Foveal

About: Foveal is a research topic. Over the lifetime, 2652 publications have been published within this topic receiving 94120 citations.

Foveal fixation and tracking in the praying mantis

Abstract: 1. Visually guided head and body movements of restrained and freely moving mantids (Tenodera australasiae) have been studied by means of closed circuit television. Interest was concentrated on the different roles of the fovea and the periphery of the eye in controlling visuo-motor behaviour. 2. The peripheral eye is mainly responsible for the detection of novel objects (preferably potential prey) and the generation of ballistic (open-loop) saccadic head movements (Fig. 2) which bring the target image to the fovea (Figs. 3, 4, 5, 6). 3. Measurements on monocular animals show that the fovea ofeach eye is encircled by a saccade sensitive periphery (Figs. 5, 6). In other words each eye is capable of measuring any retinal position of the target image in a coordinate system whose origin is at the fovea. Based on this finding, a hypothesis, which is outlined in Sect. IV.7, suggests that the binocular coordination during fixation, tracking and distance estimation is based on the comparison of angular coordinates extracted by each eye from the position vector of the target. 4. Moving targets which have been fixated are held in the fovea either by smooth or saccadic tracking eye movements. The degree to which either tracking strategy is employed depends mainly on the features of the background, but to some extent also on the velocity of the target. 5. Targets which move against a homogeneous background are tracked by smooth eye movements (Fig. 7). Low target angular velocities are closely matched by the eye velocity. At high target speeds the head lags increasingly behind the target and saccades are periodically required to reduce the position error relative to the fovea. 6. Smooth pursuit eye movements, evoked either by a single target (Fig. 7) or a disrupted background (Fig. 8), are affected primarily by the velocity of the retinal image. While the effects of target and background are similar in this respect, they differ in others. Small objects in the foreground, subtending an angle of only a few degrees on the retina, evoke strong pursuit responses only when they resemble typical prey and project onto the fovea (Fig. 9). On the other hand, the image stabilisation of the background is a stereotyped response that can be evoked whenever a large part of the background moves across the visual field (Fig. 10a). Moreover, responses caused by a moving target in the fovea, and movements of the background in the periphery, are not combined additively (Fig. 10b). The foveal tracking response is weighted more strongly, but because the target is usually small, compared with the background, competing background motion can suppress smooth foveal tracking almost completely. 7. This limitation imposed upon the smooth pursuit system by the presence of a disrupted background (either a stripe pattern in the experimental set-up or grass and other plants in a natural setting) is avoided by the adoption of a strategy of saccadic tracking (Figs. 11, 12, 15, 16). This also applies for the tracking which immediately precedes the catching of prey (Fig. 15). Therefore, the stabilisation of the target image in place on the fovea is not a prerequisite for a successful strike. 8. Up to target angular velocities of about 100 °/s, saccadic tracking is predictive, i.e. the saccades have adequate amplitudes to bring the fovea right on target at the instant the saccade is completed (Fig. 13). This implies that the saccadic system processes not only position information of the target but velocity information as well. It is suggested that this velocity information is provided by the smooth pursuit system. Saccadic tracking would then reflect interactions of two circuits, the velocity coding circuit which in the presence of a homogeneous background also generates smooth pursuit head movements, and the position coding circuit which in the absence of target movement is able to generate saccades on its own.

Astrocytes and blood vessels define the foveal rim during primate retinal development.

Abstract: Purpose To investigate the relationship between development of the perifoveal blood vessels and formation of the foveal depression. Methods Retinal sections and flatmounts from monkeys aged between fetal day (Fd)80 and 2 years of age were double labeled using antisera to CD31 or von Willebrand factor to detect vascular endothelial cells and antiserum to glial fibrillary acidic protein to detect astrocytes. Sections were studied by fluorescence or confocal microscopy. Results From Fd88 to 115, vessels on the horizontal meridian were found only at the level of the ganglion cell layer (GCL)-inner plexiform layer (IPL) border where they form the ganglion cell layer plexus (GCP). Stellate astrocytes accompany GCP vessels and extend closer to the fovea than vessels. The foveal avascular zone was present within the GCP at Fd101, and at Fd105 a shallow foveal depression encircled by the GCP was present. The GCP foveal margin had the same dimensions as the adult foveal pit. Both blood vessels and astrocytes were excluded from the emerging fovea throughout development. After Fd140, capillary plexuses in the outer retina anastomosed with the GCP on the foveal slope to form a perifoveal plexus, but this plexus did not mature until a month or more after birth. After Fd142, astrocytes rapidly disappeared from the GCP and most of central retina. Conclusions An avascular area is outlined by the GCP before the foveal pit begins to form, suggesting that molecular factors in this region exclude both vessels and astrocytes. These factors may also guide neuronal migration to form the pit. Because the perifoveal plexus is formed during late gestation, both capillary growth and foveal development may be affected adversely by prematurity.

Foveal avascular zone and its relationship to foveal pit shape.

Abstract: Purpose.To investigate the retinal microvasculature at the fovea and peripheral retina in humans using an adaptive optics scanning laser ophthalmoscope (AOSLO) and to examine the association of foveal avascular zone (FAZ) and foveal pit morphology.Methods.Retinal imaging of the foveal capill

Distribution and development of short‐wavelength cones differ between Macaca monkey and human fovea

Abstract: Macaca monkey and humans have three cone types containing either long-wavelength (L), medium-wavelength (M), or short-wavelength (S)-specific opsin. The highest cone density is found in the fovea, which mediates high visual acuity. Most studies agree that the adult human fovea has a small S cone-free area, but data are conflicting concerning S-cone numbers in the adult Macaca monkey fovea, and little evidence exists for how either primate fovea develops its characteristic cone pattern. Single- and double-label in situ hybridization and immunocytochemistry have been used to determine the pattern of foveal S cones in both the fetal and adult Macaca and human. Both labels find a clear difference at all ages between monkey and human. Adult humans have a distinct but variable central zone about 100 microm wide that lacks S cones and is surrounded by a ring in which the S-cone density is 8%. This S cone-free zone is detectable at fetal week 15.5 (Fwk15.5), shortly after S opsin is expressed, and is similar to the adult by Fwk20.5. Adult monkey foveas have an overall S-cone foveal density of 10%, with several areas lacking a few S cones that are not coincident with the area of highest cone density. A surrounding zone at 200-microm eccentricity has an S-cone density averaging 25%, but, by 800 microm, this has decreased to 11%. Fetal day 77-135 monkeys all have a distribution and density of foveal S cones similar to adults, although the high-density ring is not obvious in fetal retinas. Estimates of the numbers of S cones missing in the fetal human fovea range from 234 to 328, whereas no more than 40 are missing in the fetal monkey. These results show that, in these two trichromatic primates, S-cone distribution and the developmental mechanisms determining S-cone topography are markedly different from the time that S cones are first detected.