Properties of area 17/18 border neurons contributing to the visual transcallosal pathway in the cat

The functional characterization of callosal connections

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The functional characterization of callosal connections is informed by anatomical data.

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Callosal connections play a conditional driving role depending on the brain state and behavioral demands.

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Callosal connections play a modulatory function, in addition to a driving role.

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The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.

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The corpus callosum contributes to language processing and cognitive functions.

The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.

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.

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.

Unilateral paralytic strabismus in the adult cat induces plastic changes in interocular disparity along the visual midline: Contribution of the corpus callosum

Neurones activated through the corpus callosum (CC) in the cat visual cortex are known to be almost entirely located at the 17/18 border. They are orientation selective and display receptive fields (RFs) distributed along the central vertical meridian of the visual field (“visual midline”). Most of these cells are binocular, and many of them are activated both from the contralateral eye through the CC, and from the ipsilateral eyeviathe direct retino-geniculo-cortical (GC) pathway. These two pathways do not carry exactly the same information, leading to interocular disparity between pairs of RFs along the visual midline. Recently, we have demonstrated that a few weeks of unilateral paralytic strabismus surgically induced at adulthood does not alter the cortical distribution of these units but leads to a loss of their orientation selectivity and an increase of their RF size, mainly toward the ipsilateral hemifield when transcallosally activated (Watroba et al., 2001). To investigate interocular disparity, here we compared these RF changes to those occurring in the same neurones when activated through the ipsilateral direct GC route. The 17/18 transition zone and the bordering medial region within A17 were distinguished, as they display different interhemispheric connectivity. In these strabismics, some changes were noticed, but were basically identical in both recording zones. Ocular dominance was not altered, nor was the spatial distribution of the RFs with respect to the visual midline, nor the amplitude of position disparity between pairs of RFs. On the other hand, strabismus induced a loss of orientation selectivity regardless of whether neurones were activated directly or through the CC. Both types of RFs also widened, but in opposite directions with respect to the visual midline. This led to changes in incidences of the different types of position disparity. The overlap between pairs of RFs also increased. Based on these differences, we suggest that the contribution of the CC to binocular vision along the midline in the adult might be modulated through several intrinsic cortical mechanisms.

Interhemispheric connections between primary visual areas: beyond the midline rule

In the last five years, a number of detailed anatomical, electrophysiological, optical imaging and simulation studies performed in a variety of non-human species have revealed that the functional organization of callosal connections between primary visual areas is more elaborate than previously thought. Callosal cell bodies and terminals are clustered in columns whose correspondence to features mapped in the visual cortex, such as orientation and ocularity, are starting to be understood. Callosal connections are not restricted to the vertical midline representation nor do they establish merely point-to-point retinotopic correspondences across the hemispheres, as traditionally believed. In addition, anatomical studies have revealed the existence of an ipsilateral component of callosal axons. The aim of this short review is to propose how these new data can be integrated into an updated scheme of the circuits responsible for assembling the primary visual field map.

Callosal connections correlate preferentially with ipsilateral cortical domains in cat areas 17 and 18, and with contralateral domains in the 17/18 transition zone

Previous studies have shown that the distribution of callosal connections in the 17/18 callosal zone of the cat is patchy at a small scale, but the mechanisms that determine this periodic pattern remain unclear. The present study investigated this issue by correlating the distribution of retrogradely labeled callosal cells with the underlying patterns of ocular dominance columns (ODCs) revealed transneuronally after intraocular injections of wheat germ agglutinin-horseradish peroxidase. The density of labeled callosal cells was found to vary significantly between adjacent territories dominated by different eyes, indicating that the distribution of callosal cells is significantly biased toward domains that are eye specific. Moreover, callosal connections relate to the pattern of ODCs in a rather unique way: callosal cells correlate preferentially with contralateral ODCs within the 17/18 transition zone (TZ), and with ipsilateral ODCs in regions of areas 17 and 18 located outside the TZ. Similar results were obtained in cats raised with strabismus, indicating that the overlap between right and left ODCs present in normal cats does not influence the correlation between callosal neurons and ODCs. The results are consistent with the hypothesis that callosal linkages are stabilized during development by interhemispheric correlated activity driven by bilateral projections from temporal retina. It is proposed that developmental constraints imposed by both this retinally driven mechanism and the pattern of ODCs are likely to determine not only the association of callosal clusters with specific sets of ODCs, but also important aspects of the functional characteristics of the callosal pathway in cat striate cortex.

Visual inter-hemispheric processing: constraints and potentialities set by axonal morphology

The largest bundle of axonal fibers in the entire mammalian brain, namely the corpus callosum, is the pathway through which almost half a billion neurons scattered over all neocortical areas can exert an influence on their contralateral targets. These fibers are thus crucial participants in the numerous cortical functions requiring collaborative processing of information across the hemispheres. One of such operations is to combine the two partial cortical maps of the visual field into a single, coherent representation. This paper reviews recent anatomical, computational and electrophysiological studies on callosal connectivity in the cat visual system. We analyzed the morphology of individual callosal axons linking primary visual cortices using three-dimensional light-microscopic techniques. While only a minority of callosal axons seem to perform a strict 'point-to-point' mapping between retinotopically corresponding sites in both hemispheres, many others have widespread arbors and terminate into a handful of distant, radially oriented tufts. Therefore, the firing of a single callosal neuron might influence several cortical columns within the opposite hemisphere. Computer simulation was then applied to investigate how the intricate geometry of these axons might shape the spatio-temporal distribution of trans-callosal inputs. Based on the linear relation between diameter and conduction velocity of myelinated fibers, the theoretical delays required for a single action potential to reach all presynaptic boutons of a given arbor were derived from the caliber, g-ratio and length of successive axonal segments. This analysis suggests that the architecture of callosal axons is, in principle, suitable to promote the synchronous activation of multiple targets located across distant columns in the opposite hemisphere. Finally, electrophysiological recordings performed in several laboratories have shown the existence of stimulus-dependent synchronization of visual responses across the two hemispheres. Possible implications of these findings are discussed in the context of temporal tagging of neuronal assemblies.

Inhibition synchronizes sparsely connected cortical neurons within and between columns in realistic network models

Networks of compartmental model neurons were used to investigate the biophysical basis of the synchronization observed between sparsely-connected neurons in neocortex. A model of a single column in layer 5 consisted of 100 model neurons: 80 pyramidal and 20 inhibitory. The pyramidal cells had conductances that caused intrinsic repetitive bursting at different frequencies when driven with the same input. When connected randomly with a connection density of 10%, a single model column displayed synchronous oscillatory action potentials in response to stationary, uncorrelated Poisson spike-train inputs. Synchrony required a high ratio of inhibitory to excitatory synaptic strength; the optimal ratio was 4 : 1, within the range observed in cortex. The synchrony was insensitive to variation in amplitudes of postsynaptic potentials and synaptic delay times, even when the mean synaptic delay times were varied over the range 1 to 7 ms. Synchrony was found to be sensitive to the strength of reciprocal inhibition between the inhibitory neurons in one column: Too weak or too strong reciprocal inhibition degraded intra-columnar synchrony. The only parameter that affected the oscillation frequency of the network was the strength of the external driving input which could shift the frequency between 35 to 60 Hz. The same results were obtained using a model column of 1000 neurons with a connection density of 5%, except that the oscillation became more regular. Synchronization between cortical columns was studied in a model consisting of two columns with 100 model neurons each. When connections were made with a density of 3% between the pyramidal cells of each column there was no inter-columnar synchrony and in some cases the columns oscillated 180 degrees out of phase with each other. Only when connections from the pyramidal cells in each column to the inhibitory cells in the other column were added was synchrony between the columns observed. This synchrony was established within one or two cycles of the oscillation and there was on average less than 1 ms phase difference between the two columns. Unlike the intra-columnar synchronization, the inter-columnar synchronization was found to be sensitive to the synaptic delay: A mean delay of greater than 5 ms virtually abolished synchronization between columns.

Neuronal interactions in cat visual cortex mediated by the corpus callosum

The article summarizes three sets of physiological and anatomical studies carried out to investigate the structural basis of the functional interactions between visual cortical areas 17 and 18 in the two cerebral hemispheres of cats. (1) The visual field representations in the transcallosal sending and receiving zones are defined. (2) The consequences of severing callosal fibers on the visual field representation at the area 17/18 border are described. (3) Lastly, experiments using cooling to reversibly inactive transcallosal inputs are reported. The observations on the transcallosal sending and receiving zones show that callosal connections of area 17 are concerned with a vertical hour-glass shaped region of the visual field centered on the midline, and this region is doubly represented, once in each hemisphere. The zone represents azimuths within +/- 4 degrees of the midline at the 0 degree horizontal meridian, and azimuths out to +/- 15 to +/- 25 degrees at positions distant from the horizontal meridian. The observations suggest that, in addition to interactions between neurons concerned with positions immediately adjacent to the midline, there are positions, especially high and low in the visual field, where interactions can occur between neurons which have receptive fields displaced some distance from the midline. The extent of this double representation is reduced by approximately 2/3 when the corpus callosum is cut. The retention of some bilateral representation in these animals suggests that there are alternate routes for across-the-midline transmission of visual signals. Or, more likely, there are ganglion cells in temporal retina with crossed projections that make significant contributions to the remaining double representation of the visual field. Lastly, the results obtained using cooling inactivation of transcallosal fibers show that many excitatory and inhibitory circuits are under the direct control of transcallosal fibers in the normally functioning brain. These connections appear to be no different from intrinsic connections of area 17, and they undoubtedly contribute to the binding of the two half-field representations, one in each hemisphere, and perceptual unity across the midline.

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.

Morphology of callosal axons interconnecting areas 17 and 18 of the cat

Seventeen callosally projecting axons originating near the border between areas 17 and 18 in adult cats were anterogradely labelled with biocytin and reconstructed in 3-D from serial sections. All axons terminated near the contralateral 17/18 border. However, they differed in their diameter, tangential and radial distributions, and overall geometry of terminal arbors. Diameters of reconstructed axons ranged between 0.45 and 2.25 microns. Most of the axons terminated in multiple terminal columns scattered over several square millimetres of cortex. Thus in general callosal connections are not organized according to simple, point-to-point spatial mapping rules. Usually terminal boutons were more numerous in supragranular layers; some were also found in infragranular layers, none in layer IV. However, a few axons were distributed only or mainly in layer IV, others included this layer in their termination. Thus, different callosal axons may selectively activate distinct cell populations. The geometry of terminal arbors defined two types of architecture, which were sometimes represented in the same axon: parallel architecture was characterized by branches of considerable length which supplied different columns or converged onto the same column; serial architecture was characterized by a tangentially running trunk or main branch with radial collaterals to the cortex. These architectures may relate to temporal aspects of inter-hemispheric interactions. In conclusion, communication between corresponding areas of the two hemispheres appears to use channels with different morphological and probably functional properties.

Computational structure of visual callosal axons

We analysed the activation profiles obtained by simulating invasion of an orthodromic action potential in eleven anterogradely filled and serially reconstructed terminal arbors of callosal axons originating and terminating in areas 17 and 18 of the adult cat. This was done in order to understand how geometry relates to computational properties of axons. In the simulation, conduction from the callosal midline to the first bouton caused activation latencies of 0.9-3.2 ms, compatible with published electrophysiological values. Activation latencies of the total set of terminal boutons varied across arbors between 0.3 and 2.7 ms. Arbors distributed boutons in tangentially segregated terminal columns spanning one or, more often, several layers. Individual columns of one axon were frequently activated synchronously or else with a few hundred microseconds of each other. Synchronous activation of spatially separate columns is achieved by: (i) long primary or secondary branches of similar calibre running nearly parallel to each other for several millimetres; (ii) variations in the calibre of branches serially fed to separate columns by the same primary or secondary branch; (iii) exchange of high-order or preterminal branches across columns. The long, parallel branches blatantly violate principles of axonal economy. Simulated alterations of the axonal arbors indicate that similar spatiotemporal patterns of activity could, in principle, be obtained by less axon-costly architectures. The structure of axonal arbors, therefore, may not be determined solely by the type of spatiotemporal activation profiles it achieves in the cortex but also by other constraints, in particular those imposed by developmental mechanisms.

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.

Synapses of extrinsic and intrinsic origin made by callosal projection neurons in mouse visual cortex

Neurons in areas 17/18a and 17/18b of mouse cerebral cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) transported from severed callosal axons in the contralateral hemisphere. Terminals of the local axon collaterals of labeled neurons (intrinsic terminals) were identified in the border regions of area 17 with areas 18a and 18b, and their distribution and synaptic connectivity were determined. Also examined were the synaptic connections of extrinsic callosal axon terminals labeled by lesion-induced degeneration consequent to the severing of callosal fibers. A postlesion survival time of 3 days was chosen because by this time the extrinsic terminals were all degenerating, whereas the intrinsic terminals were labeled by horseradish peroxidase. Both intrinsic and extrinsic callosal axon terminals occurred in all layers of the cortex where, with rare exception, they formed asymmetrical synapses. Layers II and III contained the highest concentrations of intrinsic and extrinsic callosal axon terminals. Analyses of serial thin sections through layers II and III in both areas 17/18a and 17/18b yielded similar results: 97% of the intrinsic (1,412 total sample) and of the extrinsic (414 total sample) callosal axon terminals synapsed onto dendritic spines, likely those of pyramidal neurons; the remainder synapsed onto dendritic shafts of both spiny and nonspiny neurons. Thus, the synaptic output patterns of intrinsic vs. extrinsic callosal axon terminals are strikingly similar. Moreover, the high proportion of axospinous synapses formed by both types of terminal (97%) contrasts with the proportion of asymmetrical axospinous synapses that occurs in the surrounding neuropil where about 64% of the asymmetrical synapses are onto spines. This result is in accord with previous quantitative studies of the synaptic connectivities of callosal projection neurons in mouse somatosensory cortex, and lends additional weight to the hypothesis that axonal pathways are highly selective for the types of elements with which they synapse.

Visual-field map in the callosal recipient zone at the border between areas 17 and 18 in the cat

The representation of the visual field in the callosal fiber recipient zone of area 17 and the adjacent area 17/18 transition zone was determined in the cat. The callosal fiber recipient zone was identified by anterograde transport of tritiated amino acids that had been injected into transcallosal sending zone of the opposite hemisphere. Application of autoradiographic procedures revealed that transcallosal projections are densest in the area 17/18 transition zone, and that their density in area 17 diminishes within 1-2 mm of the transition zone. Of 980 sites sampled in the visual-field mapping part of the study, 507 proved to be in the zone demarcated by transcallosally transported label. In this zone, both ipsilateral- and contralateral-field positions are represented, and the representation of the visual field at the different elevations is not equal. When ipsilateral-field positions are considered, the representation extends to about 4 deg close to the visual axis, and to 15-20 deg at elevations greater than +/- 30 deg, the representation is approximately mirror-symmetric about the horizontal meridian, and the representation is concordant with that of the representation in the area 17 transcallosal sending zone of the opposite hemisphere.

Stereo perception in callosal agenesis and partial callosotomy

Stereoperception in two acallosal patients and two partial callosotomy patients was compared with that of three normal subjects. All three patients with the splenium missing, whether due to agenesis or surgical intervention, showed midline deficits and broadly similar profiles; namely, they made few uncrossed midline responses. The patient with partial callosal section but with the splenium almost totally spared performed better at the midline than in the periphery. All degrees of disconnection produced some overall loss of performance, confirming the results of Hamilton and Vermeire (In Two Hemispheres--One Brain, F. Lepore, M. Pitto and H. H. Jasper (Editors), pp. 315-333. Alan R. Liss Inc., New York, 1986) and Hamilton et al. (Suppl. Invest. Ophthal. Vis. Sci. 28, 294, 1987). The results are discussed in the context of earlier reports of human and animal studies of stereoperception. Bearing in mind reports of structural alterations in layer III of the striate cortex in acallosals (Shoumjra, K. et al. Brain Res. 93, 241-252, 1975. Also, Akert, K. et al. Trans. Am. Neurol. Assoc. 79, 151-153, 1954), it is speculated that the specific difficulties encountered by them in handling uncrossed disparities may be due to a marked reduction or absence of far neurones in acallosal brains (Poggio, G. F. and Poggio, T. Ann. Rev. Neurosci. 7, 379-412, 1984). The likely importance of the anterior commissure in the efficient integration of crossed disparity (near neurones) (Cowey, A. In Brain Mechanisms and Spatial Vision, D. Ingle (Editor), NATO Advanced Study Institute Series, Martinus Nijhoff, The Hague, 1985) is seen as a possible explanation of the acallosals relative success in making crossed disparity judgements. The variability of performance in normals documented by Hamilton and Vermeire (In Two Hemispheres--One Brain, F. Lepore, M. Pitto and H. H. Jasper (Editors), pp. 315-333. Alan R. Liss Inc., New York, 1986) and Hamilton et al. (Suppl. Invest. Ophthal. Vis. Sci. 28, 294, 1987) is, not surprisingly, even more marked amongst acallosals.