Possible functions of the interhemispheric connexions between visual cortical areas in the cat

A novel interface for cortical columnar neuromodulation with multipoint infrared neural stimulation

Cutting edge advances in electrical visual cortical prosthetics have evoked perception of shapes, motion, and letters in the blind. Here, we present an alternative optical approach using pulsed infrared neural stimulation. To interface with dense arrays of cortical columns with submillimeter spatial precision, both linear array and 100-fiber bundle array optical fiber interfaces were devised. We deliver infrared stimulation through these arrays in anesthetized cat visual cortex and monitor effects by optical imaging in contralateral visual cortex. Infrared neural stimulation modulation of response to ongoing visual oriented gratings produce enhanced responses in orientation-matched domains and suppressed responses in non-matched domains, consistent with a known higher order integration mediated by callosal inputs. Controls include dynamically applied speeds, directions and patterns of multipoint stimulation. This provides groundwork for a distinct type of prosthetic targeted to maps of visual cortical columns.

Developmental refinement of visual callosal inputs to ferret area 17

Corticocortical connections link visual cortical areas in both the ipsilateral and contralateral hemispheres. We studied the postnatal refinement of callosal connections linking multiple cortical areas with ferret area 17 during the period from just before eye opening (4 weeks) to 10 weeks of age. We aimed to determine (1) whether callosal projections from multiple visual cortical areas to area 17 refine with a similar rate and (2) whether the refinement of callosal projections parallels that of intrahemispheric cortical circuits. We injected the bidirectional tracer CTb into area 17, and mapped the areal and laminar distribution of labeled cells in visual areas of the contralateral hemisphere. Like intrahemispheric projections, callosal inputs to area 17 before eye opening are dominated by Suprasylvian area Ssy (with lesser and comparable input from areas 17, 18, 19, and 21), but within 2 weeks of eye opening are jointly dominated by area 18 and Ssy inputs; however, there are fewer labeled cells in the contralateral hemisphere. Unlike intrahemispheric projections, there is no laminar reorganization of callosal inputs; in all visual areas and at all ages studied, the greatest proportion of callosal projections arises from the infragranular layers. Also, unlike intrahemispheric projections, the peak density of callosal cells in each area projecting to area 17 declines more modestly. These results reveal important similarities and differences in the postnatal reorganization of inter- and intrahemispheric projections to area 17.

Impact of visual callosal pathway is dependent upon ipsilateral thalamus

The visual callosal pathway, which reciprocally connects the primary visual cortices, is thought to play a pivotal role in cortical binocular processing. In rodents, the functional role of this pathway is largely unknown. Here, we measure visual cortex spiking responses to visual stimulation using population calcium imaging and functionally isolate visual pathways originating from either eye. We show that callosal pathway inhibition significantly reduced spiking responses in binocular and monocular neurons and abolished spiking in many cases. However, once isolated by blocking ipsilateral visual thalamus, callosal pathway activation alone is not sufficient to drive evoked cortical responses. We show that the visual callosal pathway relays activity from both eyes via both ipsilateral and contralateral visual pathways to monocular and binocular neurons and works in concert with ipsilateral thalamus in generating stimulus evoked activity. This shows a much greater role of the rodent callosal pathway in cortical processing than previously thought.

The visual callosal pathway reciprocally connects mammalian visual cortices and is proposed to facilitate activation of binocular neurons. Here, the authors show that this pathway facilitates responses in both monocular and binocular neurons but these responses are gated by the ipsilateral lateral geniculate nucleus.

Callosal Influence on Visual Receptive Fields Has an Ocular, an Orientation-and Direction Bias

One leading hypothesis on the nature of visual callosal connections (CC) is that they replicate features of intrahemispheric lateral connections. However, CC act also in the central part of the binocular visual field. In agreement, early experiments in cats indicated that they provide the ipsilateral eye part of binocular receptive fields (RFs) at the vertical midline (Berlucchi and Rizzolatti, 1968), and play a key role in stereoscopic function. But until today callosal inputs to receptive fields activated by one or both eyes were never compared simultaneously, because callosal function has been often studied by cutting or lesioning either corpus callosum or optic chiasm not allowing such a comparison. To investigate the functional contribution of CC in the intact cat visual system we recorded both monocular and binocular neuronal spiking responses and receptive fields in the 17/18 transition zone during reversible deactivation of the contralateral hemisphere. Unexpectedly from many of the previous reports, we observe no change in ocular dominance during CC deactivation. Throughout the transition zone, a majority of RFs shrink, but several also increase in size. RFs are significantly more affected for ipsi- as opposed to contralateral stimulation, but changes are also observed with binocular stimulation. Noteworthy, RF shrinkages are tiny and not correlated to the profound decreases of monocular and binocular firing rates. They depend more on orientation and direction preference than on eccentricity or ocular dominance of the receiving neuron's RF. Our findings confirm that in binocularly viewing mammals, binocular RFs near the midline are constructed via the direct geniculo-cortical pathway. They also support the idea that input from the two eyes complement each other through CC: Rather than linking parts of RFs separated by the vertical meridian, CC convey a modulatory influence, reflecting the feature selectivity of lateral circuits, with a strong cardinal bias.

Reorganization of Visual Callosal Connections Following Alterations of Retinal Input and Brain Damage

Vision is a very important sensory modality in humans. Visual disorders are numerous and arising from diverse and complex causes. Deficits in visual function are highly disabling from a social point of view and in addition cause a considerable economic burden. For all these reasons there is an intense effort by the scientific community to gather knowledge on visual deficit mechanisms and to find possible new strategies for recovery and treatment. In this review, we focus on an important and sometimes neglected player of the visual function, the corpus callosum (CC). The CC is the major white matter structure in the brain and is involved in information processing between the two hemispheres. In particular, visual callosal connections interconnect homologous areas of visual cortices, binding together the two halves of the visual field. This interhemispheric communication plays a significant role in visual cortical output. Here, we will first review the essential literature on the physiology of the callosal connections in normal vision. The available data support the view that the callosum contributes to both excitation and inhibition to the target hemisphere, with a dynamic adaptation to the strength of the incoming visual input. Next, we will focus on data showing how callosal connections may sense visual alterations and respond to the classical paradigm for the study of visual plasticity, i.e., monocular deprivation (MD). This is a prototypical example of a model for the study of callosal plasticity in pathological conditions (e.g., strabismus and amblyopia) characterized by unbalanced input from the two eyes. We will also discuss the findings of callosal alterations in blind subjects. Noteworthy, we will discuss data showing that inter-hemispheric transfer mediates recovery of visual responsiveness following cortical damage. Finally, we will provide an overview of how callosal projections dysfunction could contribute to pathologies such as neglect and occipital epilepsy. A particular focus will be on reviewing noninvasive brain stimulation techniques and optogenetic approaches that allow to selectively manipulate callosal function and to probe its involvement in cortical processing and plasticity. Overall, the data indicate that experience can potently impact on transcallosal connectivity, and that the callosum itself is crucial for plasticity and recovery in various disorders of the visual pathway.

Effects of core auditory cortex deactivation on neuronal response to simple and complex acoustic signals in the contralateral anterior auditory field

Interhemispheric communication has been implicated in various functions of sensory signal processing and perception. Despite ample evidence demonstrating this phenomenon in the visual and somatosensory systems, to date, limited functional assessment of transcallosal transmission during periods of acoustic signal exposure has hindered our understanding of the role of interhemispheric connections between auditory cortical fields. Consequently, the present investigation examines the impact of core auditory cortical field deactivation on response properties of contralateral anterior auditory field (AAF) neurons in the felis catus. Single-unit responses to simple and complex acoustic signals were measured across AAF before, during, and after individual and combined cooling deactivation of contralateral primary auditory cortex (A1) and AAF neurons. Data analyses revealed that on average: 1) interhemispheric projections from core auditory areas to contralateral AAF neurons are predominantly excitatory, 2) changes in response strength vary based on acoustic features, 3) A1 and AAF projections can modulate AAF activity differently, 4) decreases in response strength are not specific to particular cortical laminae, and 5) contralateral inputs modulate AAF neuronal response thresholds. Collectively, these observations demonstrate that A1 and AAF neurons predominantly modulate AAF response properties via excitatory projections.

The visual callosal connection: a connection like any other?

Recent work about the role of visual callosal connections in ferrets and cats is reviewed, and morphological and functional homologies between the lateral intrinsic and callosal network in early visual areas are discussed. Both networks selectively link distributed neuronal groups with similar response properties, and the actions exerted by callosal input reflect the functional topography of those networks. This supports the notion that callosal connections perpetuate the function of the lateral intrahemispheric circuit onto the other hemisphere. Reversible deactivation studies indicate that the main action of visual callosal input is a multiplicative shift of responses rather than a changing response selectivity. Both the gain of that action and its excitatory-inhibitory balance seem to be dynamically adapted to the feedforward drive by the visual stimulus onto primary visual cortex. Taken together anatomical and functional evidence from corticocortical and lateral circuits further leads to the conclusion that visual callosal connections share more features with lateral intrahemispheric connections on the same hierarchical level and less with feedback connections. I propose that experimental results about the callosal circuit in early visual areas can be interpreted with respect to lateral connectivity in general.

Activity-dependent callosal axon projections in neonatal mouse cerebral cortex

Callosal axon projections are among the major long-range axonal projections in the mammalian brain. They are formed during the prenatal and early postnatal periods in the mouse, and their development relies on both activity-independent and -dependent mechanisms. In this paper, we review recent findings about the roles of neuronal activity in callosal axon projections. In addition to the well-documented role of sensory-driven neuronal activity, recent studies using in utero electroporation demonstrated an essential role of spontaneous neuronal activity generated in neonatal cortical circuits. Both presynaptic and postsynaptic neuronal activities are critically involved in the axon development. Studies have begun to reveal intracellular signaling pathway which works downstream of neuronal activity. We also review several distinct patterns of neuronal activity observed in the developing cerebral cortex, which might play roles in activity-dependent circuit construction. Such neuronal activity during the neonatal period can be disrupted by genetic factors, such as mutations in ion channels. It has been speculated that abnormal activity caused by such factors may affect activity-dependent circuit construction, leading to some developmental disorders. We discuss a possibility that genetic mutation in ion channels may impair callosal axon projections through an activity-dependent mechanism.

Cortical and subcortical connections of V1 and V2 in early postnatal macaque monkeys

Connections of primary (V1) and secondary (V2) visual areas were revealed in macaque monkeys ranging in age from 2 to 16 weeks by injecting small amounts of cholera toxin subunit B (CTB). Cortex was flattened and cut parallel to the surface to reveal injection sites, patterns of labeled cells, and patterns of cytochrome oxidase (CO) staining. Projections from the lateral geniculate nucleus and pulvinar to V1 were present at 4 weeks of age, as were pulvinar projections to thin and thick CO stripes in V2. Injections into V1 in 4- and 8-week-old monkeys labeled neurons in V2, V3, middle temporal area (MT), and dorsolateral area (DL)/V4. Within V1 and V2, labeled neurons were densely distributed around the injection sites, but formed patches at distances away from injection sites. Injections into V2 labeled neurons in V1, V3, DL/V4, and MT of monkeys 2-, 4-, and 8-weeks of age. Injections in thin stripes of V2 preferentially labeled neurons in other V2 thin stripes and neurons in the CO blob regions of V1. A likely thick stripe injection in V2 at 4 weeks of age labeled neurons around blobs. Most labeled neurons in V1 were in superficial cortical layers after V2 injections, and in deep layers of other areas. Although these features of adult V1 and V2 connectivity were in place as early as 2 postnatal weeks, labeled cells in V1 and V2 became more restricted to preferred CO compartments after 2 weeks of age.

The corpus callosum and the visual cortex: plasticity is a game for two

Throughout life, experience shapes and selects the most appropriate brain functional connectivity to adapt to a changing environment. An ideal system to study experience-dependent plasticity is the visual cortex, because visual experience can be easily manipulated. In this paper, we focus on the role of interhemispheric, transcallosal projections in experience-dependent plasticity of the visual cortex. We review data showing that deprivation of sensory experience can modify the morphology of callosal fibres, thus altering the communication between the two hemispheres. More importantly, manipulation of callosal input activity during an early critical period alters developmental maturation of functional properties in visual cortex and modifies its ability to remodel in response to experience. We also discuss recent data in rat visual cortex, demonstrating that the corpus callosum plays a role in binocularity of cortical neurons and is involved in the plastic shift of eye preference that follows a period of monocular eyelid suture (monocular deprivation) in early age. Thus, experience can modify the fine connectivity of the corpus callosum, and callosal connections represent a major pathway through which experience can mediate functional maturation and plastic rearrangements in the visual cortex.

Layer- and cell-type-specific subthreshold and suprathreshold effects of long-term monocular deprivation in rat visual cortex

Connectivity and dendritic properties are determinants of plasticity that are layer and cell-type specific in the neocortex. However, the impact of experience-dependent plasticity at the level of synaptic inputs and spike outputs remains unclear along vertical cortical microcircuits. Here I compared subthreshold and suprathreshold sensitivity to prolonged monocular deprivation (MD) in rat binocular visual cortex in layer 4 and layer 2/3 pyramids (4Ps and 2/3Ps) and in thick-tufted and nontufted layer 5 pyramids (5TPs and 5NPs), which innervate different extracortical targets. In normal rats, 5TPs and 2/3Ps are the most binocular in terms of synaptic inputs, and 5NPs are the least. Spike responses of all 5TPs were highly binocular, whereas those of 2/3Ps were dominated by either the contralateral or ipsilateral eye. MD dramatically shifted the ocular preference of 2/3Ps and 4Ps, mostly by depressing deprived-eye inputs. Plasticity was profoundly different in layer 5. The subthreshold ocular preference shift was sevenfold smaller in 5TPs because of smaller depression of deprived inputs combined with a generalized loss of responsiveness, and was undetectable in 5NPs. Despite their modest ocular dominance change, spike responses of 5TPs consistently lost their typically high binocularity during MD. The comparison of MD effects on 2/3Ps and 5TPs, the main affected output cells of vertical microcircuits, indicated that subthreshold plasticity is not uniquely determined by the initial degree of input binocularity. The data raise the question of whether 5TPs are driven solely by 2/3Ps during MD. The different suprathreshold plasticity of the two cell populations could underlie distinct functional deficits in amblyopia.

Transcallosal inhibition dampens neural responses to high contrast stimuli in human visual cortex

Visual cortical areas in the two hemispheres interact via the corpus callosum, but the precise role of the callosal pathway in visual processing remains controversial. Here we have investigated the function of transcallosal projections in human primary visual cortex (V1). Visual evoked potentials (VEPs) triggered by grating stimuli of different contrasts were recorded before and after functional inactivation of the occipital cortex of one hemisphere via off-line low-frequency repetitive transcranial magnetic stimulation (rTMS; 0.5 Hz stimulation for 20 min). VEPs were recorded in V1 before (T0), immediately after (T1) and 45' following the completion of rTMS (T2). We found that low-frequency rTMS had an inhibitory effect on VEPs amplitudes at all contrasts in the treated side. Remarkably, reduction of VEP amplitudes in the inhibited hemisphere at T1 was accompanied by an increase in VEP amplitudes in the contralateral side only at mid-high contrasts (50-90%). This disinhibitory effect was observed with both central and hemifield stimulation. No changes in VEP amplitudes were observed when rTMS was applied to a cortical site more anterior with respect to V1. These data provide the first evidence that a mechanism of transcallosal inhibition dampens neural responses at high contrasts in human visual cortex.

Callosal contribution to ocular dominance in rat primary visual cortex

Ocular dominance (OD) plasticity triggered by monocular eyelid suture is a classic paradigm for studying experience-dependent changes in neural connectivity. Recently, rodents have become the most popular model for studies of OD plasticity. It is therefore important to determine how OD is determined in the rodent primary visual cortex. In particular, cortical cells receive considerable inputs from the contralateral hemisphere via callosal axons, but the role of these connections in controlling eye preference remains controversial. Here we have examined the role of callosal connections in binocularity of the visual cortex in naïve young rats. We recorded cortical responses evoked by stimulation of each eye before and after acute silencing, via stereotaxic tetrodotoxin (TTX) injection, of the lateral geniculate nucleus ipsilateral to the recording site. This protocol allowed us to isolate visual responses transmitted via the corpus callosum. Cortical binocularity was assessed by visual evoked potential (VEP) and single-unit recordings. We found that acute silencing of afferent geniculocortical input produced a very significant reduction in the contralateral-to-ipsilateral (C/I) VEP ratio, and a marked shift towards the ipsilateral eye in the OD distribution of cortical cells. Analysis of absolute strength of each eye indicated a dramatic decrease in contralateral eye responses following TTX, while those of the ipsilateral eye were reduced but maintained a more evident input. We conclude that callosal connections contribute to normal OD mainly by carrying visual input from the ipsilateral eye. These data have important implications for the interpretation of OD plasticity following alterations of visual experience.

Specificity of neuronal responses in primary visual cortex is modulated by interhemispheric corticocortical input

Within the visual cortex, it has been proposed that interhemispheric interactions serve to re-establish the continuity of the visual field across its vertical meridian (VM) by mechanisms similar to those used by intrinsic connections within a hemisphere. However, other specific functions of transcallosal projections have also been proposed, including contributing to disparity tuning and depth perception. Here, we consider whether interhemispheric connections modulate specific response properties, orientation and direction selectivity, of neurons in areas 17 and 18 of the ferret by combining reversible thermal deactivation in one hemisphere with optical imaging of intrinsic signals and single-cell electrophysiology in the other hemisphere. We found interhemispheric influences on both the strength and specificity of the responses to stimulus orientation and direction of motion, predominantly at the VM. However, neurons and domains preferring cardinal contours, in particular vertical contours, seem to receive stronger interhemispheric input than others. This finding is compatible with interhemispheric connections being involved in horizontal disparity tuning. In conclusion, our results support the view that interhemispheric interactions mainly perform integrative functions similar to those of connections intrinsic to one hemisphere.

Selective potentiation of crossed vs. uncrossed inputs from lateral geniculate nucleus to visual cortex by the basal forebrain: potential facilitation of rodent binocularity

Cholinergic projections originating in the basal forebrain (BF) play important roles in the heterosynaptic facilitation of synaptic strength in various sensory cortices, including the primary visual cortex (V1). Here, using urethane-anesthetized rats, we find that pairing burst stimulation of the BF with single pulse stimulation of the lateral geniculate nucleus (LGN) does not consistently increase field postsynaptic potentials (fPSPs) in V1 elicited by ipsilateral LGN stimulation. However, longer latency fPSPs recorded in V1 in response to stimulation of the contralateral LGN, reflecting crossed, polysynaptic inputs, show significant potentiation when paired with preceding BF stimulation. This synaptic enhancement requires relatively short time intervals between paired BF burst and LGN pulse stimulation (40 ms) and is abolished by systemic or local V1 muscarinic receptor blockade (scopolamine), while systemic nicotinic receptor blockade (mecamylamine) is ineffective. Together, these data provide evidence for a differential capacity for cholinergic/muscarinic-dependent plasticity induction among different signals in V1, with inputs reaching V1 from the contralateral LGN exhibiting potentiation in the face of stable strength in ipsilateral LGN-V1 projections. This preferential readiness for potentiation in crossed fiber systems could serve to amplify binocular responses in V1 elicited by synchronized excitation of ipsi- and contralateral LGN neurons.

Interhemisphere connections of eye dominance columns in the cat visual cortex in conditions of impaired binocular vision

Data from studies of interhemisphere connections in fields 17 and 18 of cats reared in conditions of impaired binocular vision (monocular deprivation, uni- and bilateral strabismus) are presented. Monosynaptic connections between neurons were studied by microiontophoretic application of horseradish peroxidase into cortical eye dominance columns and the distributions of retrograde labeled callosal cells were analyzed. Spatial asymmetry and eye-specific interhemisphere neuron connections persisted in conditions of monocular deprivation and strabismus. Quantitative changes in connections were less marked in monocular deprivation than strabismus. In cats with impaired binocular vision, as in intact animals, the widths of callosal-receiving zones were greater than the widths of the callosal cell zones, which is evidence for the non-reciprocity of interhemisphere connections in cortical areas distant from the projection of the vertical meridian. Morphofunctional differences between cells mediating connections in the opposite directions are proposed.

'Top-down' influences of ipsilateral or contralateral postero-temporal visual cortices on the extra-classical receptive fields of neurons in cat's striate cortex

In anesthetized and immobilized domestic cats, we have studied the effects of brief reversible inactivation (by cooling to 10 degrees C) of the ipsilateral or contralateral postero-temporal visual (PTV) cortices on: 1) the magnitude of spike-responses of neurons in striate cortex (cytoarchitectonic area 17, area V1) to optimized sine-wave modulated contrast-luminosity gratings confined to the classical receptive fields (CRFs) and 2) the relative strengths of modulation of CRF-induced spike-responses by gratings extending into the extra-classical receptive field (ECRF). Consistent with our previous reports (Bardy et al., 2006; Huang et al., 2007), inactivation of ipsilateral PTV cortex (presumed homologue of primate infero-temporal cortex) resulted in significant reversible changes (almost all substantial reductions) in the magnitude of spike-responses to CRF-confined stimuli in about half of the V1 neurones. Similarly, in half of the present sample, inactivation of ipsilateral PTV cortex resulted in significant reversible changes (in over 70% of cases, reduction) in the relative strength of ECRF modulation of the CRF-induced spike-responses. By contrast, despite the fact that receptive fields of all V1 cells tested were located within 5 degrees of representation of the zero vertical meridian, inactivation of contralateral PTV cortex only rarely resulted in significant (yet invariably small) changes in the magnitude of spike-responses to CRF-confined stimuli or significant (again invariably small) changes in the relative strength of ECRF modulation of spike-responses. Thus, the ipsilateral, but not contralateral, 'higher-order' visual cortical areas make significant contribution not only to the magnitude of CRF-induced spike-responses but also to the relative strengths of ECRF-induced modulation of the spike-responses of V1 neurons. Therefore, the feedback signals originating from the ipsilateral higher-order cortical areas appear to make an important contribution to contextual modulation of responses of neurons in the primary visual cortices.

Activity-dependent development of interhemispheric connections in the visual cortex

Interhemispheric axon fibers connect the two cerebral cortical hemispheres via the corpus callosum and function to integrate information between the hemispheres. In the development of callosal connections, an early phase involves axon guidance molecules and a later phase requires neuronal activity. In addition to the well-studied role of sensory-driven neuronal activity, recent studies have demonstrated an essential role of callosal neuron firing activity in forming axonal projections and dendritic maturation during the developmental period before sensory input is available. Results suggest that factors affecting the cellular excitability of developing callosal neurons can influence the establishment of interhemispheric connections. Possible synaptic and non-synaptic mechanisms for activity-dependent axonal projections are discussed.

Evidence for activity-dependent cortical wiring: formation of interhemispheric connections in neonatal mouse visual cortex requires projection neuron activity

Neuronal activity plays a pivotal role in shaping neuronal wiring. We investigated the role of neuronal activity in the formation of interhemispheric (callosal) axon projections in neonatal mouse visual cortex. Axonal labeling with enhanced green fluorescent protein (GFP) was used to demonstrate spatially organized pattern of callosal projections: GFP-labeled callosal axons from one hemisphere projected densely to a narrowly restricted region at the border between areas 17 and 18 in the contralateral hemisphere, in which they terminated in layers 1-3 and 5. This region- and layer-specific innervation pattern developed by postnatal day 15 (P15). To explore the role of neuronal activity of presynaptic and postsynaptic neurons in callosal connection development, an inwardly rectifying potassium channel, Kir2.1, was expressed in callosal projection neurons and their target postsynaptic neurons. Kir2.1 overexpression reduced the firing rate of cortical neurons. Kir2.1 overexpression in callosal projection neurons disturbed the growth of axons and their arbors that normally occurs between P7 and P13, whereas that in postsynaptic neurons had limited effect on the pattern of presynaptic callosal axon innervation. In addition, exogenous expression of a gain-of-function Kir2.1 mutant channel found in patients with a familial heart disease caused severe deficits in callosal axon projections. These results suggest that projection neuron activity plays a crucial role in interhemispheric connection development and that enhanced Kir2.1 activity can affect cortical wiring.

Layout of transcallosal activity in cat visual cortex revealed by optical imaging

The contribution of interhemispheric connections to functional maps in cat visual cortex was investigated by using optical imaging of intrinsic signals. In order to isolate the functional inputs arriving via the corpus callosum (CC) from other inputs, we used the split-chiasm preparation. The regions activated through the CC in visual areas 17 (A17) and 18 (A18) were localized and characterized by stimulating monocularly split-chiasm cats with moving, high contrast oriented gratings. We found that the CC mediates the activation of orientation selective domains in the transition zone (TZ) between A17 and A18 and occasionally within portions of both of these areas. We observed transcallosally activated orientation domains all along the TZ without any obvious interruption, and these domains were arranged around "pinwheel" centers. Interestingly, the TZ was divided in two parallel regions, which resemble A17 and A18 in their preferred temporal and spatial frequencies. Finally, we demonstrated that orientation maps evoked through the transcallosal and geniculo-cortical pathways were similar within the TZ, indicating a convergence of inputs of matching orientations in this region. These results contribute to a better understanding of the role of the CC in visual perception of orientations and shapes, at the level of the visual cortex.

Transient synaptic silencing of developing striate cortex has persistent effects on visual function and plasticity

Neural circuits in the cerebral cortex are shaped by experience during "critical periods" early in life. For example, visual cortex is immature at the time of eye opening and gradually develops its functional properties during a sensitive period. Very few reports have addressed the role of intrinsic neural activity in cortical maturation. Here we have exploited the bacterial enzyme botulinum neurotoxin E (BoNT/E) to produce a unilateral, reversible blockade of neural activity in rat visual cortex during the sensitive period. BoNT/E is a highly selective protease that interferes with transmitter release via cleavage of the synaptic protein SNAP-25 (synaptosomal-associated protein of 25 kDa). Unilateral, intracortical injections of BoNT/E were made at the time of eye opening and resulted in the silencing of the treated, but not contralateral, hemisphere for a period of 2 weeks. We found that visual acuity was permanently reduced in the blocked hemisphere, and the critical period for ocular dominance plasticity persisted into adulthood. Unexpectedly, these effects extended equally to the contralateral, uninjected side, demonstrating a fundamental role for interhemispheric connections in cortical maturation.

Functional specificity of callosal connections in tree shrew striate cortex

Although callosal connections have been shown to link extensive regions of primary visual cortex, the distribution of these connections with respect to the map of visual space and the map of orientation preference remains unclear. Here we combine optical imaging of intrinsic signals with injection of fluorescent microspheres to assess the functional specificity of callosal connections in the tree shrew. By imaging both hemispheres simultaneously while presenting a series of spatially restricted stimuli, we find that a substantial region of visual space is represented bilaterally. Each hemisphere includes a representation of the ipsilateral visual field that is highly compressed relative to that of the contralateral visual field and is most extensive in the lower visual field, where approximately 30(o) of central visual space are represented bilaterally. Callosal connections extend throughout the region of bilateral representation but terminate in a spatially restricted manner that links visuotopically corresponding sites in the two hemispheres. In contrast, callosal connections appear to terminate without regard for the map of orientation preference, showing little sign of the orientation-specific modular and axial specificity that is characteristic of long-range horizontal connections. By coordinating the activity in the two hemispheres in a way that preserves nearest neighbor relationships, callosal connections may best be viewed as elements of local circuits that operate within a single bilateral representation of visual space.

Functional specificity of callosal connections in tree shrew striate cortex

Although callosal connections have been shown to link extensive regions of primary visual cortex, the distribution of these connections with respect to the map of visual space and the map of orientation preference remains unclear. Here we combine optical imaging of intrinsic signals with injection of fluorescent microspheres to assess the functional specificity of callosal connections in the tree shrew. By imaging both hemispheres simultaneously while presenting a series of spatially restricted stimuli, we find that a substantial region of visual space is represented bilaterally. Each hemisphere includes a representation of the ipsilateral visual field that is highly compressed relative to that of the contralateral visual field and is most extensive in the lower visual field, where approximately 30(o) of central visual space are represented bilaterally. Callosal connections extend throughout the region of bilateral representation but terminate in a spatially restricted manner that links visuotopically corresponding sites in the two hemispheres. In contrast, callosal connections appear to terminate without regard for the map of orientation preference, showing little sign of the orientation-specific modular and axial specificity that is characteristic of long-range horizontal connections. By coordinating the activity in the two hemispheres in a way that preserves nearest neighbor relationships, callosal connections may best be viewed as elements of local circuits that operate within a single bilateral representation of visual space.

Callosally projecting neurons in the macaque monkey V1/V2 border are enriched in nonphosphorylated neurofilament protein

Previous immunohistochemical studies combined with retrograde tracing in macaque monkeys have demonstrated that corticocortical projections can be differentiated by their content of neurofilament protein. The present study analyzed the distribution of nonphosphorylated neurofilament protein in callosally projecting neurons located at the V1/V2 border. All of the retrogradely labeled neurons were located in layer III at the V1/V2 border and at an immediately adjacent zone of area V2. A quantitative analysis showed that the vast majority (almost 95%) of these interhemispheric projection neurons contain neurofilament protein immunoreactivity. This observation differs from data obtained in other sets of callosal connections, including homotypical interhemispheric projections in the prefrontal, temporal, and parietal association cortices, that were found to contain uniformly low proportions of neurofilament protein-immunoreactive neurons. Comparably, highly variable proportions of neurofilament protein-containing neurons have been reported in intrahemispheric corticocortical pathways, including feedforward and feedback visual connections. These results indicate that neurofilament protein is a prominent neurochemical feature that identifies a particular population of interhemispheric projection neurons at the V1/V2 border and suggest that this biochemical attribute may be critical for the function of this subset of callosal neurons.

Functional specificity of long-range intrinsic and interhemispheric connections in the visual cortex of strabismic cats

The development of both long-range intracortical and interhemispheric connections depends on visual experience. Previous experiments showed that in strabismic but not in normal cats, clustered horizontal axon projections preferentially connect cell groups activated by the same eye. This indicates that there is selective stabilization of fibers between neurons exhibiting correlated activity. Extending these experiments, we investigated in strabismic cats: (1) whether tangential connections remain confined to columns of similar orientation preference within the subsystems of left and right eye domains; and (2) whether callosal connections also extend predominantly between neurons activated by the same eye and preferring similar orientations. To this end, we analyzed in strabismic cats the topographic relationships between orientation preference domains and both intrinsic and callosal connections of area 17. Red and green latex microspheres were injected into monocular iso-orientation domains identified by optical imaging of intrinsic signals. Additionally, domains sharing the ocular dominance and orientation preference of the neurons at the injection sites were visualized by 2-deoxyglucose (2-DG) autoradiography. Quantitative analysis revealed that 56% of the retrogradely labeled cells within the injected area 17 and 60% of the transcallosally labeled neurons were located in the 2-DG-labeled iso-orientation domains. This indicates: (1) that strabismus does not interfere with the tendency of long-range horizontal fibers to link predominantly neurons of similar orientation preference; and (2) that the selection mechanisms for the stabilization of callosal connections are similar to those that are responsible for the specification of the tangential intrinsic connections.

Functional specificity of long-range intrinsic and interhemispheric connections in the visual cortex of strabismic cats

The development of both long-range intracortical and interhemispheric connections depends on visual experience. Previous experiments showed that in strabismic but not in normal cats, clustered horizontal axon projections preferentially connect cell groups activated by the same eye. This indicates that there is selective stabilization of fibers between neurons exhibiting correlated activity. Extending these experiments, we investigated in strabismic cats: (1) whether tangential connections remain confined to columns of similar orientation preference within the subsystems of left and right eye domains; and (2) whether callosal connections also extend predominantly between neurons activated by the same eye and preferring similar orientations. To this end, we analyzed in strabismic cats the topographic relationships between orientation preference domains and both intrinsic and callosal connections of area 17. Red and green latex microspheres were injected into monocular iso-orientation domains identified by optical imaging of intrinsic signals. Additionally, domains sharing the ocular dominance and orientation preference of the neurons at the injection sites were visualized by 2-deoxyglucose (2-DG) autoradiography. Quantitative analysis revealed that 56% of the retrogradely labeled cells within the injected area 17 and 60% of the transcallosally labeled neurons were located in the 2-DG-labeled iso-orientation domains. This indicates: (1) that strabismus does not interfere with the tendency of long-range horizontal fibers to link predominantly neurons of similar orientation preference; and (2) that the selection mechanisms for the stabilization of callosal connections are similar to those that are responsible for the specification of the tangential intrinsic connections.

Non-mirror-symmetric patterns of callosal linkages in areas 17 and 18 in cat visual cortex

In the cat, callosal connections in area 17 are largely confined to a 5-6-mm-wide strip at the 17/18 border. It is commonly thought that callosal fibers extending from between the 17/18 border regions interconnect loci that are mirror-symmetric with respect to the midline of the brain, but this idea has not been tested experimentally. The present study examined the organization of callosal linkages in the 17/18 border region of normal adult cats by analyzing the patterns of connections revealed in one hemisphere after small injections of different fluorescent tracers into the opposite 17/18 callosal region. The location of the injection sites within areas 17 and 18 was assessed by examining architectonic data and by inspecting the labeling pattern in the ipsilateral visual thalamus. Area 17 and 18 were separated by a 1-1.5-mm-wide zone of cytoarchitectonic transition rather than by a sharp border. The results show that, in general, callosal fibers interconnect loci that are not mirror-symmetric with respect to the midline. Thus, area 17 injections placed nearly 3 mm away from the 17/18 transition zone produced discrete labeled areas located preferentially within the contralateral 17/18 transition zone. However, when the injection site was within the 17/18 transition zone, labeled cells were found primarily medial and lateral to, but not within, the 17/18 transition zone in the contralateral hemisphere. Previous studies have indicated that the 17/18 transition zone contains a representation of a strip of the ipsilateral visual field. Comparison of the retinotopy of the 17/18 border region with the mirror-reversed pattern of callosal linkages found in the present study suggests that callosal fibers link points that are in retinotopic correspondence in both hemispheres.

Two rules for callosal connectivity in striate cortex of the rat

In the rat, callosal cells occupy lateral as well as medial portions of striate cortex. In the region of the border between areas 17 and 18, which contains a representation of the vertical meridian of the visual field, cells projecting through the corpus callosum are concentrated throughout the depth of the cortex. In contrast, in medial portion of striate cortex, where peripheral portions of the visual field are represented, callosal cells are preferentially found in infragranular layers. These differences in topography and laminar distribution suggest that these callosal regions, referred to as medial and lateral callosal regions in the present study, subserve different functions. We explored this possibility by analyzing the patterns of callosal linkages in these two callosal regions. We charted the location of retrogradely labeled cells within striate cortex of one hemisphere after placing restricted injections of one or more fluorescent tracers into selected sites in the contralateral striate cortex. We found the medial and lateral callosal regions have distinctly different topographic organizations. Injections into medial striate cortex of one hemisphere produced labeled cells predominantly in mirror-symmetric loci in medial portions of contralateral striate cortex. The arrangement of these connections suggests that they mediate direct interactions between cortical regions representing visual fields located symmetrically on opposite sides of the vertical meridian of the visual field. In contrast, the mapping in the lateral callosal region is reversed: injections into the 17/18a border produced labeled fields located medial to the contralateral 17/18a border, while injections slightly medial to the 17/18a border produced labeled fields located at the contralateral 17/18a border.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic mapping of visual areas in the behaving cat: a [14C]2-deoxyglucose study

Visually responsive cortical areas and subcortical nuclei were studied in the awake cat using the 2-deoxyglucose technique. Visual input was confined to one hemisphere by unilaterally sectioning the optic tract, the corpus callosum and the commissura anterior. Within the intact hemisphere, numerous cortical regions were distinguishable in the autoradiographs due to differential labelling. Comparison of the intact with the visually deafferented hemisphere confirmed the visual character of eighteen cortical areas (areas 17, 18, 19, 20a, 20b, 21a, 21b, the posteromedial lateral, posterolateral lateral, anteromedial lateral, anterolateral lateral, dorsal lateral, ventral lateral, and posterior suprasylvian areas, the splenial and anterior ectosylvian sylvian areas, insular visual area and posterior area 7) and revealed the visual nature of an area in the posterior cingulate gyrus which had not been described previously. We refer to this area as cingulate visual area (CVA). This area exhibits a gradient in interhemispheric differences along a caudorostral axis similar to that observed in posterior area 7 which is in keeping with the strong and topographic connections between CVA and posterior area 7. These results support the validity of metabolic mapping for the characterisation of cortical areas.

Pyramidal and nonpyramidal callosal cells in the striate cortex of the adult rat

The aim of this study has been to determine the neuronal types (pyramidal and nonpyramidal) within the rat's visual cortex, which project through the corpus callosum. To this end, the morphology and laminar distribution of callosal cells have been investigated by combining Diamidino Yellow retrograde tracing with intracellular injection of Lucifer Yellow in slightly fixed tissue slices. The visual callosal projection arises from pyramidal cells of diverse morphology in layers II to VIb, as well as from several modified pyramids located mainly in layers II, IV (star pyramids) and VIb (horizontal or inverted pyramids and related forms of spiny stellate cells). Our results indicate that in rats, as in other mammals, several types of nonpyramidal neurons also contribute to the contralateral projection. Bitufted cells in layers II-III and V were found to project contralaterally. Moreover, a spine-free layer V cell and a sparsely spiny multipolar neuron of layer IV were also labeled. In both stellate cells, partial axonal labeling reveals that these callosal cells display a local axonal arborization. Finally, our results of retrograde transport with Diamidino Yellow and with another sensitive retrograde tracer, the beta subunit of the cholera toxin, demonstrate for the first time that the two main neuronal types of layer I participate in the callosal projection. In layer I, several small horizontal cells of the inner half of layer I and a large subpial cell displaying long radiating dendrites were also injected. The latter cell may correspond to the Cajal-Retzius cell of the adult rat. In spite of the important differences in the organization of the visual system between rodents and cats, the callosal projection in both mammals is composed of a large variety of pyramidal cells and several nonpyramidal neurons. This high morphological diversity suggests that the callosal projection is much more physiologically complex than the extracortical efferents of the visual cortex, resembling other cortico-cortical connections. The roles that the different callosal cells may play in the processing of visual information are discussed in relation to the known functions of the corpus callosum.

The role of ipsilateral and contralateral inputs from primary cortex in responses of area 21a neurons in cats

Neuronal responses in cat visual area 21a were analyzed when the primary visual cortex (areas 17 and 18) was deactivated by cooling. Ipsilateral and contralateral cortices were deactivated separately. Results established that (1) cooling the ipsilateral primary cortex diminished the activity of all area 21a cells and, in 30%, blocked responsiveness altogether, and (2) cooling the contralateral primary cortex initially increased activity in area 21a cells but, with further cooling, reduced it to below the original level although only 9% of cells ceased responding. These findings were then compared to earlier results in which bilateral deactivation of the primary cortex greatly reduced and, in most cases, blocked the activity of area 21a cells (Michalski et al., 1993). Despite the response attenuation following cooling of the primary visual cortex (either ipsilateral or contralateral), neurons of area 21a retained their original orientation specificity and sharpness of tuning (measured as the half-width at half-height of the orientation tuning curve). Direction selectivity also tended to remain unchanged. We concluded that for area 21a cells (1) the ipsilateral primary cortex provides the main excitatory input; (2) the contralateral primary cortex supplies a large inhibitory input; and (3) the nature of orientation specificity, sharpness of orientation tuning, and direction selectivity are largely unaffected by removal of the ipsilateral hemisphere excitatory input or the contralateral hemisphere inhibitory input.

Transcallosal circuitry revealed by blocking and disinhibiting callosal input in the cat

The purpose of this study was to obtain quantitative measures of the influence of callosal input to cells at the area 17/18 border region where transcallosal axons terminate most densely. Single-cell recordings were performed at the area 17/18 border region of the right hemisphere, while gamma-aminobutyric acid (GABA) or its antagonist, bicuculline, were applied to the transcallosal projecting regions of the left hemisphere to either block or overactivate the cells which projected to the neurons at the recording site. The results showed that visually evoked responses of the cells at the area 17/18 border were affected by administration of GABA or bicuculline to the contralateral hemisphere. Blockade of transcallosal input by application of GABA in the left hemisphere diminished the visually evoked responses of 51% of the neurons in the right hemisphere, and led to an increase in response magnitude for 17% of the neurons. Disinhibition of transcallosal input by application of bicuculline increased the evoked activity of 40% of the neurons and diminished the response magnitude of 20% of the neurons in the right hemisphere. GABA and bicuculline failed to show antagonistic effects on some cells. Thirty-two percent of the cells were affected by only one type of drug administration, and 13% of the cells showed either an increase or a decrease in responses after both GABA and then bicuculline administration. This study demonstrated complex interactions between neurons connected by the transcallosal pathway. A model of the transcallosal circuitry was proposed to explain the results.

Binocularity and excitability loss in visual cortex cells of corpus callosum transected kittens and cats

The contribution of the corpus callosum to binocularity of visual cortex cells and to their responsiveness was studied in cats. Electrophysiological recordings of the responses of single cells to visual stimulation was performed in the callosal projection zone, visual cortex area 17-18 boundary in callosotomized cats. Callosotomy was carried out by transection of the visual segment of the corpus callosum in 6-7-week-old kittens and in acute and chronic adult cats (postoperative recovery time: 11 days-39 months). While in our normal cats the common proportion of binocularly driven cells (79.8%) was found (66.3% in the sham controls), a remarkable diminution (29.7%) was found in the callosotomized kittens, in the acute (39.7%) and in the chronic (50.6%)-operated cats. We have also found a change in the amount of binocularity as function of postoperative recovery time. While the proportion of binocular cells was conceivable (60.7%) in the short- and intermediate-term callosotomized cats (postoperative time: 0.3-5.5 months), it was diminished (36.9%) in the long-term (6.5-39 months) chronic cats. As to the responsiveness level, it was found that visual responsive cells constituted 88% of the cells in the normal and 80.3% in the sham controls. In comparison, they constituted 69.2% in the acute, 54.4% in the chronic and 52.8% in the callosotomized kittens. Furthermore, callosal transection had produced a symmetric effect in the two hemispheres, regarding binocularity and responsiveness. It has been thus concluded that the corpus callosum is essential for the mediation of binocular functions between the two hemispheres; in addition, cortical excitability has been also found to depend on callosal integrity.

Visual callosal projections in the adult ferret

The laminar and tangential organization of visual callosal projections of areas 17 and 18 were investigated in the adult ferret, using histochemical methods to visualize axonally transported horseradish peroxidase (HRP). Normal adult ferrets were given injections of HRP throughout one visual cortex or had gelfoam soaked in HRP applied to the transected corpus callosum. The ferret callosal cell distribution has a greater tangential extent in area 18 than in area 17. In addition, the radial organization of callosal cells in areas 17 and 18 differs: three times as many infragranular cells are present in area 18 than in area 17, although the number of supragranular cells is similar for both areas 17 and 18. Since the projections of alpha retinal ganglion cells are reported to be exclusively contralateral in the ferret (Vitek et al., 1985), callosal projections may make a major contribution to the binocularity of neurons in area 18.

Topographic organization, number, and laminar distribution of callosal cells connecting visual cortical areas 17 and 18 of normally pigmented and Siamese cats

The callosal connections between visual cortical areas 17 and 18 in adult normally pigmented and "Boston" Siamese cats were studied using degeneration methods, and by transport of WGA-HRP combined with electrophysiological mapping. In normal cats, over 90% of callosal neurons were located in the supragranular layers. The supragranular callosal cell zone spanned the area 17/18 border and extended, on average, some 2-3 mm into both areas to occupy a territory which was roughly co-extensive with the distribution of callosal terminations in these areas. The region of the visual field adjoining the vertical meridian that was represented by neurons in the supragranular callosal cell zone was shown to increase systematically with decreasing visual elevation. Thus, close to the area centralis, receptive-field centers recorded from within this zone extended only up to 5 deg into the contralateral hemifield but at elevations of -10 deg and -40 deg they extended as far as 8 deg and 14 deg, respectively, into this hemifield. This suggests an element of visual non-correspondence in the callosal pathway between these cortical areas, which may be an essential substrate for "coarse" stereopsis at the visual midline. In the Siamese cats, the callosal cell and termination zones in areas 17 and 18 were expanded in width compared to the normal animals, but the major components were less robust. The area 17/18 border was often devoid of callosal axons and, in particular, the number of supragranular layer neurons participating in the pathway were drastically reduced, to only about 25% of those found in the normally pigmented adults. The callosal zones contained representations of the contralateral and ipsilateral hemifields that were roughly mirror-symmetric about the vertical meridian, and both hemifield representations increased with decreasing visual elevation. The extent and severity of the anomalies observed were similar across individual cats, regardless of whether a strabismus was also present. The callosal pathway between these visual cortical areas in the Siamese cat has been considered "silent," since nearly all neurons within its territory are activated only by the contralateral eye. The paucity of supragranular pyramidal neurons involved in the pathway may explain this silence.

Cortical cells' physiology following visual split brain in developing cats

We have studied physiologically whether visual cortex cells in areas 17 and 18 of split-brain cats preserve their performance despite the blockage of both binocularity and of interhemispheric communication. The absolute majority of the cells in cats underwent split-brain surgery as kittens and adults and were driven by the ipsilateral eye, resulting in the absence of interhemispheric interaction. Similar results were found in cats and kittens that underwent only chiasm split surgery, although some recovery of callosal transfer was found in the latter. A remarkable loss of binocularity was found when only callosal transection was performed, both in adult cats and in kittens, although some ipsilateral eye dominance was observed in the latter. As to the deprived cats, while in the inexperienced hemisphere (ipsilateral to the deprived eye), the majority of the cells was visually unresponsive, in the contralateral (experienced) hemisphere, the majority was responsive. A considerable reduction in responsiveness was found in the callosally transected cats and kittens. Generally, a degradation of function was found in the various properties as a result of chiasmal and/or callosal transection. The main effect is the increased number of cells with diffuse and incomplete receptive fields. There was also a reduction in the proportion of orientation-selective cells, mainly in the split-brain cats. It was concluded that, despite the high amount of hemispheric independency in the normal brain, the integrity and simultaneous action of the two hemispheres are needed for the normal functioning of visual cortex cells.

Effects of alternating monocular occlusion on the development of visual callosal connections

During normal postnatal development there is a partial elimination of the callosal projections of cortical areas 17 and 18 in the cat. Visual experience early in life can modulate this process. In the present study, we investigated how restricting visual experience to alternating monocular occlusion affects the development of the callosal connections of cortical areas 17 and 18. Alternating monocular occlusion exaggerates the normally occurring partial elimination of immature callosal projection: it causes a significant reduction in the total number of neurons in the supragranular layers that send an axon through the corpus callosum and marginally increases the distribution of these neurons across areas 17 and 18. Examination of these data in the context of the effects of other types of abnormal early visual experience on the corpus callosum and on the anatomy and physiology of areas 17 and 18 indicates that the postnatal development of the corpus callosum is under the control of multiple, interacting influences which differ in the magnitude and quality of their effects. The data also support the conclusion, drawn from our results in prior studies, that normal visual stimulation is necessary for the stabilization of the normal complement of callosal projections.

Transcallosal non-pyramidal cell projections from visual cortex in the cat

Non-pyramidal cells with transcallosal projections were identified in the area 17/18 border region of the cat by retrograde transport of horseradish peroxidase injected into border region of the opposite hemisphere. From several hundred neurons filled with a Golgi-like diaminobenzidine (DAB) reaction product, seven cells were identified by their radially oriented smooth dendrites as possible non-pyramidal cells. Following thin-sectioning and examination with the electron microscope, four of the neurons proved to be layer IV spiny stellate cells with incompletely filled dendritic spines, and two proved to be layer III pyramidal cells with an incompletely labelled apical dendrite and dendritic spines. The remaining neuron was a non-pyramidal cell whose essentially smooth dendrites were covered with synapses, and whose cell body formed both symmetric and asymmetric synapses with presynaptic terminals. To better assess how many non-pyramidal cells might be labelled, thin sections of the area 17/18 border were surveyed using material processed with tetramethylbenzidine (TMB), and another five labelled non-pyramidal cells with transcallosal projections were identified by the needle-like crystals of TMB reaction product they contained. During the study it became evident that both the DAB and TMB reaction products in the lightly labelled neurons tended to be associated with granules that are 0.5 microns or larger in diameter and that had the characteristics of lysosomes. These granules are also visible in the light microscope as dark puncta. The numbers of puncta in profiles of pyramidal and of non-pyramidal cells in layers II/III and IVa of the area 17/18 border region and in the control acallosal region of area 17 were counted and compared. These comparisons revealed that labelled transcallosally projecting non-pyramidal cells may constitute 10-32% of the non-pyramidal cell population at the area 17/18 border region. Similar values were also obtained for pyramidal cells in this region. Consequently, it is concluded that significant numbers of non-pyramidal cells have axons that project through the corpus callosum to the contralateral hemisphere.

Morphological evidence for callosally projecting nonpyramidal neurons in rat visual cortex

This investigation shows that some of the callosally projecting neurons in rat visual cortex are nonpyramidal cells. Callosally projecting neurons were labeled by injections of horseradish peroxidase (HRP) into the area 17/18a border zone of the contralateral hemisphere. The retrogradely transported HRP was visualized with diaminobenzidine or with tetramethylbenzidine. In some of the labeled neurons the reaction product was diffuse, so that the neurons had a Golgi-like appearance, but in others the reaction product was granular, or punctate. The majority of neurons with a Golgi-like appearance were pyramidal cells, but one callosally projecting neuron from layer V area 18a was confirmed by electron microscopy to be a nonpyramidal neuron. This dearth of well-filled nonpyramidal cells suggested that callosally projecting nonpyramidal neurons may not transport sufficient HRP to show Golgi-like filling, and so punctately labeled neurons from areas 17, 18a and 18b were examined. Reacted sections from areas 17, 18a and 18b of control animals, into which no tracer had been injected, were also examined, but in these control preparations no granules similar to the HRP granules within the neuronal profiles of the experimental animals were encountered. In methylene blue-stained 1-microns sections, neuronal profiles from the control animals possessed only blue staining lysosomes, while neuronal profiles from the experimental animals exhibited both lysosomes and HRP granules. It was determined, from the counts of HRP granules in neurons from the experimental animals, that in selected regions of areas 17, 18a, and 18b similar percentages of the pyramidal and nonpyramidal neuronal populations (ranging from 100% to 34%) contained HRP granules, and so had callosally projecting axons. However, most callosally projecting nonpyramidal neurons had far fewer HRP granules than the pyramidal neurons, again indicating that they transport less HRP. This could account for the fact that callosally projecting nonpyramidal neurons only rarely show a Golgi-like filling, and this could be one reason why such cells have been overlooked in most previous studies.

Representation of the ipsilateral visual field in the transition zone between areas 17 and 18 of the cat's cerebral cortex

The representation of the visual field in the architectonically defined transition zone between areas 17 and 18 of cat cerebral cortex was assessed by recording the activities and plotting the receptive fields of neurons at 2327 sites along 148 electrode penetrations made in 19 cats. The results show that the transition zone contains a significant representation of the ipsilateral visual hemifield although not all elevations in the visual field are represented to the same extent. The shape of the field region represented resembles an hour glass, for the region represented is narrowest on the 0-deg horizontal meridian and increasingly wider at progressively more positive and negative elevations. When receptive-field centers are considered, the extent of the representation reaches to -2.5 deg on the 0-deg horizontal meridian and to 10 or more degrees towards the field periphery. When receptive-field areas are considered, the representation at the 0-deg horizontal meridian extends to -3.6 deg and to beyond 20 deg at other elevations. In contrast, the visual-field representations in flanking areas 17 and 18 are essentially limited to the contralateral hemifield. The presence of a distinct representation of part of the ipsilateral hemifield in the transition zone suggests that the zone may have connections distinctly different from those of the adjacent areas. The observations bear on the problems of understanding the visual pathways in hypopigmented cats and binocular disparity mechanisms about the midline.

Functional organization of the cortical 17/18 border region in the cat

The representation of the visual field in the 17/18 border region of the cat's visual cortex, and the layout of orientation and ocular dominance columns, were studied by making many closely spaced electrode penetrations into the superficial layers of the flattened dorsal region of the marginal gyrus and recording response properties at each location. The 17/18 border region was defined by measuring the change in the horizontal component of receptive field position within the gyrus: as the position of the recording electrode moved from medial to lateral, the receptive fields moved towards the vertical midline, indicating that the electrode was in area 17; as penetrations were made in increasingly lateral positions, the trend reversed, and receptive field positions moved away from the midline, indicating that the electrode was in area 18. The receptive fields of cells close to the border straddled, or lay within 2 degrees-3 degrees on either side of the vertical midline. In addition, patches of cortex were sometimes encountered in which cells had receptive field centers located up to 7 degrees in the ipsilateral visual field. Experiments in which maps were made in the left and right hemispheres of a single animal showed that these patches had a complementary distribution in the two hemispheres. Cells within the patches behaved as though driven by Y-cell inputs: they usually had large receptive fields and responded to rapidly-moving stimuli. They were broadly tuned for orientation and strongly dominated by the contralateral eye. Fourier spectral analysis of orientation selectivity maps showed that iso-orientation bands had an average spacing of 1.14 +/- 0.1 mm and tended to be elongated in a direction orthogonal to the 17/18 border. Individual bands crossed the border without obvious interruption, although singularities (points of discontinuity in the layout of orientations) were more frequently observed in the border region than in adjacent areas. Two dominant periodicities could be measured in the maps of ocular dominance, one at around 0.8 +/- 0.2 mm and a second at 2.0 +/- 0.3 mm. No constant direction of elongation was noted. These are close to the periods present within areas 17 and 18 respectively.

The callosal projection in cat visual cortex as revealed by a combination of retrograde tracing and intracellular injection

The neuronal composition of callosally projecting cells in cat visual cortex was determined with a combination of retrograde labelling and intracellular injection. Fluorescent tracers were stereotaxically injected into the proximity of the area 17/18 border, corresponding to the representation of the visual vertical meridian. In fixed slice preparations of homotopic regions of the contralateral hemisphere retrogradely labelled cells were filled with Lucifer Yellow. Of more than a hundred injected cells a morphological variety of pyramidal cells, located in cortical layers II-IV and VI, constituted the prevalent cell class in the contralateral projection. A minor proportion of spiny stellate cells was encountered in layer IV. Despite the presence of a contralaterally projecting smooth stellate cell, presumed to be a basket cell, it is concluded that the efferents to contralateral visual cortex predominantly arise from pyramidal and spiny stellate cells. Thus, in agreement with findings from anterograde degeneration studies, the interhemispheric pathway most likely conveys a direct excitatory input to postsynaptic target cells.

Electrophysiological evidence for interhemispheric transmission of visual information in man

1. Electrophysiological evidence is presented of interactions between two stimuli (sinusoidal gratings of equal spatial frequency but different contrast, phase-reversed sinusoidally at different temporal frequencies) located on opposite side of, and within a few degrees from, the vertical meridian. 2. These interactions are revealed by a depression of the cortical visual evoked potential (VEP) evoked by the grating of lower contrast in the presence of the grating of higher contrast. They are similar to, albeit weaker than, those obtained with superimposed asynchronously modulated gratings. 3. The VEP reduction occurs also if the stimuli are presented dichoptically. 4. It does not occur if the gratings are located one above the other either on the same or on opposite sides of the vertical meridian. 5. The strength of the VEP reduction depends on the relative contrast of the two gratings and vanishes for spatial frequencies beyond 4 cycles/deg and temporal frequencies of the high-contrast grating beyond 10 Hz. 6. The results are in agreement with data on visual callosal connections in animals and confirm previous psychophysical findings (Berardi & Fiorentini, 1987) indicating the particular properties of the interhemispheric cross-talk between symmetric regions of the visual field astride the vertical meridian in man.

Global stereopsis in human albinos

The presence of global stereopsis was examined in 18 clinically diagnosed albinos; four non-albino controls were also tested including two observers with congenital nystagmus. Stereopsis was evaluated with standard clinical stereo tests and with TV generated random dot stereograms. The latter test involved electrophysiological measures of vertical eye movement tracking in response to a stimulus target. For either test procedure, global stereopsis could be demonstrated in a significant number of albinos across varying phenotypes. These results are of interest in view of electrophysiological investigations in albino animal models which indicate a paucity of binocularly driven cortical neurons in visual areas 17, 18 and 19. While stereopsis may be mediated in our albinos via residual appropriately projecting retino-geniculo-cortical fibres, we suggest that inter and intra cortical communication via corpus callosal connections may play a primary role in providing the adequate neural substrate for albino binocularity.

Corpus callosum transection reduces binocularity of cells in the visual cortex of adult cats

The possible involvement of the corpus callosum in binocular functions of the visual cortex was studied in adult cats. Unit recording was made in areas 17, 18 boundary following posterior or complete transection of the corpus callosum, acutely as well as chronically, after short (3-4 months) and long (5.5-39 months) survival periods. A considerable reduction of binocularly driven cells was found in the posteriorly callosally transected cats (acute: 41% cells; short-chronic: 65%; long-chronic: 32%). Similar results, albeit smaller in the long-survival group, were found following complete callosal transection. In comparison, the proportion of binocular cells in the normal cats was 85%. It was concluded that the corpus callosum is involved in interhemispheric integration and enhancement of binocularity in visual cortex cells. No recovery occurs as function of time following cancellation of the interhemispheric interaction by callosal transection.

Callosal projections between areas 17 in the adult tree shrew (Tupaia belangeri)

In the primary visual cortex (area 17) of the tree shrew (Tupaia belangeri) neurons projecting to the contralateral area 17 via the corpus callosum were identified by horseradish peroxidase histochemistry (HRP, WGA-HRP). The distribution of homotopic and heterotopic connections was studied. We found that a narrow stripe of area 17 close to the dorsal area 17/18 border - which corresponds to the visual field along the vertical meridian - is connected via homotopic callosal projections. The adjacent dorsal part of area 17, which largely corresponds to the binocular visual field, is connected via homotopic as well as heterotopic projections. Heterotopic projections originate in the cortical stripe along the area 17/18 border and their contralateral targets are displaced medially. Callosal neurons are located mostly in supragranular but also occur in infragranular layers. The supragranular neurons in general are pyramidal cells. In addition to these findings, we confirmed earlier reports on ipsilateral projections of the primary visual area to the dLGN, the claustrum, area 18 and other visual areas.

Mechanisms for binocular depth sensitivity along the vertical meridian of the visual field

By removing the visual cortex unilaterally, and recording along the intact 17/18 border, we have investigated the influence of the corpus callosum on the tuning curves for stimulus disparity of cat cortical neurons. Responses to binocular stimulation were examined to movement in the same (in-phase) and in the opposite direction (antiphase) across the two retinae. In lesioned cats, as in normal cats, units were encountered which showed high sensitivity and narrow tuning for stimulus disparity. In contrast to normal cats, however, lesioned cats showed a reduced proportion of units displaying moderate binocular interactions, as well as a substantial increase in disparity-insensitive cells. The loss of disparity sensitivity after decortication was associated with a reduced incidence of both selectivity for the direction of stimulus motion and binocular activation. No large differences between the preparations were seen, however, in the ocular dominance of the total populations of cells. Differences between the normal and lesioned cats were found in binocular responses to in-phase but not to antiphase stimulation motion. Tuning curves in lesioned cats showed reduced binocular inhibition but no changes in binocular facilitation. Our findings indicate that callosal input contributes to unit disparity sensitivity by enhancing direction-selective binocular inhibition. The corpus callosum generates disparity sensitivity in a population of units in the superficial cortical layers which may play a particular role in the perception of stereoscopic depth.

Unilateral visual cortex deafferentation induces changes in receptive field properties of cortical cells in the intact hemisphere of normal and of monocularly deprived cats

Receptive field properties and the selectivity of cortical cells to visual stimulation were studied in the visual cortices (the boundary between areas 17 and 18) of both hemispheres following unilateral deafferentation in normal and in early monocularly deprived cats. Almost no visual activity was encountered in the deafferented hemisphere and a considerable diminution in visual responsiveness was found in the intact hemisphere of all experimental cats. Responsiveness had increased with recovery time; unresponsive cells had consisted 44.6% of the cells in the intact hemisphere of the acute, 34.7% in the 3-month chronic and 14.5% in the normal cats. Disregarding the bias due to an early monocular deprivation, the ocular dominance distribution of the cells in the intact hemispheres of these cats was unaffected. Selectivity was markedly reduced in the intact hemisphere of the deafferented cats as expressed mainly in the increased orientation tuning range and in the proportion of orientation-selective cells. The proportion of these cells was 45.5% in the deafferented cats, 38.6% in the deafferented monocularly deprived cats, 65.3% in the monocularly deprived and 81.9% in the normal control cats. A similar trend, although less prominent, was found in respect to the proportion of direction-selective cells. The size of receptive fields was only slightly affected in the deafferented deprived cats. Receptive fields were considerably larger in size (length) in the deafferented cats in comparison to their respective normal and monocularly deprived controls. It is concluded that the absence of visual input from the deafferented hemisphere is bidirectionally affecting the corpus callosum.(ABSTRACT TRUNCATED AT 250 WORDS)

The transfer of visual information across the corpus callosum: spatial and temporal properties in the cat

1. The spatial and temporal characteristics of the visual information transmitted across the corpus callosum have been studied in normal cats by recording directly from the corpus callosum and in split-chiasm cats by means of visual evoked potentials (v.e.p.s) and single-unit recordings at the 17/18 border. 2. The modulation transfer functions (m.t.f.s) obtained by recording from the corpus callosum are comparable to the m.t.f.s evaluated by various techniques for the whole visual system of the cat. The spatial and temporal acuities, however, do not reach the values obtained behaviourally or estimated with cortical evoked potentials. 3. In split-chiasm cats, both v.e.p.s and single-unit recordings indicate that the contrast gain of the callosal pathway is considerably lower than the gain of the direct, geniculo-cortical system. Spatial and temporal acuities are lower for the callosal than for the direct system. 4. The same differences in contrast gain between the spatial m.t.f. obtained for the callosal and the direct system have been found in alert split-chiasm cats. 5. Our data suggest that the cross-talk between the hemispheres taking place across the corpus callosum is nearly abolished at low contrasts and high spatial and temporal frequencies.