Biomedical subjects

R Malach

Publications and source records attributed to R Malach.

44 records · Page 3Linked to original sources

PubMed1986

Mosaic architecture of the somatic sensory-recipient sector of the cat's striatum.

The striatum is known to have a compartmental organization in which histochemically defined zones called striosomes form branched 3-dimensional labyrinths embedded within the surrounding matrix. We explored how fiber projections from cortical somatic sensory areas representing cutaneous and deep-receptor inputs are organized in relation to this striatal architecture. Areas SI and 3a were mapped electrophysiologically, and distinguishable anterograde tracers (wheat germ agglutinin-HRP and 35S-methionine) were injected into physiologically identified loci. Primary somatic sensory corticostriatal projections were confined to a small, well-defined sector in the dorsolateral corner of the ipsilateral striatum. The somatic sensory afferents were arranged according to a coherent global body map in which rostral body parts were represented more laterally than caudal body parts. Single cortical loci innervated branched and clustered striatal zones that were reminiscent of the striosomes in their range of sizes and shapes yet lay strictly within the extrastriosomal matrix. In contrast to the global orderliness of the striatal body map, there were clear examples of locally complex patterns in which functionally distinct inputs interdigitated with each other. These patterns were often, but not always, produced when corticostriatal afferents carrying different submodality types were labeled. These findings demonstrate the existence of striosome-like striatal compartments within the seemingly uniform extrastriosomal matrix. The principle of mosaic organization thus holds throughout the tissue of the somatic sensory striatum. The striatal architecture delineated here could provide the anatomical substrate for computations requiring cross-modality comparisons within the framework of an overall somatotopy. If a similar multicompartmental architecture also characterizes other striatal regions, as seems likely, it may set general constraints on the nature of associative processing within the striatum as a whole.

Afferent Pathways↗

PubMed1984

Horizontal optokinetic nystagmus in the cat: recovery from cortical lesions.

We have examined recovery from the acute effects of unilateral and bilateral visual cortex lesions on horizontal optokinetic nystagmus (OKN) in the cat. A single bilateral cortical lesion virtually abolishes monocular OKN in response to temporalward stimulation, and severely reduces OKN in response to nasalward stimulation at higher drum velocities. A unilateral cortical lesion causes deficits in OKN toward the side of the lesion, but these changes are less substantial than those observed after a bilateral lesion. The nature of the acute changes in OKN caused by cortical lesions supports the notion that the visual cortex has two important roles in the OKN system of the cat: first, it mediates OKN in response to temporalward motion of the visual environment during monocular viewing; second, it expands the response range of the OKN system to include higher stimulus velocities. Finally, the results from unilaterally lesioned cats suggest that a single hemisphere can mediate OKN in both directions, with an emphasis on the direction ipsilateral to that hemisphere. The patterns of recovery of OKN in lesioned cats follow two major trends: a small, gradual increase in the overall gain of OKN from its severely depressed level immediately after a large bilateral lesion, and a 'balancing' process that reduces the marked asymmetry between rightward and leftward OKN caused by a unilateral lesion. This balancing of the reflex is accomplished by an improvement in performance in one direction while performance in the opposite direction actually declines. Recovery is faster and more pronounced in lesioned cats with one cortical hemisphere intact, suggesting that the surviving visual cortex compensates for the effects of the lesion by modifying activity in subcortical pathways. These experiments indicate that the visual system of the cat has the capacity to recover at least partially from the deficits in OKN behavior caused by cortical lesions, and that this recovery is the outcome of an interaction between cortical and subcortical pathways mediating this important visuomotor reflex.

Adaptation, Physiological↗

PubMed1984

Horizontal optokinetic nystagmus in the cat: effects of long-term monocular deprivation.

The effects of prolonged monocular deprivation (MD) on horizontal optokinetic nystagmus (OKN) have been examined in cats subjected to unilateral or bilateral visual cortex lesions. Presurgically , OKN elicited through the deprived eye was substantially weaker than that through the non-deprived eye. This effect was most prominent for OKN in response to temporalward stimulation, which essentially was abolished in the deprived eye. In addition, OKN elicited by temporalward stimulation of the non-deprived eye tended to be weaker in comparison to nasalward stimulation of that eye. A bilateral cortical lesion severely disrupted OKN behavior through the non-deprived eye but left OKN through the deprived eye relatively unaffected, with the result that the marked interocular differences in OKN that were present presurgically disappeared. During the recovery period following this lesion, there was a small gradual improvement in OKN through both eyes, so that OKN performance through the deprived eye ultimately exceeded that observed presurgically . Unilateral cortical lesions had little effect on OKN through the deprived eye, but they produced substantial changes in OKN through the non-deprived eye. Both the immediate effects of cortical lesions, and the patterns of recovery observed following these lesions, in many ways resemble those observed when normally reared cats are subjected to similar lesions. These behavioral experiments indicate that while subcortical OKN pathways are spared from the effects of long-term monocular deprivation, cortical pathways mediating OKN through the deprived eye are severely disrupted.

Animals↗

PubMed1984

Retino-pretectal projections in monocularly deprived cats.

Experiments using double-label anterograde tracers in normally reared and monocularly deprived cats reveal that the overall pattern of retino-pretectal projections appears similar for the deprived and non-deprived eyes of monocularly deprived cats, and no different from that observed in normally reared cats, in terms of both overall extent and amount of binocular overlap. These findings indicate that this pathway is unaffected by prolonged periods of monocular deprivation, suggesting that the presence of binocular overlap per se is not a sufficient condition for binocular competition to occur.

Animals↗

PubMed1984

Recovery from effects of brief monocular deprivation in the kitten.

The potential for recovery from the cortical effects of monocular deprivation (MD) was studied in kittens that were briefly deprived and then exposed to various periods of normal binocular vision. In eight kittens, recordings from the hemisphere ipsilateral to the deprived eye revealed that at 4 wk of age, exposure to 12 h of MD (six 2-h sessions spread over 2 days) was sufficient to cause a massive shift in the ocular dominance of striate cortex neurons in favor of the nondeprived eye. Six of these MD kittens were allowed 3 wk of normal binocular vision and then recorded from a second time to assess the extent to which their cortex could recover from the effects of this brief period of deprivation. Data from these animals indicated that now approximately equal numbers of cortical neurons were dominated by each eye and that, while the overall level of binocularity was somewhat lower than that found in normally reared animals, the majority of cells had regained functional binocular connections. The possibility that cortical binocularity could recover even further was explored by allowing four of these six MD kittens to experience an additional 4 wk of binocular vision and then recording from them a third time. These final recordings indicated that following a total of 7 wk of binocular vision, the level of cortical binocularity was no different from that found in normally reared animals. Having demonstrated that normal binocular function can be restored to a cortex in which it had been severely disrupted, we next attempted to characterize the earliest stages of this recovery process by examining the pattern of cortical binocularity in 10 MD kittens that were allowed to experience either 6 or 12 h of binocular vision (given over 1 or 2 days, respectively). Our results indicate that, during the initial day of binocular vision, recovery seems to involve a noncompetitive expansion of functional cortical input from the deprived eye, which joins with input from the nondeprived eye in driving cortical neurons. The level of cortical binocularity continues to increase during the next day of binocular vision, but now there is also a small increase in the proportion of cells driven exclusively by the initially deprived eye--suggesting that there may be an additional competitive component to the early stages of recovery. The results of this study complement our previous report of complete recovery of binocularity following exposure to a brief period of optically induced strabismus.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1981

Analysis of monocular optokinetic nystagmus in normal and visually deprived kittens.

The asymmetry of monocular OKN was assessed in normal, monocularly deprived (non-deprived eye) and strabismic cats. No significant differences were observed between the responses of these 3 groups. In young kittens, the gain of OKN for nasalward stimulus movement was larger than that for temporalward. This asymmetry declined to a low residual level by about one year of age. Neurophysiological recordings indicated that the symmetry of OKN is not related to cortical binocularity.

Aging↗

PubMed1981

Emmetropization: a vision-dependent phenomenon.

Anomalous visual experience during development has been shown to induce myopia in several species of animals. A retrospective analysis of refractive error among humans subjected to various ocular anomalies that disrupt pattern vision revealed a significant degree of myopia. This result suggests that emmetropization is a vision-dependent phenomenon.

Adult↗

PubMedDate not supplied

Relationship between orientation domains, cytochrome oxidase stripes, and intrinsic horizontal connections in squirrel monkey area V2.

Area V2, the main target of primary visual cortex projections, is characterized by a striking functional and connectional compartmentalization. Many aspects of this organization are correlated to three sets of stripes (thick, thin, and pale) revealed by cytochrome oxidase (CO) staining. Several questions related to the physiological properties of these compartments, their intrinsic connections, and points of similarity with area V1 modules are still unresolved. We have addressed some of these questions by combining the techniques of optical imaging of intrinsic signals, tract tracing, and CO histochemistry in the same patches of areas V1 and V2 of the squirrel monkey. The following observations were made. Orientation domains: in area V1 these are organized in narrow bands, while in area V2 they form patches. In area V2, domain width and distance between domains are approximately double that found in area V1. Orientation and CO stripe organization: orientation tuning was organized so that highly selective regions were centered on thick CO stripes while regions of broad orientation selectivity were centered on thin CO stripes. However, the orientation domains appeared to ignore borders between thick and pale stripes. Intrinsic connections: injections of the sensitive tracer biocytin into area V2 labeled a dense network of horizontally projecting fibers that were organized in columnar patches. Patches were small (mean width, 211 microns; mean length, 342 microns) and the labeling pattern extended over 4-5 mm. Axonal patches and CO stripes: Axonal patches found were in all three stripe compartments. However, injections that straddled the borders of thick/pale stripe compartments produced axonal projections that tended to cluster around border regions. Axonal patches and orientation domains: V2 injections produced labeling in V1 that appeared to be organized in narrow bands, reminiscent of orientation domain distribution in V1. Within area V2, axonal patches targeted a wide range of orientation domains, but appeared to avoid domains having orthogonal orientation preference to that found at the injection site. To conclude, our results show, on the one hand, a measure of functional specificity for the CO stripes and the intrinsic connections. On the other hand, they indicate additional substructures within area V2, whose precise relationship to the known compartmental organization remains to be clarified.

Animals↗