Electrophysiological evidence for interhemispheric connections in the anterior ectosylvian sulcus in the cat

Summary We report electrophysiological data regarding the contribution of the corpus callosum to visual responses in the cortex around the anterior ectosylvian sulcus (AES). The experiments were performed in cats in which the optic input from each eye was surgically restricted to the ipsilateral hemisphere (split-chiasm cats), and where neuronal responses to stimulation of the contralateral eye were mediated by interhemispheric connections. A very high proportion of cells were driven by stimuli presented to either eye indicating that they were activated not only through an intrahemispheric pathway from the ipsilateral eye, but also through an interhemispheric pathway from the contralateral eye. With few exceptions, both receptive fields (RFs) of each binocular neuron abutted or were in the vicinity of the vertical meridian. All neurons responded well to moving stimuli and most of them showed directional selectivity. A few cells were activated by stimuli moving in depth. Following an additional section of the posterior half of the corpus callosum, cells in AES responded only to stimulation of the ipsilateral eye, demonstrating thus that the input from the contralateral eye was conveyed by this part of the corpus callosum. By contrast following a section of the anterior half of the corpus callosum, all visually responsive AES neurons were binocularly activated. These results suggest that the interhemispheric visual input to this ectosylvian region is conveyed via a polysynaptic loop involving visual cortical areas that are connected through the posterior portion of the corpus callosum.     

Indirect, across-the-midline retinotectal projections and representation of ipsilateral visual field in superior colliculus of the cat

1. In agreement with previous work, we have found that the ipsilateral visual field is represented in an extensive rostral portion--from one-third to one-half--of the superior colliculus (SC) of the cat. This representation is binocular. The SC representation of the ipsilateral visual field can be mediated both directly, by crossed retinotectal connections originating from temporal hemiretina, and indirectly, by across-the-midline connections relaying visual information from one-half of the brain to contralateral SC. 2. In order to study the indirect, across-the-midline visual input to the SC, we have recorded responses of SC neurons to visual stimuli presented to either the ipsilateral or the contralateral eye of cats with a midsagittal splitting of the optic chiasm. Units driven by the ipsilateral eye, presumably through the direct retinotectal input and/or corticotectal connections from ipsilateral visual cortex, were found throughout the SC, except at its caudal pole, which normally receives fibers from the extreme periphery of the contralateral nasal hemiretina. Units driven by the contralateral eye, undoubtedly through an indirect across-the-midline connection, were found only in the anterior portion of the SC, in which is normally represented the ipsilateral visual field. Receptive fields in both ipsilateral and contralateral eye had properties typical of SC receptive fields in cats with intact optic pathways. 3. All units having a receptive field in the contralateral eye had also a receptive field in the ipsilateral eye; for each of these units, the receptive fields in both eyes invariably abutted the vertical meridian of the visual field. The receptive field in one eye had about the same elevation relative to the horizontal meridian and the same vertical extension as the receptive field in the other eye; the two receptive fields of each binocular unit matched each other at the vertical meridian and formed a combined receptive field straddling the vertical midline of the horopter...     

Comparison of receptive-field organization of the superior colliculus in Siamese and normal cats

1. The superior colliculus has been studied in Siamese and normal cats by recording the responses of single tectal units to visual stimuli.

2. The retinotopic organization of the superior colliculus has been compared in the two breeds. In the normal cat, the contralateral half-field is represented in the central and caudal part of the colliculus, and a vertical strip of the ipsilateral half-field, 15-20° wide, is represented at the anterior tip. The Siamese cat superior colliculus receives an abnormally large projection from the ipsilateral half-field so that units with visual receptive fields which extend as far as 40° into the ipsilateral half-field can be found. The area of the tectal surface devoted to the representation of the ipsilateral half-field is about twice as large in Siamese cats as in normal cats. The enhanced representation of the ipsilateral half-field in Siamese cats is reflected in a displacement of the vertical meridian and the area centralis on the tectal surface.

3. The area centralis in the Siamese cat is located at about the same point on the tectal surface as would be occupied by a point in the visual field about 6-7° contralateral to the area centralis in the normal cat. The smallest receptive fields in both breeds are located near the area centralis. The size of the receptive field for a tectal unit seems to be determined by the retinal location of the receptive field and not by the absolute position of the unit on the tectal surface.

4. The receptive-field characteristics of tectal units show many similarities in the two breeds. The receptive fields of individual units consist of activating regions flanked by suppressive surrounds. Units respond well to stimuli of different shapes and orientation provided they are moving. The optimum stimulus for a given unit can be much smaller than the size of the activating region. About two thirds of the units studied in both breeds show directional selectivity. Most of the units studied in normal cats can be activated by stimulation of either eye, while in Siamese cats, 80% of the units studied can be driven only by the contralateral eye. A few monocularly driven units with two separated receptive fields have been observed in Siamese cats.

5. In the left tectum of both breeds, units respond well to left-to-right stimulus movement. The reverse situation obtains in the right tectum. In Siamese cats, units located at the anterior tip of the tectum with their receptive fields located in the visual half-field ipsilateral to the tectum under study respond better to stimulus movement toward the area centralis than away from it. The preferred direction for a tectal unit seems to be determined by its tectal location rather than by the location of its receptive field in the retina.

6. Visual cortex lesions in both breeds increase the responsiveness of tectal units to flashing spots and almost entirely remove the directional selectivity exhibited by tectal units, although units with asymmetric surrounds are still found. In normal cats, the lesions change the ocular dominance distribution, skewing it more strongly toward the contralateral eye. In Siamese cats, the ocular dominance distribution remains unchanged after a visual cortex lesion.

7. The squint commonly exhibited by Siamese cats is regarded as a compensation for the anomalous retinotectal topography. It is suggested that, in the absence of an adaptive modification, the anomalous retinotectal projection would lead to mislocalization in Siamese cats just as it does in frogs and hamsters whose retinotectal projection has been experimentally altered. The convergent strabismus which Siamese cats commonly exhibit may be a cure for the abnormal retinal projections rather than a disease.

     

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

Summary 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.     

Effects on binocular activation of cells in visual cortex of the cat following the transection of the optic tract

Cells in area 17 of the cortex are generally activated either directly through a retino-thalamic pathway or indirectly via a contralateral hemisphere-callosal pathway. The aim of the present experiment was to evaluate the effects of eliminating this second pathway on the binocular activation of cells in the primary visual cortex. The optic tract was sectioned on one side in 18 cats and unit activity was recorded in the contralateral hemisphere. This hemisphere should receive normal thalamo-cortical inputs but no visual callosal input. These animals were compared to 21 normal cats. Extracellular electrophysiological recordings were carried out in the conventional way using tungsten microelectrodes and N2O anaesthesia. Results indicated that the proportion of binocular cells found in the cortex of tract sectioned animals was lower than that found in normal animals. However, this decrease in binocularity could be essentially attributed to cells having receptive fields situated to within 4 degrees of the vertical meridian of the visual field. These results are interpreted as being congruent with the demonstrated anatomo-physiological projections of the callosal system.     

A quantitative analysis of the direction-specific response of neurons in the cat's nucleus of the optic tract

Summary All cells in the nucleus of the optic tract (NOT) of the cat, that Bcould be activated antidromically from the inferior olive, were shown to be direction-specific, as influenced by horizontal movements of an extensive visual stimulus. Cells in the left NOT were activated by leftward and inhibited by rightward movement, while those in the right NOT were activated by rightward and inhibited by leftward movement. Vertical movements did not modulate the spontaneous activity of the cells. The mean spontaneous discharge rate in 50 NOT cells was 30 spikes/s.This direction-specific response was maintained over a broad velocity range (<0.1 ° – >100 °/s). Velocities over 200 °/s could inhibit NOT cells regardless of stimulus direction.All cells in the NOT were driven by the contralateral eye, about half of them by the ipsilateral eye also. In addition, activation through the contralateral eye was stronger in most binocular units. Binocular cells preferred the same direction in the visual space through both eyes.An area approximately corresponding to the visual streak in the cat's retina projected most densely onto NOT cells. This included an extensive ipsilateral projection. No clear retinotopic order was seen. The most sensitive zone in the very large receptive fields (most diameters being >20 °) was along the horizontal zero meridian of the visual field.The retinal input to NOT cells was mediated by W-fibers.The striking similarities between the input characteristics of NOT-cells and optokinetic nystagmus are discussed. The direction selectivity and ocular dominance of the NOT system as a whole can provide a possible explanation for the directional asymmetry in the cat's optokinetic nystagmus when only one eye is stimulated.This work was supported by DFG-Grants No 450/3 and 450/7 to K.-P. Hoffmann     

Response properties of striate cortex neurons in cats raised with divergent or convergent strabismus

Recordings were made from striate cortex in five groups of cats that had been raised with strabismus produced by sectioning the extraocular muscles. These groups included animals reared with exotropia, unilateral or bilateral esotropia, and esotropia combined with lid suture of the unoperated eye. In addition, a group of esotropes was studied in which the unoperated eye was removed a few hours prior to recording. For comparison, five normal adult cats were also studied. In each of the above groups, cells were sampled in the representations of the central and peripheral visual fields in area 17 ipsilateral and contralateral to the deviated eye. We mapped the receptive field of each responsive cell, determined its ocularity, and tested it for selectivity. Confirming previous work, we found a marked loss of cortical binocularity in cats raised with strabismus. On average only 7% of the neurons that we recorded could be driven by both eyes. This percentage was relatively constant at all cortical locations that were studied and was not influenced by whether cats had been reared with exotropia, unilateral esotropia, or bilateral esotropia. The percentage of selective cells driven by the deviated eye in exotropes or esotropes did not appear to be different from normal at most cortical locations (but see 5, below). In addition, we did not observe any bias in the axial preference of selective cells in strabismic cats when compared with normal adult cats. In both exotropes and esotropes the deviated eye drove fewer cells when compared with the proportion that are driven by one eye in normal cats. In exotropes this deficit did not vary at different cortical representations of the visual field. In esotropes, however, this deficit was graded, being least in the representation of the peripheral visual field in area 17 contralateral to the deviated eye, intermediate in the representations of the central visual field in the contralateral and ipsilateral hemispheres, and greatest in the representation of the peripheral visual field in ipsilateral area 17. Furthermore, only when recording from the peripheral field representation in the ipsilateral hemisphere did we encounter significant numbers of cells driven by the deviated eye that lacked normal selectivity. Since it is possible that deprivation of the converged eye during development might account for the deficits noted above, we attempted to evaluate this factor using several independent lines of evidence. First, we could find no correlation between the angle of esotropia and the ability of the deviated eye to drive ipsilateral cortical cells representing the peripheral visual field.(ABSTRACT TRUNCATED AT 400 WORDS)     

Aberrant visual projections in the Siamese cat

1. Guillery has recently shown that the Siamese cat has a grossly abnormal lateral geniculate body. His anatomical study suggested that certain fibres originating in the temporal retina of each eye cross in the chiasm instead of remaining uncrossed. They thus reach the wrong hemispheres, but in the geniculate they terminate in the regions that the missing fibres from the ipsilateral eye would normally have occupied. The result is that each hemisphere receives an input from parts of the ipsilateral field of vision, this input being entirely from the opposite eye. The purpose of the present work was to study the physiological consequences of this aberrant projection, in the lateral geniculate body and visual cortex.2. Single-cell recordings from the lateral geniculate body confirmed the presence of projections from the ipsilateral visual field of the contralateral eye. The part of layer A(1) receiving these projections was arranged so that the receptive fields of the cells were situated at about the same horizontal level and at the same distance from the vertical meridian as the fields of cells in the layers above and below (layers A and B), but were in the ipsilateral visual field instead of the contralateral. They thus occupied a region directly across the mid line from their normal position.3. In the cortex of all animals studied, we found a systematic representation of part of the ipsilateral visual field, inserted between the usual contralateral representations in areas 17 and 18. When the visual cortex was crossed from medial to lateral the corresponding region of visual field moved from the contralateral periphery to the mid line, and then into the ipsilateral field for 20 degrees . The movement then reversed, with a return to the mid line and a steady progression out into the contralateral field. The entire double representation was, with some possible exceptions, a continuous one. The point of reversal occurred at or near the 17-18 boundary, as judged histologically, and this boundary was in about the same position as in ordinary cats.4. Cells in the part of the cortex representing the ipsilateral fields had normal receptive fields, simple, complex, or hypercomplex. These fields tended to be larger than those in corresponding parts of the contralateral visual fields. Receptive-field size varied with distance from the area centralis, just as it does in the normal cat, so that cells with the smallest fields, in the area centralis projection, were situated some distance from the 17-18 border.5. Projections originating from the first 20 degrees from the midvertical in both visual half-fields had their origin entirely in the contralateral eye, as would be expected from the abnormal crossing at the chiasm. Beyond this visual-field region, and out as far as the temporal crescents, there were projections from both eyes, but we found no individual cells with input from the two eyes. The cells were aggregated, with some groups of cells driven by one eye and some by the other.6. From previous work it is known that ordinary cats raised with squint show a decline in the proportion of cells that can be driven binocularly, whereas animals raised with both eyes closed show little or no decline. A Siamese cat raised with both eyes closed had binocular cells in the regions of 17 and 18 subserving the peripheral visual fields, suggesting that the absence of binocular cells seen in the other Siamese cats was indeed secondary to the squint.7. In two Siamese cats there were suggestions of an entirely different projection pattern, superimposed upon that described above. In the parts of 17 and 18 otherwise entirely devoted to the contralateral visual field, we observed groups of cells with receptive fields in the ipsilateral field of vision. The electrode would pass from a region where cells were driven from some part of the contralateral visual field, to regions in which they were driven from a part of the ipsilateral field directly opposite, across the vertical mid line. The borders of these groups were not necessarily sharp, for in places there was mixing of the two groups of cells, and a few cells had input from two discrete regions located opposite one another on either side of the vertical mid line. The two receptive-field components of such cells were identical, in terms of orientation, optimum direction of movement, and complexity. Stimulation of the two regions gave a better response than was produced from either one alone, and the relative effectiveness of the two varied from cell to cell. These cells thus behaved in a way strikingly reminiscent of binocular cells in common cats.8. The apparent existence of two competing mechanisms for determining the projection of visual afferents to the cortex suggests that a number of factors may cooperate in guiding development. There seems, furthermore, not to be a detailed cell-to-cell specificity of geniculocortical connexions, but rather a tendency to topographic order and continuity, with one part of a given area such as 17 able to substitute for another. Whether or not these tentative interpretations are ultimately proved correct, it seems clear that this type of genetic anomaly has potential usefulness for understanding mechanisms of development of the nervous system.     

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

Summary The present experiment examined the receptive field (R.F.) properties of cortical cells which receive part of their input from the contralateral hemisphere via the corpus callosum. Two groups of cats were used for recording unit activity: a normal control group, and an experimental group consisting of cats which had their optic chiasmas split across the midline prior to the recording sessions. Acute recordings were carried out in the conventional manner using tungsten microelectrodes and N₂O: O₂ anaesthesia. The recording site was the 17–18 border. The stimulus consisted of a thin bar generated on an oscilloscope screen by a computer. The bar, whose orientation was varied automatically from 0 ° to 345 ° in 15 ° steps, was swept across the screen at constant speed orthogonal to the orientation. Various R.F. properties were studied using both quantitative and qualitative criteria. Thus, in the normal cat, simple, complex and hypercomplex type R.F.'s were found, whereas no callosally activated cell was of the simple type. The ocular dominance distribution found in the split chiasma cat was skewed towards the ipsilateral eye, although a fairly large number of cells could be driven with the two eyes. The R.F.'s of the callosally activated neurons were all situated close to the vertical meridian, which they sometimes straddled. Both in the normal and in the chiasma sectioned cats, the complex cells had larger R.F.'s than the other cell types. However, the R.F.'s determined through the ipsilateral eye was essentially of the same dimensions as those obtained through the indirect interhemispheric pathway, and this irrespective of cell type. Orientation specificity was similar for the two eyes in the split chiasma cats as it was for the normal cats although in the former the orientation tuning curve was narrower for the callosal pathway than for the more direct thalamo-cortical pathway. The results are interpreted within the context of the different functions ascribed to the corpus callosum in vision.Supported in part by grants from the Conseil de Recherches en Sciences Naturelles et en Génie du Canada and from the Ministère de l'Education du Québec     

Learning and interhemispheric transfer of visual pattern discriminations following unilateral suprasylvian lesions in split-chiasm cats

Summary A suprasylvian lesion removing cortical areas 7 and 21 and portions of area 19 and of the lateral suprasylvian area was placed in one hemisphere of split-chiasm cats. By comparison with the normal side and with cortically intact split-chiasm and split-brain cats, form discrimination learning with the eye on the injured side was severely retarded. This deficit could not be attributed to an unintentional undercutting of areas 17 and 18, since in three cases the laminae of the lateral geniculate nucleus showed little retrograde atrophy; marked degeneration was found in the medial interlaminar nucleus and the pulvinar complex. In addition, interocular transfer of form discriminations to the eye on the injured side was absent or poor, while transfer in the opposite direction was normal. A cat with a suprasylvian lesion undercutting areas 17 and 18 was unable to learn pattern discriminations with the eye on the injured side, in spite of prolonged training with that eye and normal learning with the other eye. Another cat with a suprasylvian lesion selectively removing the anteromedial and posteromedial portions of the lateral suprasylvian area showed no learning deficit on the injured side, but poor transfer to that side. A learning deficit on the side of the lesion emerged in this cat after forebrain commissurotomy.The results support the hypothesis of a major involvement of cortical areas outside of 17 and 18 in the processes of abstraction and generalization of visual information necessary for learning and interhemispheric transfer of form discrimination in the cat.     

Visual experience and the maturation of the ipsilateral visuotectal projection in Xenopus laevis

The effect of visual deprivation upon the maturation of the ipsilateral visuotectal projection has been studied in Xenopus laevis. This topographically ordered projection is polysynaptic. The first stage involves the retinal projection to the contralateral optic tectum. The tectum projects to the nucleus isthmi on the same side. The final stage is the crossed isthmotectal projection from the nucleus isthmi to the tectum ipsilateral to the eye. The topographic precision of connections at various points in this polysynaptic pathway has been investigated by quantifying single-unit and multi-unit receptive field sizes in the contralateral and ipsilateral visuotectal projections. Observations have been made on normal animals of different ages to plot the normal maturational course of events. The effects of visual deprivation on this maturational process has been studied. Between one week and one year after metamorphosis there is an increase in the precision of connections in both the contralateral and ipsilateral visuotectal projections. Visual deprivation had no effect upon the parameters of the contralateral visuotectal projection. Ipsilateral visuotectal single units in dark-reared animals had normal receptive field sizes. Ipsilateral multi-unit receptive fields in dark-reared animals were considerably larger than in normal animals. It was concluded that the effects of visual deprivation are limited to effects on the crossed isthmotectal component of the intertectal system. In this component, however, visual experience seems to play an important role in the normal development and modification of connections. It is suggested that visual experience is utilized to accommodate changes in the system required to respond to normal changes in interocular geometry that take place with development in Xenopus.     

Binocular neuronal responsiveness in Clare-Bishop cortex of Siamese cats

Summary Retinotopy and binocular responsiveness were studied extracellularly in a total of 278, 61, 110 and 275 cells sampled in areas 17, 18, 19 and Clare-Bishop (CB) of Siamese cats. The misalignment of the visual axes of the two eyes was determined by the pupil reflex method in the behaving animal. The recording sessions were conducted under N₂O anesthesia, supplemented with continuous infusion of short-lasting anesthetics (Saffan, Glaxo) and muscle relaxants (Gallamine triethiodide) using two types of visual stimulators presenting two-dimensional (2D) motion stimuli and the visual cues for three-dimensional (3D) motion. All of the nine Siamese cats demonstrated Boston type retinotopic abnormalities in all of cortical areas 17–19 and CB. Very few binocular cells were present in areas 17–19 and the posterior (A1-P2) CB but they were numerous in most of CB (A9-4). A significant fraction (36/78) of binocular cells in the major CB of the Siamese cats demonstrated similar response selectivity to that reported in normal CB cortex for stimulation with the 3D motion cues under both null disparity and strabismic conditions (binocular receptive fields for two eyes were optically superposed or separated by the strabismic angles estimated in the individual animals). These findings indicate that the binocular signals converging to the CB cells through different pathways (signals coming from the contralateral eye via the ipsilateral hemisphere including the interlaminar nucleus and areas 17–19, and commissural signals from the ipsilateral eye via the contralateral areas 17–19 and CB) were integrated to yield useful information for the recognition of 3D motion, and that the major CB is an actual site of binocular integration at least in Siamese cats, rather than being merely a reflection of the information processing before the CB cortex.     

Investigations into the source of binocular input to the nucleus of the optic tract in an Australian marsupial,the wallaby Macropus eugenii

Recordings from direction-selective neurons in the nucleus of the optic tract (NOT) of the marsupial wallaby, Macropus eugenii, show that 53% of cells are sensitive to visual stimulation of both eyes. Anatomical tracing studies using horseradish peroxidase reveal many retinal terminals in the contralateral NOT but very few in the ipsilateral nucleus. There was no convincing evidence of cortical inputs to the ipsilateral NOT despite large injections of tracer into the visual cortex. During visual stimulation in the visual field of the contralateral eye with moving patterns, the excitatory responses in the NOT generated by ipsiversive motion (right-to-left when recording from the left NOT) were usually larger than the inhibitory responses produced by contraversive motion. Conversely, during ipsilateral eye stimulation, the negative motion components to contraversive motion were usually larger than the positive components to ipsiversive motion. This response pattern resembles that observed in the NOT of the American opossum, Didelphis aurita, where binocularity appears to arise through a commissural subcortical pathway that connects the two nuclei and inverts the directional tuning of the transmitted signals. We propose that the lack of significant input from the ipsilateral eye and cortex in the wallaby suggests that binocularity must arise from another pathway, possibly a commissural route between the nuclei. As directional information appears not to be carried by the internucleus pathway in rats and cats, our results suggest that binocularity in the NOT arises from different sources in marsupials as compared to eutherians. Electronic Publication     

Residual tectal projection from the contralateral central retina of the frog after homolateral optic nerve and main optic tract section

Summary After homolateral (right) optic nerve and main optic tract section a residual visual activity originating from the contralateral (left) central retina was recorded in the right optic tectum. Units were classified in three groups according to their receptive field properties: (1) slow-adapting units analogous to class 3 retinal ganglion cells; (2) fast-adapting post-synaptic units; (3) visual neurons. All of these units have in common a receptive field located near the projection of the left eye optic axis. Evidence that these units belong to the same visual pathway (i.e., the axial optic tract) is discussed.Supported by a grant from the CNRS (AI 3313)     

Electrophysiological correlates of presaccadic remapping in humans

Saccadic eye movements cause rapid displacements of space, yet the visual field is perceived as stable. A mechanism that may contribute to maintaining visual stability is the process of predictive remapping, in which receptive fields shift to their future locations prior to the onset of a saccade. We investigated electrophysiological correlates of remapping in humans using event-related potentials. Subjects made horizontal saccades that caused a visual stimulus to remain within a single visual field or to cross the vertical meridian, shifting between visual hemifields. When an impending saccade would shift the stimulus between visual fields (requiring remapping between cerebral hemispheres), presaccadic potentials showed increased bilaterality, having greater amplitudes over the hemisphere ipsilateral to the grating stimulus. Results are consistent with interhemispheric remapping of visual space in anticipation of an upcoming saccade.     

'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.     

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

Summary 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.The authors wish to dedicate this paper to Prof. W. Lierse in honour of his 60th birthday     

Chronic asymmetry in the extraocular muscles of adult cats: Stability in binocularity of cortical neurons

Summary During the first 2 weeks following unilateral severance of the 6 extraocular eye muscles in adult cats, the operated eye is partially immobile as shown by electrooculographic (EOG) recordings of horizontal eye movements. Although the motility of the operated eye improves with time (mainly in terms of amplitude but also with regard to direction and frequency of movements), it does not reach (up to 2 months) the level of the normal eye.Unit recording was done in visual cortex area 17 of these cats while paralyzed, either immediately (acute group, 5 cats), or 3–60 days (chronic group, 6 cats) following the operation.The number of visually inactive cells was slightly higher in the operated cats (13.8%) than in normal cats (8.3%), but the number of nonoriented cells or cells with disorganized receptive fields was similar in both groups of cats. The proportion of binocularly activated cells in the operated cats, especially in the chronic group (>10 days after the operation, 71.2% of 153 cells), was similar to that of the normal control cats (72.8% of 236 cells). No ocular dominance shift was found when either the operated eye was compared to the normal one or the ipsilateral eye to the contralateral one.It was concluded that distortion of afferent proprioceptive input from the extraocular eye muscles to visual centers has no effect on binocularity of cortical neurons in adult cats.     

The role of the corpus callosum in the development of interocular eye alignment and the organization of the visual field in the cat

Summary In young cats, the posterior portion of the corpus callosum was sectioned 13–29 days after birth. The animal's eyes were photographed at weekly intervals for six months using the pupil-reflex method. From the corneal reflection evident in the photographs the degree of alignment for the optical axes of each cat was estimated (Sherman, 1972). The 17 experimental cats all showed a significant tendency toward permanent divergent strabismus, as compared to six normal cats. The limits of the visual field were determined for both groups of cats using a perimetry technique similar to that of Sprague and Meikle (1965) and Sherman (1973). With one eye open normal cats responded from 90 ° ipsilateral to 45 ° past the vertical midline into the contralateral visual field. With either eye the experimental cats responded from 90 ° ipsilateral to approximately the vertical midline. The loss of visual responsiveness is within the contralateral region of the normally binocular zone. Three cats received the same operation at 9, 13, or 20 months old. Eye alignment and visual field perimetry were unaffected by the surgery. It is not known whether the observed abnormalities result from arrested development, or disruption of intrinsically determined ocular alignment.     

Topographic relationships of ipsi- and contralateral visual inputs to the rostral tectum opticum in the salamander Plethodon jordani indicate the presence of a horopter

Topographies of ipsilateral and contralateral retinal inputs to the optic tectum were studied by means of electrophysiological multi-unit recordings from the superficial layers. It was found that the nasotemporal coordinate of the visual field is represented along a rostrocaudal axis on the contralateral tectal map and along a caudorostral axis on the ipsilateral map. Electrical stimulation of one tectal hemisphere and recording of the response in the other hemisphere revealed that the ipsilateral map is most probably established by an intertectal pathway. In one tectal hemisphere, inputs from the ipsilateral eye match those from the contralateral eye if stimuli are located at a certain distance at arbitrary horizontal angles within the binocular field. This distance varied from 11.8 to 29.5 mm between the 5 individuals examined.