Visual receptive field properties of cells innervated through the corpus callosum in the cat

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

In a series of physiological experiments, a total of 203 neurons at the Area 17/18 border were recorded with a callosal link either demonstrated by antidromic or transsynaptic activation from stimulating electrodes located in the homotopic contralateral hemisphere (CH), or in the splenial segment of the corpus callosum (CC). Forty-four percent of the transcallosal cells could also be driven from stimulating electrodes in or just above the lateral geniculate nucleus (OR1). The majority (69%) of transcallosal neurons were classifiable as belonging to the complex family (B and C cells) and most of these were found in the supragranular laminae and in lamina 4A. The ocular dominance distribution of transcallosal cells was trimodal, consisting of roughly equal numbers of monocularly dominated and binocularly balanced neurons. Estimates of conduction time and synaptic delay were obtained for neurons driven from CH, CC, and from OR1, and in most instances the response latency was short enough to suggest a monosynaptic input from either the ipsi- or contra-lateral hemisphere. The distribution of transcallosal conduction times showed that S cells, as a class, had significantly faster conduction than cells of the complex family but otherwise there was no obvious signs of multimodality in the distribution curve. An analysis of the synaptic delays in transcallosal activation produced a mean of 0.6 to 0.7 ms but some were too short to be consistent with a transsynaptic drive, suggesting that some cells with an antidromic drive may have been included in the transsynaptic category. Results are interpreted in terms of the contribution made by the corpus callosum to stereoscopic vision.

Post-critical period plasticity of callosal transfer to visual cortex cells of cats following early conditioning of monocular deprivation and late optic chiasm transection

We studied whether plasticity-induced callosal transfer exists after the critical period for sensitivity of visual cortex cells in kittens postnatally monocularly deprived and in which interocular competition was cancelled by chiasm transection during adulthood. Callosal transfer was studied acutely (n = 3 cats) and chronically (n = 7) following the chiasm transection (OCAMD). For comparison, adult cats in which chiasm transection only was performed (OCA) were also studied acutely (n = 3) and chronically (n = 9). The results were also compared to cats in which monocular deprivation and chiasm transection were simultaneously performed (OCKMD) during development (n = 6) and to normal control cats (n = 18). Unit recording was extracellularly carried out in visual cortex areas 17 and 18 and their boundary region, where the corpus callosum is represented. When no interocular competition was allowed between the non-deprived and the deprived eye via the thalamocortical direct visual pathways on cortical cells, such as in the OCKMD cats, the absolute majority of the cells were ipsilaterally driven, regardless of which hemisphere was studied. Only a minor proportion (4.1%) of the cells had some contralateral input from the non-deprived eye in the hemisphere ipsilateral to the deprived eye, indicating almost no interhemispheric callosal transfer. A slight increase in the proportion of cells callosally driven from the non-deprived eye (9.8%), was found in this hemisphere in cats in which interocular competition was allowed via the direct visual pathways prior to its cancellation by chiasm transection (OCAMD), if studied acutely after the chiasm transection. A remarkable increase in callosal transfer was found in this hemisphere under chronic conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

The ocular dominance and receptive field properties of visual cortex cells of cats following long-term transection of the optic chiasm and monocular deprivation during adulthood

Plasticity-induced interhemispheric transfer of visual information to cortical cells was studied in adult cats. The direct contralateral visual pathway was surgically eliminated permitting binocularity only by callosal transfer. In order to enhance the interhemispheric transfer, one hemisphere was made less visually active by depriving it chronically from visual input. Single cell recording was made in areas 17-18 boundary, the callosal projection zone, of operated (OC), operated and deprived (OCMD), and normal control cats. In the OCMD cats, greater than 90% of the cells in each hemisphere reacted ipsilaterally to the deprived or non-deprived eye. Only 3.1% of the cells in both hemispheres of the OCMD cats and 3.9% in the OC cats had contralateral input via the corpus callosum. The two hemispheres were similar in the selectivity of their cells to stimulus orientation and direction. The average receptive field area of the OCMD cats was also similar for the ipsilaterally driven cells in the two hemispheres; it was 1.2 degrees 2 for the deprived eye and 1.1 degrees 2 for the normal eye. The receptive fields (greater than 95%) of both eyes of the OCMD cats were found in the nasal visual hemifields and greater than 70% of them were at eccentricities of less than 5 degrees from the vertical meridian. The disappearance of the temporal (contralateral) hemifields in these cats and the physiological properties of their cortical cells were determined merely by the chiasm transection which had thus induced nearly complete interhemispheric separation. No effect of the monocular deprivation, in normal adult cats or in cats with chiasm transection was found, even after long periods (greater than 7 months). Therefore, plasticity-induced interhemispheric transfer of visual information was not found during adulthood.

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.

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.

Split chiasm developmentally induced in kittens: plasticity of interhemispheric transfer in visual cortex cells

Visual callosal transfer during development was studied in order to reveal plasticity-related compensation for the absence of direct contralateral inputs. The optic chiasm was midsagittally sectioned in 6-8 weeks old kittens (OCK) and for comparison, in adult cats (OCA). Unit recording was made during adulthood in the border area between visual cortex areas 17 and 18, namely the callosal projection zone. The proportion of cells showing interhemispheric transfer in the OCK group, as indicated by the presence of visual input from the contralateral eye was 10.5%; in the OCA cats their proportion was 4.0%. Moreover, 2.3% of the cells showed a pure transfer of input from the contralateral eye in the OCK, although none was seen in the OCA cats. Thus, during the developmental period, a plasticity induced process, albeit limited, takes place in the enhancement of interhemispheric transfer of visual information.

Comparative development of cell properties in cortical area 18 of normal and dark-reared kittens

The development of visual cell properties was studied in cortical Area 18 (A18) of normal (NRs) and dark-reared kittens (DRs), from 2 weeks of age to adulthood. In addition to the orientation selective (S) and non-selective (NS) cells, we describe a new type of non-selective cell with a peripheral zone (NSp), which could be either an intermediate form between NS and S cells and included in a sequential model or an immature form of the S cells whose responses are affected by peripheral stimulations. Using accurate coordinates for the area centralis position relative to the optic disc projection as a function of age, we show that: a) the extent of the visual field increases with age in DRs and NRs; b) the retinotopic organization is always present; c) receptive fields, large in the NS cells, reduce to the size of mature S cells as soon as the cells acquire orientation selectivity. This process can occur after only 6 h of visual experience; d) velocity preference shifts toward high velocities, though more so in NRs than in DRs. An interpretation of the development of these properties is proposed, taking into account eye growth, the growth of dendritic fields and the formation of new connections. A comparison with previous results obtained in Area 17 (A17) shows a similar time course of the specification (NRs) and of the despecification (DRs) processes, although the development of A18 is postponed by about 2 weeks. Moreover, the "adult-like" binocular distribution of ocular dominance depends upon visual experience in A18, while it does not in A17.

Somatosensory receptive fields of fibres in the rostral corpus callosum of the cat

The corpus callosum is the principal neocortical commissure which transmits lateralized information between the hemispheres. The aim of the present experiment was to study the receptive field (RF) properties of somatosensory callosal fibres in the cat. The callosum was approached under direct visual control and axonic responses were recorded under N2O anaesthesia using tungsten microelectrodes or, mostly, glass micropipettes. RFs representing all the sensory submodalities tested (light touch, medium and deep pressure, joint movement and light pinches) were found to be present in the axons which travelled through the callosum. Rapidly adapting units were more common than slowly adapting ones. The axial and para-axial portions of the body accounted for about three-fifths of all RFs, followed by the head (about one-fifth), with the rest responding to stimulation of the extremities. The medial borders of most of the unilateral RFs situated on the trunk and, to a lesser degree, the head, extended to the mid-line. The results are interpreted in terms of the roles of the corpus callosum in mid-line fusion and interhemispheric transfer.

Receptive field properties of somatosensory callosal fibres in the monkey

The corpus callosum is the principal neocortical commissure which transmits lateralized information between the hemispheres. The aim of the present experiment was to study the receptive field properties of somatosensory callosal fibres in rhesus macaque monkeys. The callosum was approached under direct visual control and axonic responses were recorded using tungsten microelectrodes. All sensory submodalities which could be examined with the available instruments were found (light touch, medium and deep pressure, joint movement and light pinches). Most fibres had receptive fields concerned with the trunk, followed by the head, with only a few responding to stimulation of the extremities. The medial borders of the unilateral receptive fields situated on the trunk and the head extended to the midline. The results are interpreted in terms of the roles of the corpus callosum in midline fusion and interhemispheric transfer.

Transfer of visual information after unilateral input to the brain

Visual callosal connections are more numerous and widespread in the association areas than in the primary visual cortex and adjoining visual areas. In keeping with this, the amount of "wrong" ipsilateral visual field that is represented in the various cortical areas of primates and cats increases as one goes from primary visual cortex to extraoccipital areas. Therefore it can be argued that transfer of unilaterally presented visual stimuli occurs mainly at the temporal and parietal cortical level.

Interhemispheric connections of cortical sensory areas in tree shrews

Interhemispheric connections were studied in tree shrews (Tupaia belangeri) after multiple injections of horseradish peroxidase or horseradish peroxidase conjugated to wheat germ agglutinin into the cortex of one cerebral hemisphere. After an appropriate survival period, the areal pattern of connections was revealed by flattening the other hemisphere, cutting sections parallel to the cortical surface, and staining with tetramethylbenzidine. Architectonic boundaries were identified by using sections stained for myelinated fibers. Labeled cells and axon terminations formed largely overlapping distributions that covaried in density, although labeled cells appeared to be more evenly distributed than labeled terminations. Connections were concentrated along the border of area 17 (V-I) with area 18 (V-II). However, connections also extended as far as 2 mm into area 17 to include cortex representing parts of the visual field 10 degrees or more from the zero vertical meridian. Clusters of dense connections spanned the width of area 18, where they alternated with regions of fewer connections. These clusters roughly corresponded in location to regions with heavier myelination. In the visually responsive temporal cortex, connections were also unevenly distributed. The organization of most of this cortex is not understood, but one subdivision, the temporal dorsal area (TD), has been identified on the basis of reciprocal connections with area 17. The central part of the TD had few interhemispheric connections, while most of the outer border had dense connections. The auditory cortex had dense and patchy connections throughout. The pattern in the primary somatosensory cortex (S-I) varied according to the representation of body parts, so that the cortex related to the forepaw had sparse connections, while connections were dense but uneven over much of the representation of the face, nose, and mouth. A focus of connections was found at the border of the forepaw and face representations, where the myelination of S-I cortex is interrupted. Dense, uneven connections also characterized the second somatosensory area, S-II. The motor cortex was densely connected, with only slightly fewer terminations rostral to the forepaw region of S-I. Other parts of frontal cortex had dense connections. The distribution of cortical connections varied with depth for at least some areas, so that clusters of cells and terminations were found in supragranular layers in S-I, S-II, and TD, while infragranular labeled cells were more evenly distributed.(ABSTRACT TRUNCATED AT 400 WORDS)

Optic chiasm split and binocularity diminution in cortical cells of acute and of chronic operated adult cats

The ocular dominance, responsiveness level and receptive field properties of single cortical cells were studied in 12 acute and chronic split chiasm adult cats (729 cells) and in 13 normal controls (544 cells). Recording was made from the border between visual areas 17/18. Responsive cells in the operated cats were obtained exclusively (87.1%) following stimulation of the ipsilateral eye, except for a very few cells (2.5%) which were binocularly driven. In comparison, only few (10.9%) of the cells in the normal control cats were driven ipsilaterally and the majority of them (74.5%) were binocularly driven. Relatively small proportions of cells (46.1%) were visually responsive in the acute (less than 1 week postoperatively) and in the most chronic (greater than 6 months) cats, in comparison to the normal cats (87.3%). No consistent change was found in the responsiveness level of cortical cells as function of length of the survival time (correlation coefficient: -0.45). Only a very slight tendency for a relative increase in binocularly driven cells with survival time was found as well as a reduction in the proportion of nonspecific cells. However, in view of the general absence of binocularity and responsivity in these cats, it was concluded that no recovery was found, even long after the elimination of the contralateral inputs.

Further investigation of visual evoked potentials elicited by lateralized stimuli: effects of stimulus eccentricity and reference site

Visual evoked potentials (VEPs) to small lateralized flashes were recorded from the parietal midline, homotopic lateral central and occipital electrodes, and from left and right mastoid processes, all referred to a balanced non-cephalic reference. Two stimulus eccentricities, 4 degrees and 10 degrees, were employed, and a comparison made between the non-cephalic and linked mastoid references. P120 (measured at lateral occipital sites only) peaked earlier and was of smaller amplitude at the electrode contralateral to the visual field of stimulus exposure. N160 peaked earlier at central than occipital sites, was larger from electrodes over the contralateral hemisphere, and at the occipital sites only, peaked earlier in the electrode contralateral to the visual field of stimulus exposure. These effects were virtually unaffected by the eccentricity manipulation and it is concluded that light scatter across the visual midline is unlikely to be responsible for the observed pattern of ipsilateral-contralateral VEP asymmetries. The mastoids were found to detect significant stimulus-locked activity in the same latency range as the occipital N160 component. However, comparison of the non-cephalic and linked mastoid references revealed only non-specific effects, and no sign of any change in the pattern of ipsilateral-contralateral VEP asymmetries, or the magnitude of the associated latency differences. It is concluded that these effects may be of value in the study of callosal transfer.

The critical period for corpus callosum section to affect cortical binocularity

The period of time during which surgical section of the corpus callosum (CC) is effective in altering the physiological properties of cells in cat striate cortex was investigated. Cats which had the CC transected between 13 days and 24 weeks of age were studied using extracellular, single-unit recording procedures. Analysis of the results from 1,747 cortical units indicate that when the CC was sectioned prior to 19 days of age there was a reduction in the encounter rate of binocularly activated neurons and an increase in the proportion of neurons dominated by the contralateral eye. The decrease in cortical binocularity was observed in both simple and complex cell populations, and at all receptive field eccentricities studied (0-39 degrees). However, when the CC was sectioned after 19 postnatal days, no physiological changes were detected. Thus, in contrast with previous studies (Payne et al. 1980a, b) no changes were found following CC section in adult cats. The results therefore define a critical period which ends before 3 weeks of age during which corpus callosum section reduces striate cortex binocularity. Although the corpus callosum critical period is much shorter than the critical period for experiential alterations in cortical binocularity, the physiologically determined limits of the callosal critical period agree with the behaviorally determined limits previously found for the callosal critical period (Elberger 1984).

Perfect interocular transfer of visual pattern discriminations in split-chiasm cats trained with fading

It has been reported that the interocular transfer of visual pattern discrimination is imperfect in split-chiasm cats. Perhaps the efficiency of callosal transfer is to some extent determined by the training procedure employed. To test that possibility, we trained 3 split-chiasm cats with a Classical Procedure used in interocular transfer studies (CP cats) and 4 split-chiasm cats with Fading (Fading cats). The Fading procedure consisted of the presentation of two stimuli which differed maximally at the beginning of training; through a series of gradual steps their difference was reduced to a minimal value required to control the desired discriminative behavior. Confirming previous studies, CP cats showed a drop in the discriminative performance when the viewing eye was changed from trained to untrained and they needed additional trials to reattain criterion with the untrained eye. Fading cats, however, showed no difference when the viewing eye was changed; criterion was immediately reached with the untrained eye. It is suggested that the perfect interocular transfer shown by the Fading paradigm represents efficient callosal transmission and stabilized, well organized memory traces.

Binocularity in the visual cortex of the adult cat does not depend on the integrity of the corpus callosum

The importance of the corpus callosum for binocular interaction in areas 17 and 18 of the adult cat is still a matter of controversy, since its specific role in integrating information from the two eyes has been suggested by some and questioned by others. We have reanalyzed the problem by assessing binocular interaction for single neurons in areas 17 and 18 of adult cats submitted to section of the posterior two-thirds of the corpus callosum. In 5 cats this interhemispheric disconnection was performed from 10 days to 7 weeks before the electrophysiological recordings; in another cat callosal afferents to the recording sites were at first partially eliminated by an acute lesion of corresponding cortical zones in the other hemisphere, and thereafter completely interrupted by a posterior callosal section performed in the same recording session. Recordings were mainly aimed at the callosal zone of areas 17 and 18, which coincides with the border between these two areas and corresponds to visual field regions bordering the vertical meridian. Electrophysiological recordings were carried out in awake, unanesthetized animals in which all nociceptive pathways were previously interrupted by a midpontine pretrigeminal transection. The results indicate that the interhemispheric disconnection, whether acute or chronic, does not disrupt binocularity in areas 17 and 18; moreover, the analysis of the ocular dominance for binocular neurons did not reveal any imbalance between the inputs from the two eyes, since at all levels of eccentricity the majority of binocular neurons was equally activated by both eyes. Since in previous experiments on anesthetized cats, section of the corpus callosum apparently reduced binocular interaction in areas 17 and 18, we suggest that such an effect, which was lacking in our unanesthetized cats, was probably due to an interaction or summation between callosotomy and anesthesia.

Lack of binocular activation of cells in area 19 of the Siamese cat

Single cells were recorded in area 19 of 8 Siamese cats. Receptive fields (RFs) were typical for this area in terms of size, directional specificity and type. However, 69 out of the 70 units found were monocularly driven through the contralateral eye. Moreover, the amount of excursion of RFs into the ipsilateral visual field was more limited than that generally demonstrated for areas 17 and 18, extending to a maximum of 5 degrees with very few cells having RFs situated completely within the ipsilateral hemifield.

Interhemispheric transfer of visual pattern discriminations: evidence for a bilateral storage of the engram

The present experiment was carried out to determine whether the memory trace of a pattern discrimination learned with one hemisphere is also transmitted to the second hemisphere via the corpus callosum or whether the trace is limited to the trained hemisphere and becomes accessible to the second during recall via this route. Split-chiasma cats learned two pattern discriminations with one eye (and hemisphere), then were subjected to a mid-sagittal transection of their corpus callosum, followed by learning with the other untrained eye (and hemisphere). Ten cats were separated into two groups: one group learned the discrimination to criterion (the non-overtrained group) while the other received 1600 overtraining trials over and beyond those needed to reach criterion (the overtrained group). Results indicated that there was little bilateral storage in the non-overtrained group (as determined by the number of trials needed to attain criterion with the second eye). Most subjects from the overtrained group showed chance performance during the first transfer session but learned the pattern discrimination much more rapidly with the second eye than with the first. These results are taken as indicating that memory transcription is possible through the callosum but that this route is slower and its readout is possibly contaminated by secondary non-specific factors which affect the initial utilization of the trace.