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At least 649 records · Page 36Linked to original sources
PubMed1987

Scleral flap surgery for modification of corneal astigmatism.

We performed resections and recessions of scleral flaps on human cadaver eyes in order to measure the induced change in keratometric astigmatism. We prepared 3-mm scleral flaps that were 3, 5, 7, or 10 mm long and resected or recessed these flaps up to 1 mm in 0.25-mm increments. Scleral flap resection resulted in up to 10.1 D of net corneal steepening along the meridian of the incision. Scleral flap recessions resulted in up to 7.7 D of net corneal flattening along the meridian of the incision; the effect increased with increasing flap length in the recessed eyes. There was a tendency toward mean corneal flattening with resections and mean corneal steepening with recessions.

Astigmatism↗
PubMed1978

Visuotopic organization of the cebus pulvinar: a double representation the contralateral hemifield.

The projection of the visual field in the pulvinar nucleus was studied in 17 Cebus monkeys using electrophysiological techniques. Visual space is represented in two regions of the pulvinar; (1) the ventrolateral group, Pvlg, comprising nuclei P delta, P delta, P gamma, P eta and P mu 1; and (2) P mu. In the first group, which corresponds to the pulvinar inferior and ventral part of the pulvinar lateralis, we observed a greater respresentation of the central part of the visual field. Approximately 58% of the volume of the ventrolateral group is concerned with the visual space within 10 degrees of the fovea. This portion of the visual field is represented at its lateral aspects, mainly close to the level of the caudal pole of the lateral geniculate nucleus (LGN). Projection of the vertical meridian runs along its lateral border while that of the horizontal one is found running from the dorsal third of the LGN's hilus to the medial border of the ventro-lateral group. The lower quadrant is represented at its dorsal portion while the upper quadrant is represented at the ventral one. In Pmu the representation is rotated 90 degrees clockwise around the rostrocaudal axis: the vertical meridian is found at the ventromedial border of this nucleus. Thus, the lower quadrant is represented at the later portion of Pmu and the upper at its medial portion. Both projections are restricted to the contralateral hemifield.

Animals↗
PubMed1982

Response of raphe magnus neurons after acupuncture stimulation in rat.

Extracellular recording was made from single cells in the nucleus raphe magnus (NRM) of Wistar albino rats. The spontaneous firing rate of these cells increased, decreased or was not altered after acupuncture stimulation. Excitatory response of the cells was obtained more often than the other ones. There were no significant differences between the effects induced by stimulation of meridian and nonmeridian points. It is suggested that only the excited neuron may participate directly in the descending analgesic system through NRM cells activated by acupuncture stimulation. Presumably, meridian point is not a specific one for activation of NRM cells in rat.

Acupuncture Therapy↗
PubMed1988

The cortico-cortical connections within the parieto-temporal lobe of area PG,7a, in the monkey.

After injections of HRP into area PG(7a) labelled cells have been found in architectonic areas OA, PE, the cingulate and retrosplenial areas medial to area PG; posteriorly areas MST, OA (V4), V2, V3 and the cortex in the walls and floor of the superior temporal sulcus have also been labelled. Small injections placed in PG have resulted in different parts of these areas being labelled, suggesting that these cortico-cortical connections are well organized and raising the possibility of an ordered representation of the visual field in PG. It is suggested that the vertical meridian is around the boundary and the horizontal meridian passes antero-posteriorly across about the middle of its medio-lateral extent; the central part of the visual field is in the depths of the intraparietal sulcus, and the periphery is on the surface of the inferior parietal lobule and in the anterior wall of the upper part of the superior temporal sulcus. The lower visual field is medial and the upper field is lateral.

Animals↗
PubMed1986

Intracellular pattern reversal in Tetrahymena thermophila. II. Transient expression of a janus phenocopy in balanced doublets.

Homopolar doublets of Tetrahymena thermophila which have two normal oral systems directly opposite one another may undergo a global transformation of cell surface geometry to create transient imitations of mirror-image configurations brought about by mutations at janus gene loci. The process by which a typical doublet transforms into a janus-like organization involves loss of capacity to form oral structures at one of the two normal oral meridians, followed by interpolation of reversed oral structures at a new location to the cell's right of the disappearing normal oral meridian. At the same time, the contractile vacuole pore (CVP) set on the side of the cell that is undergoing the transformation shifts to the left. The combination of these events creates a symmetrical large-scale organization in which both of the CVP sets are situated on one side of the cell, between the normal and the partially reversed oral apparatus. This unilateral positioning of CVP sets is commonly manifested even when reversed oral structures are absent. These configurations probably represent intermediate stages in the transformation of balanced typical doublets into singlets. We propose that this pathway of regulation from the doublet to the singlet state, like the more common one that starts from unbalanced typical doublets (described in the preceding paper), involves reverse intercalation. The remarkable resemblance between the transient configuration described here and the stable configuration of janus mutant cells leads us to suggest that the phenotype of the mutant is also a consequence of reverse-intercalation, in that case provoked by a loss of capacity to maintain positional values rather than by a geometrical instability in the system of positional values.

Animals↗
PubMed1982

The early wave of the visual evoked potential to sinusoidal gratings: responses to quadrant stimulation as a function of spatial frequency.

VEPs were elicited by sinusoidal grating patterns of differing spatial frequencies presented in each of the 4 quadrants of the visual field. Analysis of the early wave of the VEP indicated a pattern of polarity inversions when wave forms were compared across the horizontal meridian of the visual field. With the exception of low spatial frequencies, clear polarity inversions also occurred across the vertical meridian of the visual field in the case of laterally placed electrodes. Comparison of the clearly identifiable early peak with records for the same subject using central field stimulation showed the complexity of the latter records and the inherent difficulty of isolating and interpreting specific component waves of the VEP without adequate topographical data from quadrant stimulation.

Brain Mapping↗
PubMed1991

The distribution and kinetics of visual pigments in the owl monkey retina.

An imaging fundus reflectometer has been used to study the distribution and regeneration of visual pigments in the retina of the owl monkey, Aotes trivirgatus. Measurements were made over an area of retina from 10 degrees nasal to 30 degrees temporal on the horizontal meridian, and from 5 degrees inferior to 30 degrees superior on the vertical meridian. The measured density differences vary with retinal location, with values in the central retina higher than those in more peripheral regions. The area of high density differences is roughly circular, with the highest values (approximately 0.3 log units) centred on or near the area centralis. Spectral measurements are consistent with a rod visual pigment absorbing maximally at about 518 nm, and indicate that the contribution of cone pigments to the imaging fundus reflectometer (IFR) data is negligible everywhere within the retinal area studied. The distribution of density differences is shown to correlate well with anatomical data for receptor and ganglion cell populations. Bleaching the visual pigment with brief intense lights leads to the extensive formation of the long-lived photoproduct metarhodopsin 3. Complete regeneration of rhodopsin following a full bleaching exposure (whether of brief or extended duration) takes more than 60 min and the time course of its recovery cannot be described accurately by first order kinetics.

Animals↗
PubMed1996

Residual vision with awareness in the field contralateral to a partial or complete functional hemispherectomy.

Two patients with unilateral disconnection or removal of the entire occipital lobe were tested for residual vision in their blind field. Using image stabilization to eliminate eye motion artifacts, the central portion of each subject's visual field was tested, beginning 1 degree from fixation and extending outward to 13.5 degrees. A narrow zone of residual vision was identified along the retinal vertical meridian of each patient. The lateral edge of this zone was generally within 3.5 degrees of the vertical meridian, though extended farther outward (but not beyond 6 degrees) at one field location for each subject. In one patient, it was present in both superior and inferior quadrants; in the other, it was confined to the superior quadrant. Within their zones of residual vision, both patients could detect stimuli and perform simple shape discriminations, but could not name complex line drawings. The patients were aware of their vision within this zone. No residual vision, with or without awareness, was found in areas tested outside these zones. Given the complete absence of visual cortex contralateral to the observed residual vision, alternate structures must be mediating these abilities.

Adolescent↗
PubMedDate not supplied

A nineteenth century view of the optic commissure.

Historically, ophthalmologists and anatomists have been intrigued by the optic nerve and its decussation. Isaac Newton was the first to describe and suggest the significance of the chiasmal crossing, while W.H. Wollaston defined the importance of hemiretinal differentiation in 1824. However, at that time, the vertical meridian separating nasal from temporal retina was believed to be located at the optic papillae. This theory was disproved in 1870 by two Philadelphia ophthalmologists, who demonstrated and documented the vertical meridian at the macula during their examination of an injured Civil War soldier.

History, 19th Century↗
PubMed1982

Why the cortical magnification factor in rhesus can not be isotropic.

Using the data of Hubel and Wiesel (1974) J. comp. Neurol. 158, 295-306, on macaque, the shape of the operculum of striate cortex (area 17) is calculated from their magnification factor values assuming that the cortical magnification factor is locally isotropic with respect to displacement on the cortical surface (from a fixed point of origin). Using geometric considerations, the shape predicted from the isotropic assumption is inconsistent with the actual shape given by anatomical reconstruction by LeVay, Hubel and Wiesel (1975) J. comp. Neurol. 159, 559-576. Rather the true shape is more consistent with the hypothesis that on the average the magnification factor along a meridian is twice the value as orthogonal to it. Except in the region of the horizontal meridian, this is often the same as assuming that the magnification factor across ocular dominance columns is twice that going parallel to them.

Animals↗
PubMed1983

Astigmatism of oblique incidence in the human model eye.

Two cylindrical lenses of equal power, one positive and one negative, were used to correct astigmatism of oblique incidence in a wide-angle optical model of the human eye. Tangential and sagittal vergences were calculated at different visual angles from the posterior pole to the peripheral retina. The positive cylinder was placed with its axis parallel to the tangential meridian while the negative cylinder was placed with its axis parallel to the sagittal meridian. Improvement in the quality of the retinal image at different angles of incidence was obtained by varying the angle between the axes of the correcting cylinders. Corresponding gains in resolution have been calculated and confirmed by peripheral fundus photography.

Astigmatism↗
PubMed1987

The projection of the visual field upon the retina of the pigeon.

Co-ordinates in the visual field of the pigeon eye were located in the eye cup anatomically by marking the position of the trans-scleral image. The retinal horizon and vertical meridian at azimuths 0 deg (frontal field) and 90 deg (lateral field) were located. Approximately 18 deg of binocular field are available in frontal vision and the frontal meridian projects to temporal retina outside the red field. Spatial distributions of ganglion cells, displaced ganglion cells, and centrifugal terminals were located in the eye cup. Retinal magnification factors in the posterior pole and temporal periphery are close to 120 microns/deg.

Animals↗
PubMed1991

A horizontal stripe of displacement sensitivity in the human visual field.

Displacement thresholds of peripheral sites in monocular human vision were obtained. The average of 12 directional thresholds at different visual field sites was used to define isometric lines of average displacement threshold about central vision. Isometric lines extended further into the temporal visual fields along the horizontal meridian than along other meridians. At any single site in the peripheral visual field the thresholds were not the same in all directions; they were larger toward and away from central vision. These two psychophysical findings vary in a qualitatively similar manner across the retinal field, as does the average size and the collected orientation bias of dendritic fields of retinal ganglion cells.

Fixation, Ocular↗
PubMed1995

The form of the human pupil.

The purpose of this study was to characterize the form of the pupil in normal human subjects. Using a modified slitlamp, photographs of pupils were taken in steady illumination and 10-20 sec after darkness. Transparencies were projected and digitized, and the pupil margin was represented as a circular Fourier series. Best-fit ellipses were also determined. The placement of the pupil relative to the limbus was determined in a number of subjects. Results from 23 subjects indicated that in both darkness and light, average pupil noncircularity was 0.0166. (A value of 0.0200 is easy to detect with the unaided eye from the photographs.) On average, the best-fit ellipse accounted for about half of the noncircularity (59.6% in darkness; 47.7% in light). Most of the contribution to shape was made by the first 4 or 5 harmonics. Shapes were usually stable within a session and could remain fairly stable for at least a year; however, shapes for different subjects were not very similar, especially in the light. (Average pairwise similarity: 0.106 in darkness; 0.034 in light; similarity can have values from -1 to 1.) For a given subject, shapes in light and dark were often fairly similar (average similarity 0.260), but there were systematic differences: in eyes where the ellipse contributed > 20% of noncircularity, ellipse major axes clustered around vertical in darkness, and horizontal in light, implying greater contraction near the vertical meridian. Even pupils with little elliptical contribution turned out to contract more near the vertical meridian. There was some tendency for left and right eyes of an individual to show mirror symmetry of shape. In the dark, pupils were located 0.27 +/- 0.09 mm nasal and 0.20 +/- 0.15 mm superior to the limbus center, and usually moved slightly further nasal or superior in the light. Noncircularity increased with age (0.0015/decade). It was concluded that pupils show individuality of shape, with significant regularities within and across subjects.

Adult↗
PubMed1994

Directional anisotropy of motion sensitivity across the visual field.

There is an implicit assumption in most models of movement direction perception that sensitivity to motion is directionally isotropic, i.e. equal for movement in all directions. However, most previous research on directional effects on human motion perception have measured responses (e.g. manual reaction times) to suprathreshold stimuli rather than motion sensitivity. I investigated this possibility by measuring monocular sensitivity to motion coherence in small field random-dot kinematograms with global leftwards or rightwards motion as a function of eccentricity along the horizontal meridian in each eye of six observers. Although foveal motion sensitivity was isotropic, small but significant differences (about 0.1 log units) in sensitivity in favour of centripetal motion were observed at eccentricities between 5.0 and 12.5 deg. Motion anisotropy was significantly larger in the temporal than the nasal visual hemifield. In a second experiment, motion coherence thresholds for upwards and downwards movement were measured foveally and at +/- 5 deg on the vertical meridian. At the fovea, no difference in sensitivity to upwards vs downwards motion was observed but these vertical axis thresholds were significantly higher than foveal thresholds for movement along the horizontal axis. Directional anisotropy in favour of centripetal motion was present in the inferior visual field but directional isotropy was found for the superior visual field. Perhaps this generally heightened sensitivity to centripetal directions may function normally to facilitate figure/ground segmentation in the presence of centrifugally-moving background contours that typically accompany forward locomotion.

Adolescent↗
PubMed1994

A polar coordinate system for describing binocular disparity.

When a meridional magnifier is introduced in front of one eye, a planar surface is perceived as slanted about a vertical axis. If the horizontal meridian is magnified, the perceived slant is away from the eye with the magnifier (geometric effect). If the vertical meridian is magnified, the slant is towards the eye with the magnifier (induced effect). While the geometric effect can be explained by the binocular horizontal disparities introduced by the horizontal magnifier, the induced effect has to be explained differently. Various models have been developed and the induced effect has generally been explained as a reinterpretation of horizontal disparity under a new reference of stereoscopic localization which is resultant from the vertical positional disparity introduced by the vertical magnifier. In this paper we describe binocular disparity in a polar coordinate system. Under this system, horizontal and vertical disparities are combined into a single stimulus variable, polar angle disparity. We show that the spatial distribution of polar angle disparity can faithfully describe the three-dimensional slant and inclination of a planar surface relative to the gaze normal plane. Both geometric and induced effects can be explained as direct responses to the polar angle disparity map distorted by the magnifier. Theoretical predictions based on the polar angle disparity are compared with experimental findings.

Depth Perception↗
PubMed1986

A right hemispatial field advantage on a verbal free-vision task.

Within a fixed time limit for each stimulus item, right handers searched, in a top-to-bottom scan and from right to left for half the subjects and from left to right for the other half, among randomly placed and oriented words for target words that rhymed with a designated sound. A right hemispatial field advantage emerged, as well as an advantage for targets above as compared to below the horizontal meridian. A lateral X vertical location interaction showed a right-side advantage only for targets below the horizontal meridian and an upper target advantage only for targets on the left. Even though the task was presented in free vision, results were highly similar to prior findings with lateralized tachistoscopic presentation of verbal stimuli.

Adolescent↗
PubMed1989

Molecular organization of type IV collagen: polymer liquid crystal-like aspects.

A new X-ray diffraction pattern from type IV collagen is described, which can be interpreted on the basis of crystalline and liquid crystalline origins of the reflections. Bovine anterior lens capsules extracted with 1 M NaCl and oriented by extension of 60% under constant load gave medium angle X-ray diffraction patterns showing many of the characteristics typical of liquid crystals. Prominent features, apart from those wide angle features attributable to the collagen triple helix, are (1) a four-point pattern of broad reflections at d-spacing 3.9 nm, and layer line spacing near 5 nm. (2) A broad intense equatorial peak centred at 1.24 nm, indicative of liquid-like lateral molecular associations. (3) A set of five sharp, streaked meridional reflections (previously obscured by the broad peak near 5 nm in unextracted capsules). (4) A further six higher angle reflections of a diffuse, arced and broad appearance on the meridian. The sharp streaked meridional reflections emanate from a long-range periodicity of units 8-9 nm in diameter. These features form a self-consistent system if interpreted on the basis of a staggered liquid crystal-like array of collagen molecules, in which case the first five meridionals and remaining broad reflections, sampled on the meridian, can all be indexed as orders of 21 nm.

Collagen↗