the left superior temporal lobe in all cases or the side opposite the loudest tinnitus. Regardless of targeting technique, beneficial results are obtained in about 50% of patients who receive active treatment. While treatment effects have been reported to persist for 6 months in some cases,2 most treatment studies have demonstrated tinnitus suppression effects lasting only a few days to a few weeks.3,5,6 This article will briefly review rTMS, the rationale for its use to treat tinnitus, and data relevant to its efficacy in treating tinnitus.

Effect of auditory feedback differs according to side of hemiparesis: a comparative pilot study

BackgroundFollowing stroke, patients frequently demonstrate loss of motor control and function and altered kinematic parameters of reaching movements. Feedback is an essential component of rehabilitation and auditory feedback of kinematic parameters may be a useful tool for rehabilitation of reaching movements at the impairment level. The aim of this study was to investigate the effect of 2 types of auditory feedback on the kinematics of reaching movements in hemiparetic stroke patients and to compare differences between patients with right (RHD) and left hemisphere damage (LHD).Methods10 healthy controls, 8 stroke patients with LHD and 8 with RHD were included. Patient groups had similar levels of upper limb function. Two types of auditory feedback (spatial and simple) were developed and provided online during reaching movements to 9 targets in the workspace. Kinematics of the upper limb were recorded with an electromagnetic system. Kinematics were compared between groups (Mann Whitney test) and the effect of auditory feedback on kinematics was tested within each patient group (Friedman test).ResultsIn the patient groups, peak hand velocity was lower, the number of velocity peaks was higher and movements were more curved than in the healthy group. Despite having a similar clinical level, kinematics differed between LHD and RHD groups. Peak velocity was similar but LHD patients had fewer velocity peaks and less curved movements than RHD patients. The addition of auditory feedback improved the curvature index in patients with RHD and deteriorated peak velocity, the number of velocity peaks and curvature index in LHD patients. No difference between types of feedback was found in either patient group.ConclusionIn stroke patients, side of lesion should be considered when examining arm reaching kinematics. Further studies are necessary to evaluate differences in responses to auditory feedback between patients with lesions in opposite cerebral hemispheres.

Individual Differences in Stroop and Local-Global Processing: A Possible Role of Interhemispheric Interaction

Two experiments are reported examining individual differences in the processing of centrally presented stimuli containing two dimensions of information lateralized to opposite cerebral hemispheres. Left-handers, arising from (a) their lesser degree of functional lateralization and (b) their greater degree of callosal connectivity, were hypothesized to exhibit greater interdimensional (and presumably interhemispheric) interaction. Experiment 1 utilized local-global stimuli, and left-handers were found to be impaired at keeping the two dimensions independent and superior at integrating the two dimensions . Experiment 2 used Stroop stimuli, and left-handers again were impaired at keeping the two dimensions independent (i.e., showed greater Stroop interference). Correlational analyses indicated that the mechanisms of interdimensional integration versus independence are at least partially independent from one another. Results suggest that aspects of interhemispheric interaction can be addressed via the use of nonlateralized input.

Subdivisions of auditory cortex and processing streams in primates.

The auditory system of monkeys includes a large number of interconnected subcortical nuclei and cortical areas. At subcortical levels, the structural components of the auditory system of monkeys resemble those of nonprimates, but the organization at cortical levels is different. In monkeys, the ventral nucleus of the medial geniculate complex projects in parallel to a core of three primary-like auditory areas, AI, R, and RT, constituting the first stage of cortical processing. These areas interconnect and project to the homotopic and other locations in the opposite cerebral hemisphere and to a surrounding array of eight proposed belt areas as a second stage of cortical processing. The belt areas in turn project in overlapping patterns to a lateral parabelt region with at least rostral and caudal subdivisions as a third stage of cortical processing. The divisions of the parabelt distribute to adjoining auditory and multimodal regions of the temporal lobe and to four functionally distinct regions of the frontal lobe. Histochemically, chimpanzees and humans have an auditory core that closely resembles that of monkeys. The challenge for future researchers is to understand how this complex system in monkeys analyzes and utilizes auditory information.

Why a journal of hand surgery?

Joseph Boyes, in the inaugural editorial of The Journal of Hand Surgery (reprinted on the previous pages), addressed the question “Why a Journal of Hand Surgery?” by noting that published articles concerning the hand were scattered in various textbooks, societal transaction papers, and tissue-oriented periodicals. Although hand surgery had met many of its educational obligations as an established medical discipline, such as sponsoring meetings and courses related to the upper extremity, defining requirements for fellowship training, and publishing a continuously updated bibliography of hand surgery, these activities alone were not sufficient to fulfill hand surgery’s educational mission. A specialty journal devoted exclusively to the upper limb was needed to accomplish its commitment “to improve and develop surgery of the hand.” By all standards, the publication established by Dr Boyes and his founding colleagues has been successful. Nevertheless, the question “Why a Journal of Hand Surgery?” is still pertinent today. It begs, however, a more expanded response, one that focuses not on explaining the need for such a journal, but rather on what we have come to expect of it; that is, what is its purpose and what are its objectives? As envisioned by Dr Boyes, the Journal serves as a principal repository for thoughts and ideas related to surgery of the hand. The 3,804 published articles are the historical record—the archives, so to speak—of the development of our specialty, the authority to which students of hand surgery turn for information. Some proved to be longlasting, some have been modified by ensuing notations and entries, and some became mere footnotes in the ongoing chronicles of hand surgery. Nevertheless, all are essential to defining what hand surgery is and what it will become. Additionally, The Journal of Hand Surgery provides a forum for authors to present new ideas and concepts. From a retrospective view point, ideas that have endured include the development of a classification system to categorize congenital anomalies, the identification of different patterns of carpal bone instability, the importance of tendon mobilization in the rehabilitation of sutured flexor tendons, the use of tubulization to restore continuity to injured peripheral nerves, and the microvascular transfer of composite tissues. Few ideas, however, remain unaltered; most are expanded on, clarified, or even rebutted by subsequent publications. In contrast to textbooks, review articles, and monographs, journal articles represent “current” data and do not enjoy the luxury of being shaped by the reflective perspective of time. No idea is forever and no concept is absolute; all will in time be altered by subsequent information and the presentation of new data. A third principal objective, and perhaps the most important, is to educate the reader. As noted in a 1994 ASSH member survey, The Journal of Hand Surgery is the hand surgeon’s most important instrument in addressing continuing medical education needs, exceeding even that of the annual meeting. At the most basic level,

Effect of unilateral perinatal hypoxic-ischemic brain damage on the gross development of opposite cerebral hemisphere.

The effect of unilateral perinatal cerebral hypoxic-ischemic damage on the gross development of the opposite hemisphere was investigated in immature rats. Eight-day-old rats underwent unilateral common carotid artery ligation followed by exposure to hypoxia (8% oxygen) ranging from 1 to 2 h at 37 degrees C. The animals were sacrificed 3 weeks later, and brain damage was assessed histologically, by weighing the cerebral hemispheres, and by measuring hemispheric cross-sectional area and diameter as well as the thickness of neocortex. There were no significant differences in either the size or cortical thickness of the cerebral hemisphere contralateral to the damaged hemisphere compared to those of controls. The results are comparable to the effect of unilateral perinatal hemispheric decortication and hemispherectomy on the growth of the contralateral hemisphere and suggest that: (1) perinatal damage to one cerebral hemisphere does not significantly alter the size of the other hemisphere, and (2) the contralateral hemisphere may be used as a 'normal' reference in evaluating the extent of atrophy or infarction of a cerebral hemisphere damaged earlier by perinatal hypoxia-ischemia.

Effect of GABAmimetics on electrocorticographic spike discharges induced by guanidinoethanesulfonic acid (amidino-taurine) in the rat

The effect of guanidinoethanesulfonic acid (GES) on rat electrocorticograms (ECoG) and the effects of gamma-aminobutyric acid (GABA) and GABA-agonists on the ECoG changes induced by GES were studied. Sporadic spike discharges began 2-5 min after 1 mumol GES/10 microliters on filter paper was applied to the pia mater of the left sensorimotor cortex; spike discharges extended to the opposite cerebral hemisphere 60 min after the onset of the ipsilateral spike discharges. The spike discharges with a frequency of 5-10 spikes/min lasted until the end of the 4 hour recording. The induced spike discharges were suppressed when the original GES soaked filter paper was replaced by one containing GES (1 mumol) supplement combined with taurine (1 mumol/10 microliters). GABA (1 mumol) and its receptor agonist, muscimol (10 nmol) and (3R)-(-)-4-amino-3-hydroxybutyric acid (1 mumol) also suppressed the GES-induced spike discharges when applied topically. Diazepam (DZP) (10 mg/kg) suppressed the GES-induced spike discharges 10 min after i.p. injection, but phenobarbital (20 mg/kg) increased the frequency and voltage of spike discharges 100 min following subcutaneous administration. Intraperitoneal injection of either valproate (200 mg/kg) or phenytoin (25 mg/kg), after the completion of the spike discharges, showed no effect. These findings suggest that neurotransmission or neuromodulatory effects of taurine participate in GES-induced seizure activity, and that GABAA and DZP receptors may play a role in the mechanism that suppresses GES-induced seizures.

Plastic changes in the intact cerebral cortex as a result of local injury to a symmetrical division of it.

The representation of the radial nerve in a symmetrical division of the opposite cerebral hemisphere was studied using an evoked potentials technique at various periods (from 1.5 months to 1 year 8 months) following the electrolytic destruction of the associative parietal cortex in chronic experimental conditions in cats. An expansion of the zone of the representation of the radial nerve was identified, starting from three to four months following the operative intervention. The activity of lactate dehydrogenase and malate dehydrogenase, as well as the spectrum of lactate dehydrogenase isoenzymes in the indicated time periods. The possible factors underlying the changes observed are discussed.

Astrocytoma with widespread calcification along axonal fibres

We describe a 44-year-old man found to have a diffusely calcified astrocytoma originating in the left frontoparietal region and extending along the axonal fibres into the opposite cerebral hemisphere and brain stem. Computed tomography and magnetic resonance imaging clearly demonstrated the spread of the tumour. The tumour was partially resected and histologically diagnosed as an astrocytoma. A correct preoperative diagnosis was difficult, because the lesion was very slow-growing and its widespread calcification mimicked congenital or inflammatory calcium deposits.

Early phenotype expression of cortical neurons: evidence that a subclass of migrating neurons have callosal axons.

The use of [3H]thymidine labeling in combination with various axonal transport tracers has revealed that a subset of migrating neurons in the fetal monkey cerebrum issue axons to the opposite cerebral hemisphere while still migrating to their final positions in the cortical plate. Other cortical neurons with the same "birthdate" (i.e., that underwent their last round of DNA synthesis on the same day) are not retrogradely labeled by tracer injections of the opposite hemisphere. These findings suggest that the cardinal distinction between projection and local circuit neurons may be specified in postmitotic neurons before they acquire their final positions in the cortex.

Cerebrocerebellar Relationship during Behavioral Activation: A PET Study

The effect of behavioral activation on cerebral and cerebellar glucose metabolism was studied in normal subjects when performing either a verbal memory task or a tactile somatosensory task. Each subject was also studied in a resting state control condition, either 1 h earlier or later than the activation task. Compared to the resting state, both tasks produced asymmetrical metabolic activation, which was opposite in direction within the cerebral and cerebellar hemispheres. In both tasks, the difference of activation of CMRglc in the right and left hemispheres in the cerebellum was negatively correlated with that in the sensory-motor region. This apparently coupled metabolic activation of one cerebellum and areas within the opposite cerebral hemisphere represents the inverse of the crossed cerebellar diaschisis phenomenon commonly observed when a vascular lesion affects one cerebral hemisphere and hypometabolism occurs in the opposite cerebellum. Because these correlations were selective and concordant with known anatomical connections, and were found in two different tasks, they suggest strong functional connections between these specific brain regions.

Cytological and quantitative characteristics of four cerebral commissures in the rhesus monkey

The number, types, and distribution of distinct classes of axons and glia in four cerebral commissures of the adult rhesus monkey (Macaca mulatta) were determined using electron microscopic and immunocytochemical methods. The two neocortical commissures, the corpus callosum, and the anterior commissure contain small but cytologically distinct archicortical components: the hippocampal commissure, which lies ventral to the splenium of the corpus callosum, and the basal telencephalic commissure, which forms a small crescent at the anterior margin of the anterior commissure. Each archicortical pathway is delineated from the adjacent neocortical commissure by a glial capsule. The glia cells that form this border are immunoreactive with antisera directed against glial fibrillary acidic protein (GFAP) and issue long processes that form numerous desmosomal junctions with one another. Braids of these glial processes envelop axonal fascicles within the archicortical commissures. In contrast, the GFAP-positive cells of the corpus callosum and anterior commissure are randomly distributed cells with relatively short stellate processes that do not form boundaries around axon fascicles. Quantitative electron microscopic analysis reveals that approximately 60 million axons connect the two cerebral hemispheres: the corpus callosum contains 56.0 million +/- 3.8 million axons (n = 8), the anterior commissure contains 3.15 million +/- 0.24 million axons (n = 8), the hippocampal commissure has 237,000 axons +/- 31,000 (n = 6), and the basal telencephalic commissure has 193,000 axons +/- 28,000 (n = 5). The number of axons is not directly proportional to the cross-sectional area in any of the commissures because of variation in axonal composition. On the basis of an estimate of approximately 3 billion neurons in the monkey cortex (Shariff, '53), we estimate that between 2 and 3% of all cortical neurons project to the opposite cerebral hemisphere. Subregions of the corpus callosum as well as each of the other commissures consist of characteristic subsets of five classes of axons and contain different proportions of myelinated to unmyelinated fibers. The largest myelinated axons and the smallest proportion of unmyelinated axons (approximately 6%) are found in regions of the corpus callosum that carry projections from primary sensory cortices, whereas the smallest myelinated axons and largest proportion of unmyelinated axons (approximately 30%) are found in regions of the corpus callosum that carry projections from association cortices. Axon composition in the anterior commissure is uniform and resembles that of callosal sectors that contain association projections.(ABSTRACT TRUNCATED AT 400 WORDS)