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S M Zeki

Publications and source records attributed to S M Zeki.

At least 19 recordsLinked to original sources
PubMed1991

The brightness-sense comparison in patients with optic nerve hypoplasia.

The brightness-sense has been assessed for a cohort of individuals with optic nerve hypoplasia, and for a group of normal subjects. It was found to be significantly impaired in the worst eye as compared to the fellow eye in patients with asymmetrical bilateral optic nerve hypoplasia. No difference in brightness between the two eyes was perceived in two patients with segmental optic nerve hypoplasia. The brightness-sense comparison test has not hitherto been applied to patients with optic nerve hypoplasia. The test is simple to use and provides an additional investigative tool for the clinician to detect optic nerve dysfunction.

Adolescent↗
PubMed1991

Reappraisal of the ratio of disc to macula/disc diameter in optic nerve hypoplasia.

The ratio of disc to macula/disc diameter is characteristically increased in eyes with optic nerve hypoplasia. We present the largest reported series of patients with a definitive diagnosis of optic nerve hypoplasia for whom this ratio has been determined. All measurements were made by an independent masked observer. Our results are in accordance with previous reports. A ratio of 2.94 provides a one-tailed upper population limit of 95%. An attempt has been made to correlate optic disc size and visual acuity. In 75% of bilateral cases the eye with the relatively smaller optic disc was found to have a better Snellen visual acuity than the fellow eye. This suggests that additional pathogenic mechanism(s) may have determined the eventual visual outcome in such eyes. Such mechanisms include macular hypoplasia, high refractive error, refractive amblyopia, central scotoma, and optic atrophy.

Adolescent↗
PubMed1990

Optic nerve hypoplasia and astigmatism: a new association.

Thirty-one patients with optic nerve hypoplasia (ONH) or septo-optic dysplasia and two patients with segmental ONH underwent retinoscopy. The results were compared with those of 20 normal subjects. There was a higher prevalence of astigmatism in the patients than in the controls. Two patients with segmental ONH had no evidence of astigmatism. The association of astigmatism with ONH has not hitherto been reported. The close association of ONH with astigmatism highlights the importance of performing careful retinoscopy in children with ONH or septooptic dysplasia in order to identify and correct errors of refraction, thereby optimising visual function and diminishing the likelihood of the development of superadded amblyopia.

Adolescent↗
PubMed1990

Optic nerve hypoplasia in children.

Optic nerve hypoplasia (ONH) is characterised by a diminished number of optic nerve fibres in the optic nerve(s) and until recently was thought to be rare. It may be associated with a wide range of other congenital abnormalities. Its pathology, clinical features, and the conditions associated with it are reviewed. Neuroendocrine disorders should be actively sought in any infant or child with bilateral ONH. Early recognition of the disorder may in some cases be life saving.

Abnormalities, Multiple↗
PubMed1988

Photocoagulation of raised new vessels by long-duration low-energy argon laser photocoagulation--a preliminary study.

Direct laser treatment of retinal neovascularisation is indicated when regression has not been brought about by retinal photocoagulation. Current treatment regimens involve multiple laser applications to the neovascular complex. Long-duration low-energy burns have the advantage of slowly heating the tissue without engendering disruption. We report the use of this treatment as a means of occluding the feeder vessels to a neovascular network. The application of argon blue-green laser treatment at 0.1 watt for 60 seconds at two adjacent points on a feeder vessel was found to give rise to permanent vascular occlusion without causing complications.

Adult↗
PubMed1979

Fucntional specialization and binocular interaction in the visual areas of rhesus monkey prestriate cortex.

If is is believed that neural mechanisms mediating stereoscopic vision may be localized in specific areas of the visual cortex, then it becomes necessary to be able to define these areas adequately. This is no easy matter in the rhesus monkey, an animal close to man, where the cytoarchitecturally uniform prestriate cortex is folded into deep sulci with secondary gyri. One way around this awkward problem is to use the callosal connections of the prestriate cortex as the anatomical landmarks. Callosal connections are restricted to regions at which the vertical meridian is represented. Since the visual fields, including the vertical meridian, are separately represented in each area, each has its own callosal connections. These are of great help in defining some of the boundaries of these areas, since the boundaries often coincide with the representation of the vertical meridian. With the visual areas thus defined anatomically, it becomes relatively easy to assign recordings to particular areas. Studies of binocular interactions in these areas reveal that most cells in all prestriate areas are binocularly driven. Hence, theoretically, all of the prestriate areas are candidates for stereoscopic mechanisms. The degree of binocular interaction varies from cell to cell. At the two extremes are cells which either respond to monocular stimulation only and are inhibited by binocular stimulation or ones which respond to binocular stimulation only. Changing, as opposed to fixed, disparity is signalled by two types of cells. In one category are cells activated in opposite directions for the two eyes. Such cells are always binocularly driven. In the other category are cells, some of which are monocularly activated, that are capable of responding to changing image size. In the monkey, both these categories of cells have so far been found in the motion area of the superior temporal sulcus only.

Animals↗
PubMed1978

The topographic organization of rhesus monkey prestriate cortex.

1. The topographic organization of prestriate visual cortex in the rhesus monkey has been studied both anatomically, by determining the pattern of termination of fibres passing through the corpus callosum, and physiologically, in the same animals, by plotting receptive field positions for different recording sites. Results are displayed on two-dimensional, "unfolded" maps of the cortex in the dorsal half of the occipital lobe. 2. Transcallosal fibres terminate in a narrow strip of cortex along the boundary between striate and prestriate areas and in a separate, broader, zone occupying much of the anterior bank of the lunate sulcus, the annectant gyrus, and the parietooccipital sulcus. The detailed pattern of inputs is highly complicated but shows considerable similarities from one animal to the next. 3. Physiological recordings confirmed earlier reports that regions where transcallosal fibres terminate correspond to representations of the vertical meridian in the visual field. This relationship is most precise along the striate-prestriate boundary and along the boundary of area V3 farthest from V1; it is less precise within area V4, where the pattern of transcallosal inputs is more complex. 4. A distinct, topographically organized visual area, named V3A, was found in the region between areas V3 and V4 in the lunate and parieto-occipital sulci. Area V3A differs from V2 and V3 in that both superior and inferior visual quadrants are represented in a single region of the dorsal occipital lobe. 5. The contralateral visual field is represented in a suprisingly complex fashion in areas V3A and V4. Within each area there are multiple representations of some, but perhaps not all, parts of the visual hemifield. It is unclear whether V3A and V4 should be more appropriately considered as sets of distinct sub-areas, each representing only a portion of the hemifield, or as larger areas with complicated internal topographies. 6. Most cells in areas V2, V3 and V3A are orientation selective but not selective for stimulus colour or direction of movement. In contrast, area V4 contains a higher incidence of colour selective cells and a lower incidence of orientation selectivity. These results support the notion of a functional division of labour within the prestriate cortex.

Animals↗
PubMed1978

The cortical projections of foveal striate cortex in the rhesus monkey.

1. The cortical projections of the foveal and extrafoveal parts of the striate cortex have been compared, using conventional degeneration techniques, as well as combinations of anatomical methods. While both foveal and extrafoveal striate cortex share a common pattern of projections (to areas V2, V3 and the visual area in the medial part of the posterior bank of the superior temporal sulcus), foveal striate cortex was found to have an additional projection (to part of the cortex of the fourth visual areas, V4). The latter projection includes the posterior lip of the inferior occipital sulcus which, on anatomical grounds, is regarded as the ventral extension of V4. 2. Anatomical studies using double tracers were employed to clarify the nature of the projections from the striate cortex and from V2 to V4. In one such experiment, tritiated proline was injected into extra-foveal striate cortex and a small lesion was made in that part of V2 receiving a direct projection from the region of the striate cortex into which the radioactive tracer was injected. Only degenerating fibres (due to the lesion), and no radioactive label, was found in V4. Such an experiment showed that, unlike foveal striate cortex, the projections from extrafoveal striate cortex to V4 are not direct, but through V2. 3. In another type of anatomical experiment using double tracers, the corpus callosum was sectioned and tritiated proline was injected into foveal striate cortex. Such an experiment allowed a more accurate determination of the extent of V4, as judged from its callosal connexions, to which foveal striate cortex projects. 4. Considering the projections of V1 to areas V2, V3 and the visual area in the medial part of the posterior bank of the superior temporal sulcus, and considering the differences in the projections of foveal and extrafoveal striate cortex, it is suggested that, among other functions, the striate cortex acts as a distribution centre for the information coming over the retino-geniculo-cortical pathways, parcelling this information out to different visual areas of the prestriate cortex for further analysis.

Animals↗
PubMed1978

The third visual complex of rhesus monkey prestriate cortex.

1. Two independent but neighbouring visual areas, V3 and V3A, sharing a common cytoarchitectural plan, but in each one of which the visual fields are separately represented, have been studied anatomically, functionally, and in combined anatomico-physiological experiments. 2. The properties of single cells in the two areas are so similar, judged by the techniques used in this study, that it is often impossible to tell whether any one penetration was sampling from cells in V3 or V3A. This is especially so if the cells have receptive fields in the lower hemi-quadrants, since the vertical meridian of the lower visual fields is represented along the V3-V3A boundary and since a transition from V3 to V3A along this border is not accompanied by a shift in receptive field positions of cells. 3. Since the visual fields, including the vertical meridian, are separately represented in these two areas, and since regions of vertical meridian representation are callosally connected, a simple and certain method of specifying the boundary between V3 and V3A is to examine the degeneration following section of the callosal splenium. A heavy patch of degeneration then marks the V3-V3A boundary. Within this patch, however, is a sub-patch containing fewer callosal fibres, or none at all. The boundary between V3 and V3A was taken to be at this subpatch. 4. Since the horizontal meridian is represented at the V2-V3 boundary, and since V1 projects to both these areas, sending coarse fibres to V3 and fine fibres to V2, it was found that the boundary between V2 and V3 could be precisely drawn by making a lesion in the horizontal meridian representation in V1 and noting where, in the prestriate cortex, fine fibres give way to coarse ones, without an intervening gap. 5. Double tracer anatomical experiments, in which tritiated proline was injected into V1 of animals whose callosal splenium had been sectioned, showed that whereas V3 receives a direct input from V1, V3A does not. V3A, instead, was found to receive an input from V3. Double tracer anatomical experiments were undertaken to study a possible input from V2 to V3A. Although such experiments did not reveal a direct input from V2 to V3A, they were not entirely conclusive. 6. The vast majority of cells in V3 and V3A were binocularly driven, without obvious monocular preferences. Some cells, however, though responding to stimulation of the individual eyes, summated their responses to binocular stimulation. Others responded only when both eyes were simulataneously stimulated. In any oblique penetration, cells preferring binocular stimulation only occurred either singly or in groups. 7. In an oblique penetration, the shift from a cell responding to binocular stimulation only to one responding equally well to stimulation of either eye was not necessarily accompanied by a shift in orientational preferences, shifts in the former...

Animals↗
PubMed1978

Uniformity and diversity of structure and function in rhesus monkey prestriate visual cortex.

1. Recordings were made from single neurones, or small clusters of cells, in five prestriate visual areas of rhesus monkey cortex. The cells were studied for their binocularity, as well as for their orientational, motion and colour preferences. In all, 1500 cells were studied, 250 cells for each of the areas V2, V3, V3A and the motion area of the posterior bank of the superior temporal sulcus, and 500 cells for V4. All the cells referred to in this study can be placed in one prestriate area or another unambiguously. 2. The great majority of cells in all areas were binocularly driven, without monocular preferences. Within each area, there were cells that either preferred binocular stimulation markedly, or were responsive to binocular stimulation only. The ocular interaction histograms for all areas are remarkably similar when tested at a fixed disparity. 3. Over 70% of the cells in areas V2, V3 and V3A were selective for orientation. The receptive fields of cells were larger in V3 and V3A than in V2. By contrast, less than 50% of the cells in V4 and the motion area of the superior temporal sulcus were orientation selective. 4. Directionally selective cells were found in all areas. But they were present in small numbers (less than 15%) in areas V2, V3, V3A and V4. By contrast, 90% of the cells in the motion area of the superior temporal sulcus were directionally selective. 5. 8% of the cells in V2 had opponent colour properties. Cells with such properties were not found in V3, V3A or in the motion area of the superior temporal sulcus. By contrast, 54% of the cells in the V4 complex had opponent colour properties. 6. It is argued that despite its uniformity in cytoarchitectural appearance and in ocular interaction patterns, there is a functional division of labour within the prestriate cortex. Evidence for this is seen not only in the different concentrations of functional cell types in distinct areas of the prestriate cortex, but also in the differential anatomical and callosal connexions of each area.

Animals↗