Deafferentation and axotomy of neurons in cat striate cortex: time course of changes in binocularity following corpus callosum transection

Cortical Neuromodulation of Remote Regions after Experimental Traumatic Brain Injury Normalizes Forelimb Function but is Temporally Dependent

Traumatic brain injury (TBI) results in well-known, significant alterations in structural and functional connectivity. Although this is especially likely to occur in areas of pathology, deficits in function to and from remotely connected brain areas, or diaschisis, also occur as a consequence to local deficits. As a result, consideration of the network wiring of the brain may be required to design the most efficacious rehabilitation therapy to target specific functional networks to improve outcome. In this work, we model remote connections after controlled cortical impact injury (CCI) in the rat through the effect of callosal deafferentation to the opposite, contralesional cortex. We show rescue of significantly reaching deficits in injury-affected forelimb function if temporary, neuromodulatory silencing of contralesional cortex function is conducted at 1 week post-injury using the γ-aminobutyric acid (GABA) agonist muscimol, compared with vehicle. This indicates that subacute, injury-induced remote circuit modifications are likely to prevent normal ipsilesional control over limb function. However, by conducting temporary contralesional cortex silencing in the same injured rats at 4 weeks post-injury, injury-affected limb function either remains unaffected and deficient or is worsened, indicating that circuit modifications are more permanently controlled or at least influenced by the contralesional cortex at extended post-injury times. We provide functional magnetic resonance imaging (MRI) evidence of the neuromodulatory effect of muscimol on forelimb-evoked function in the cortex. We discuss these findings in light of known changes in cortical connectivity and excitability that occur in this injury model, and postulate a mechanism to explain these findings.

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

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

Visual hemispheric dominance induced in split brain cats during development: a model of deficient interhemispheric transfer derived from physiological evidence in single visual cortex cells

The effects of cancellation of both interhemispheric callosal transfer and interocular interactions, were studied in early monocularly deprived cats. The main purpose of this study was therefore to prove whether unilateral hemispheric dominance would result under these conditions and to what extent each hemisphere will be functionally independent. Secondly, we have attempted to establish such an experimental model physiologically, on the single cell level. Interhemispheric transfer was surgically canceled by sagittal transection of the corpus callosum. In addition, the ocular projections were separated by sagittal transection of the optic chiasm in the transbuccal approach. This condition had practically induced visual split brain condition in these cats. These manipulations were carried out concurrently with monocular deprivation (SBDK group) which was surgically done by eye closure during the critical period of development of the visual system. Thus, the hemisphere ipsilaterally to the visually deprived eye had developed under conditions of deficient visual experience while the hemisphere ipsilaterally to the normal eye had developed under conditions of unaltered visual experience. A group of cats (SBK) similarly operated but equally binocularly exposed during development was served as controls. In addition, adult cats similarly operated during adulthood either chronically or acutely were studied to evaluate the effects of interhemispheric and interocular separation. Other groups of cats were also studied for comparison, and included sham operated and normal adult cats. At adulthood, electrophysiological studies were done on these cats, in which action potentials were extracellularly recorded from single cells in the visual cortex (area 17-18 boundary) following anesthesia and paralysis. Stimulation was carried out manually and by a computer driven optical system, presenting on a tangent screen light bars at various spatial positions, orientations and directions. Receptive fields were thus mapped for all neurons and their dimensions and eccentricities were measured. The responsiveness, ocular dominance and other parameters were also studied for these cells. The results in the early deprived cats and in their controls, had shown a full separation between the two hemispheres, as reflected in the almost absolute ipsilateral eye responsiveness (> 97.0% cells). In comparison, in the sham operated and in the normal control cats only minor proportions of cells (13.0-18.7%) have been found as ipsilaterally and monocularly driven, showing almost full interhemispheric and interocular interaction. The main difference, however, in the results between the early monocularly deprived cats and their controls is that in the first group the two hemispheres were asymmetric concerning the amount of visual activation and in the second one they were very symmetric.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

Cortical cells' physiology following visual split brain in developing cats

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

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.

Global stereopsis in human albinos

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

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

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

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

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

The deafferented visual cortex: neuronal activity and visual evoked potentials

The callosal transfer of information to the visual cortex following its unilateral deafferentation from its geniculate input was studied in both hemispheres. Deafferentation was performed in adult cats by sectioning the optic tract. Action potentials of single cortical cells and visual evoked potentials were recorded from area 17-18 boundary in acute and chronic operated cats. In the deafferented hemisphere, cortical cells were usually visually inactive. However, some recovery of function took place in this hemisphere in the chronic cats, as expressed by the increase in the proportion of S-cells. In the intact hemisphere diminution of responsiveness and reduction of selectivity to the stimulus orientation and direction were found. The responsiveness and selectivity level in the intact hemisphere increased with postoperative time. The ocular dominance distribution in this hemisphere was similar to that of our normal control cats. The characteristics of the visual evoked potentials were in keeping with the hemispheric dominance of the cortical cells found in the experimental cats. It was concluded that a plasticity related mechanism is involved in the recovery of callosal activation of visual cortical cells following deafferentation.