Schizophrenia, neurodevelopment and corpus callosum.

The Zeitgeist favors an interpretation of schizophrenia as a condition of abnormal connectivity of cortical neurons, particularly in the prefrontal and temporal cortex. The available evidence points to reduced connectivity, a possible consequence of excessive synaptic pruning in development. A decreased thalamic input to the cerebral cortex appears likely, and developmental studies predict that this decrease should entail a secondary loss of both long- and short-range cortico-cortical connections, including connections between the hemispheres. Indeed, morphological, electrophysiological and neuropsychological studies over the last two decades suggest that the callosal connections are altered in schizophrenics. However, the alterations are subtle and sometimes inconsistent across studies, and need to be investigated further with new methodologies.

Vision after early-onset lesions of the occipital cortex: I. Neuropsychological and psychophysical studies.

We analyzed the visual functions of two patients (MS, FJ) with bilateral lesion of the primary visual cortex, which occurred at gestational age 33 wk in MS and at postnatal month 7 in FJ. In both patients basic visual functions--visual acuity, contrast sensitivity, color, form, motion perception-are similarly preserved or modestly impaired. Functions requiring higher visual processing, particularly figure-ground segregation based on textural cues, are severely impaired. In MS, studied longitudinally, the deficits attenuated between the ages of 4.5 and 8 y, suggesting that the developing visual system can display a considerable degree of adaptive plasticity several years after the occurrence of a lesion. In FJ (age 18:9 to 20:6 y), who is more impaired, the recovery, if any, was less.

Vision after early-onset lesions of the occipital cortex: II. Physiological studies.

In one of two patients (MS and FJ) with bilateral, early-onset lesion of the primary visual cortex, Kiper et al. (2002) observed a considerable degree of functional recovery. To clarify the physiological mechanisms involved in the recovery, we used fMRI and quantitative EEG to study both patients. The fMRI investigations indicated that in both patients, isolated islands of the primary visual cortex are functioning, in the right hemisphere in MS and in the left in FJ. The functional recovery observed in MS roughly correlated with the functional maturation of interhemispheric connections and might reflect the role of corticocortical connectivity in visual perception. The functionality of interhemispheric connections was assessed by analyzing the changes in occipital inter-hemispheric coherence of EEG signals (ICoh) evoked by moving gratings. In the patient MS, this ICoh response was present at 7:11 y and was more mature at 9:2 y. In the more visually impaired patient, FJ, a consistent increase in ICoh to visual stimuli could not be obtained, possibly because of the later occurrence of the lesion.

EEG coherence studies in the normal brain and after early-onset cortical pathologies.

Visual corpus callosum (CC) preferentially interconnects neurons selective for similar stimulus orientation near the representations of the vertical meridian. These properties allow studying the CC functionality with EEG coherence analysis. Iso-oriented and orthogonally-oriented gratings were presented to the two hemifields, either close to the vertical meridian or far from it. In animals with intact CC, and in man, interhemispheric coherence (ICoh) increased only with iso-oriented gratings presented near or crossing the vertical meridian. The increase was localized to occipital electrodes and was specific for the beta-gamma frequency band. Visual-stimulus induced changes in ICoh were studied in patients with early pathologies of the visual areas. From a girl with abnormal vision and severe bilateral lesion of the primary visual areas at 3 weeks, after premature birth at 30 weeks, we obtained no ICoh response until 9 years. In control children visual stimulation increased occipital ICoh at 6-7 years. From a young man having suffered similar lesions when he was 9 months older than the girl, no consistent increase in ICoh could be obtained. In a 14-year-old girl with congenital visual agnosia, no visible lesions, but with a temporal-occipital epileptic focus, ICoh responses were evoked both by iso-oriented, and by orthogonally-oriented gratings. In a young man with bilateral parieto-occipital microgyria extending into the calcarine sulcus, visual stimuli increased ICoh as in normal individuals, but the response was weaker. These cases are discussed in terms of development of CC connections and point to a variety of plastic changes in the cortical connectivity of children.

Functional activation of microgyric visual cortex in a human.

We report on the case of a 20-year-old man with bilateral parasagittal parieto-occipital polymicrogyria and epilepsy. Functional magnetic resonance imaging responses to reversing checkerboard and interhemispheric electroencephalogram coherence changes to moving gratings were investigated. Results of both studies indicate that the polymicrogyric cortex was activated by visual stimuli, suggesting preserved function in the dysplastic area.

Visual stimulus-dependent changes in interhemispheric EEG coherence in ferrets.

In recent years, the analysis of the coherence between signals recorded from the scalp [electroencephalographic (EEG) coherence] has been used to assess the functional properties of cortico-cortical connections, both in animal models and in humans. However, the experimental validation of this technique is still scarce. Therefore we applied it to the study of the callosal connections between the visual areas of the two hemispheres, because this particular set of cortico-cortical connections can be activated in a selective way by visual stimuli. Indeed, in primary and in low-order secondary visual areas, callosal axons interconnect selectively regions, which represent a narrow portion of the visual field straddling the vertical meridian and, within these regions, neurons that prefer the same stimulus orientation. Thus only isooriented stimuli located near the vertical meridian are expected to change interhemispheric coherence by activating callosal connections. Finally, if such changes are found and are indeed mediated by callosal connections, they should disappear after transection of the corpus callosum. We perfomed experiments on seven paralyzed and anesthetized ferrets, recording their cortical activity with epidural electrodes on areas 17/18, 19, and lateral suprasylvian, during different forms of visual stimulation. As expected, we found that bilateral iso-oriented stimuli near the vertical meridian, or extending across it, caused a significant increase in interhemispheric coherence in the EEG beta-gamma band. Stimuli with different orientations, stimuli located far from the vertical meridian, as well as unilateral stimuli failed to affect interhemispheric EEG coherence. The stimulus-induced increase in coherence disappeared after surgical transection of the corpus callosum. The results suggest that the activation of cortico-cortical connections can indeed be revealed as a change in EEG coherence. The latter can therefore be validly used to investigate the functionality of cortico-cortical connections.

Visual stimulus-dependent changes in interhemispheric EEG coherence in humans.

We analyzed the coherence of electroencephalographic (EEG) signals recorded symmetrically from the two hemispheres, while subjects (n = 9) were viewing visual stimuli. Considering the many common features of the callosal connectivity in mammals, we expected that, as in our animal studies, interhemispheric coherence (ICoh) would increase only with bilateral iso-oriented gratings located close to the vertical meridian of the visual field, or extending across it. Indeed, a single grating that extended across the vertical meridian significantly increased the EEG ICoh in normal adult subjects. These ICoh responses were obtained from occipital and parietal derivations and were restricted to the gamma frequency band. They were detectable with different EEG references and were robust across and within subjects. Other unilateral and bilateral stimuli, including identical gratings that were effective in anesthetized animals, did not affect ICoh in humans. This fact suggests the existence of regulatory influences, possibly of a top-down kind, on the pattern of callosal activation in conscious human subjects. In addition to establishing the validity of EEG coherence analysis for assaying cortico-cortical connectivity, this study extends to the human brain the finding that visual stimuli cause interhemispheric synchronization, particularly in frequencies of the gamma band. It also indicates that the synchronization is carried out by cortico-cortical connection and suggests similarities in the organization of visual callosal connections in animals and in man.

The role of pattern vision in the development of cortico-cortical connections.

The development of cortico-cortical connections was studied in kittens deprived of vision by binocular eyelid suture during the formation of axonal arbors and synaptogenesis, i.e. between the second postnatal week and the end of the third postnatal month. Axons originating in area 17 and terminating either in ipsilateral or contralateral visual areas were visualized with biocytin. In ipsilateral areas 17 and 18, distinct clusters of branches begin to form, distally from the injection, during the second half of the first postnatal month, independently of pattern vision. More proximal clusters differentiate during the second postnatal month, and this seems to involve elimination of exuberant axonal branches. In kittens deprived of vision for 3 or more months, beginning before natural eye opening, the distal clusters regress and the proximal ones fail to differentiate. In extrastriate areas, distinct clusters of branches have segregated by the end of the second postnatal month, independently of visual experience; however, in kittens deprived of vision for 2 or more months, one of the clusters was selectively eliminated. In contralateral areas 17 and 18, we found stunted terminal arbors in kittens continuously deprived of vision. This was already noticeable at the end of the first postnatal month. Apparently, in the absence of pattern vision, most axons undergo only limited growth and do not form their characteristic terminal columns. Many of these axons are subsequently eliminated. In contrast, 8 days of vision beginning at natural eye opening and followed by visual deprivation caused a nearly normal development of intrahemispheric and interhemispheric connections. In conclusion, pattern vision appears to validate connections at early stages of their development; this validation is necessary for their further growth and differentiation that can then continue autonomously.

Typology, early differentiation, and exuberant growth of a set of cortical axons.

The corpus callosum interconnects both corresponding (homotopic) and noncorresponding (heterotopic) cortical sites of the two hemispheres. We have studied the axons that establish heterotopic connections from visual areas 17 and 18 (E axons) by using anterogradely transported biocytin and three-dimensional serial reconstructions in adult cats and in kittens. Their site of termination distinguished four types of axons. Type EI ends near the border between areas 19/21a or 7, and type EII near the PMLS/PLLS border (posteromedial and posterolateral lateral suprasylvian areas). Type EIII and EIV terminate the first near the PMLS/PLLS and PMLS/21a borders, and the second near the PMLS/PLLS and 19/21a or 7 borders. Taking into account the previously studied homotopic axons (O axons; Houzel et al. [1994] Eur. J. Neurosci. 6:898-917), it can be concluded that areas 17 and 18 are interhemispherically connected by at least five types of axons, three of which (O, EI, and EII) terminate near one areal border, the other two (types EIII and EIV), near two areal borders. All types terminate near representations of the vertical meridian of the visual field. The different types of axons can be identified already during the first postnatal week; at this age, unlike in the adult, they originate not only near the 17/18 border, but also, transiently, in area 17. This suggests that the developing cortex contains sets of neurons destined to send their axon to different targets; however, the axons grow beyond their sites of adult termination. Indeed, exuberant growth takes place at the stage of axonal elongation, and at subsequent stages of axonal differentiation, i.e., during subcortical branching, intracortical branching and synaptogenesis. The growth is progressively more constrained in its topographic distribution and the axons are subsequently reshaped by regressive events.

On nature and limits of cortical developmental plasticity after an early lesion, in a child.

MS is a little girl who suffered severe, bilateral destruction of her primary visual areas at six weeks, after premature birth at 30 weeks. Between the ages of 4.5 and 5.5 years she partially recovered different aspects of visual function, and, in particular, the ability to segregate figures from background, based on texture cues. The recovery might have been due to the compensatory role of the remaining visual areas that could have acquired response properties similar to those of the primary visual areas. This is not supported by the available FMRI (functional magnetic resonance imaging) responses to visual stimuli. Instead, abnormalities in the pattern of stimulus-induced changes of interhemi-spheric EEG-coherence in this patient suggest that her visual callosal connections, and possibly other cortico-cortical connections have re-organized abnormally. Since cortico-cortical connections, including the callosal ones appear to be involved in perceptual binding and figure-background segregation, their reorganization could be an important element in the functional recovery after early lesion, and/or in the residual perceptual impairment.

Constant and variable aspects of axonal phenotype in cerebral cortex.

In order to determine to what extent the terminal arbors of phylogenetically and functionally distant axons are constructed according to common rules, we have compared visual callosal axons in cats (CCC axons) with thalamocortical axons to the whisker representation in mice (MTC axons). Both similarities and differences were found. Maximal order of branching, branching angles, topological distribution of branches and boutons are similar for all axons, indicating strong constraints in arbor formation. CCC and MTC axons are indistinguishable for total arbor length and number of branches, although these parameters can vary across individual axons of each group. MTC axons have longer and bouton-richer end-branches (the 'transmission compartment') while, in CCC axons, proximal, boutonless branches (the 'conduction compartment') predominate. Therefore, the two classes of axons appear to be specialized for performing different types of operations, in agreement with the available electrophysiological data and computer simulations. Differences in the length of branches were also observed between MTC axons of normal and 'barrelless' mice, suggesting that this parameter can be regulated by conditions at the terminal sites.

The development of auditory callosal connections in normal and hypothyroid rats.

Previous studies have shown that hypothyroidism modifies the development of callosal connections. In particular, adult hypothyroid rats have fewer callosally projecting neurons in layers II-III of the auditory cortex and more in layer V. This might be due to disturbance in the stabilization/elimination of juvenile callosal axons, or to abnormal neuronal migration during cortical histogenesis. To distinguish between these possibilities we have studied the distribution of callosally projecting auditory neurons at different postnatal ages using retrogradely transported tracers, and the cortical neurogenetic gradients using DNA labelling with 5-bromo-2'-deoxiuridine. In hypothyroid rats, injected at postnatal day 5 (P5) and killed at P18-20, most of the neurons retrogradely labelled from the contralateral hemisphere are distributed between layers IV and VI, as in older rats. In hypothyroid rats, many neurons are at locations inappropriate for their birthdate, including the subcortical white matter, resulting in more diffuse radial neurogenetic gradients. These results indicate that early induced hypothyroidism alters neuronal migration and prevents the establishment of callosal connections from cortical layers II-III.

Maxsim, software for the analysis of multiple axonal arbors and their simulated activation.

In order to analyze the structural organization of complex axonal arbors reconstructed from histological serial sections, and to investigate the functional implications of their geometrical properties, we developed software providing the following facilities: (1) direct importation of data files generated by a commercially available 3-D light-microscopic reconstruction system, including routine procedures for identification and correction of data acquisition errors; (2) real-time 3-D rotations of the arbors in the stack of serial sections; (3) multiple interactive display modes; (4) possibility of modifying diameter and/or connectivity of different branches; (5) simulation of the invasion of the arbor by a single action potential initiated at any chosen point, and visualization of spatio-temporal profiles of activation; (6) extraction of quantitative data converted to standard file formats compatible with available mathematical software. All these tools can be applied to single or multiple axons, individually or simultaneously. The software, called Maxsim, is a highly flexible C-written program running on graphical workstations using the UNIX operating system and X-Window environment.

Growth of callosal terminal arbors in primary visual areas of the cat.

In kittens ranging in age between postnatal day (P) 5 and P150, callosal axons originating near the 17/18 border were anterogradely labelled with biocytin and reconstructed from serial sections. At the end of the first postnatal week most of the axons begin to invade the cortex near the 17/18 border with multiple branches; some axons already span the grey matter up to layer 1. Branches tend to grow into the grey matter in loose bundles

Exuberant development of connections, and its possible permissive role in cortical evolution.

The callosal visual connections of the cat provide a model for studying the phenotypes of cortical axons and their differentiation. The terminal arbor of a callosal axon develops in several successive stages. At each stage, the arbor approximates the adult phenotype more closely. This is achieved through two mechanisms: (1) exuberant, but increasingly constrained, growth and (2) partial deletion of previously generated parts of the arbor. This differentiation is controlled by interactions of the axon with its cellular environment, and by visual experience. It might have played a permissive role in the evolution of the cerebral cortex by enabling adjustments of cortical connectivity to changes in the number, size, internal organization and cellular composition of cortical areas.

Cellular aspects of callosal connections and their development.

Detailed visualization, three-dimensional reconstruction, and quantification of individual callosal axons interconnecting the visual areas 17 and 18 of the cat was undertaken in order to clarify the structural basis for interhemispheric interaction. These studies have generated the notion of macro- vs micro-organization of callosal connections. The first refers to the global distribution of callosal connections in the hemisphere as well as to the pattern of area-to-area connections. The latter refers to the fine radial and tangential distributions of individual callosal axons. A discrete disjunctive, 'columnar' pattern of termination of callosal axons, previously unknown for the visual areas, was found. The consequence of caliber and distribution of callosal axons and their branches on the dynamic properties of interhemispheric interactions were analyzed by computer simulations. These studies suggested that callosal axons could synchronize activity within and between the hemispheres in ways relevant for the 'binding' of perceptual features. These new concepts prompted a reexamination of the normal development of callosal connections. The central issue is whether intrinsic developmental programs, or else cellular interactions open to environmental information specify the morphological substrate of interhemispheric interactions. The answer to this question is still incomplete. In development, transient, widespread arbors of callosal axons, which could provide the basis for plastic changes of callosal connections were found in the white matter and the deep cortical layers. On the other hand, growth into the cortex and synaptogenesis of callosal axons appear to be highly, topographically specific albeit not necessarily independent of visual experience.

Juvenile visual callosal axons in kittens display origin- and fate-related morphology and distribution of arbors.

In kittens, callosal axons originating either from medial area 17 (transient axons) or near the 17/18 border (mostly permanent axons) were labelled with anterogradely transported biocytin; they were reconstructed by computer from serial sections, and their morphologies compared at different ages. During the first and second postnatal weeks both sets of axons branched profusely in the white matter of the lateral gyrus and the number of branches increased with age. The most common type of axon ending was the growth cone; others may have been collapsing growth cones, branches in the process of elimination or early synaptic boutons. Axons from medial area 17 distributed over a broad territory, including the 17/18 border where callosal axons terminate in the adult cat, but without aiming specifically at any one area. The majority of axons and their branches terminated in the white matter or at the bottom of layer VI; exceptionally they extended further into the cortex. Most of the axons originating near the 17/18 border were different from those described above, and the difference increased with age. Although they also terminated profusely in the white matter of the lateral gyrus, most of the branches terminated near the contralateral 17/18 border; they frequently entered the grey matter up to the superficial layers and branched into it. During the third week, axons from medial area 17 were rarely found to extend beyond the corpus callosum, probably because they were in the process of being eliminated. In contrast, axons originating near the 17/18 border had increased their number of branches in the grey matter. In conclusion, during the first and second postnatal weeks axons grew and differentiated according to their origin, and this anticipated whether they would be maintained or eliminated. Neurotrophic signals, possibly from the white matter or the subplate, and growth-inhibiting signals from area 17 may be involved in this process.