Foveal

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

Foveal tracking cells in the superior temporal sulcus of the macaque monkey

Abstract: Visual responses were recorded from neurons in the superior temporal sulcus (STS) of awake, behaving cynomolgus monkeys trained to fixate a small spot of light. Visual receptive fields, directionality, and responses during visual tracking were examined quantitatively for 50 cells in the foveal portion of the middle temporal (MT) visual area and surrounding cortex. Directionality indices and preferred directions for tracked and nontracked stimuli were compared. Eighteen cells (18/50 = 36%) were found to respond preferentially during tracking (tracking cells), 7 within MT, 9 in area FST on the floor of the STS, and 2 in unidentified areas. Three distinctly different tracking response profiles (VTS, VTO, and T) were observed. VTS and VTO cells had foveal receptive fields and gave directionally selective visual responses. VTS cells (3 in foveal MT, 6 in FST, 1 in an unidentified area) had a preferred visual direction that coincided with the preferred tracking direction, and began responding 50–100 ms before the onset of tracking. VTO cells (4 in foveal MT, 0 in FST, 1 in an unidentified area) had a preferred visual direction opposite to the preferred tracking direction, and began responding 0–100 ms after the onset of tracking. T cells (0 in MT, 3 in FST) had no visual responses and began responding simultaneously with the onset of tracking. It is suggested that this region of the brain could be the primary location for converting direction-specific visual responses into signals specifying at least the direction of an intended pursuit movement.

Visual acuity at different retinal loci of eccentrically fixating functional amblyopes.

Abstract: Visual acuity was measured in eccentrically fixating amblyopic subjects to determine the contribution of sensory (inhibitory) and motor (retinal-locus) components of the acuity loss. A unique aspect of this research involved the subjects' use of auditory feedback to control their unsteady and eccentric fixation while responding to flashed (800 msec) or continuously presented targets at various eccentricities in the visual field. All 4 amblyopic eyes had maximum visual acuity at the fovea; from there, the acuity declined approximately symmetrically into the nasal and temporal periphery. Foveal acuity for these amblyopic eyes was depressed from the acuity at the fovea of the normal eye. We conclude that Worth's explanation of eccentric fixation (eccentric fixation develops in an attempt by the amblyopic eye to fixate with a peripheral retinal locus having higher acuity than the fovea) cannot apply to our amblyopes. The acuity losses exhibited by the amblyopic eyes studied have a sensory (inhibition) component and a motor (retinal-locus) component, the sensory component being greater for small degrees of eccentric fixation and the motor component being greater for large amounts of eccentric fixation.

Effects of foveal ablation on the pattern of peripheral refractive errors in normal and form-deprived infant rhesus monkeys (Macaca mulatta).

Abstract: PURPOSE. The purpose of this study was to determine whether visual signals from the fovea contribute to the changes in the pattern of peripheral refractions associated with form deprivation myopia in monkeys. METHODS. Monocular form-deprivation was produced in 18 rhesus monkeys by securing diffusers in front of their treated eyes between 22 ± 2 and 155 ± 17 days of age. In eight of these form-deprived monkeys, the fovea and most of the perifovea of the treated eye were ablated by laser photocoagulation at the start of the diffuser-rearing period. Each eye's refractive status was measured by retinoscopy along the pupillary axis and at 15° intervals along the horizontal meridian to eccentricities of 45°. Control data were obtained from 12 normal monkeys and five monkeys that had monocular foveal ablations and were subsequently reared with unrestricted vision. RESULTS. Foveal ablation, by itself, did not produce systematic alterations in either the central or peripheral refractive errors of the treated eyes. In addition, foveal ablation did not alter the patterns of peripheral refractions in monkeys with form-deprivation myopia. The patterns of peripheral refractive errors in the two groups of form-deprived monkeys, either with or without foveal ablation, were qualitatively similar (treated eyes: F = 0.31, P = 0.74; anisometropia: F = 0.61, P = 0.59), but significantly different from those found in the normal monkeys (F = 8.46 and 9.38 respectively, P < 0.05). CONCLUSIONS. Central retinal signals do not contribute in an essential way to the alterations in eye shape that occur during the development of vision-induced axial myopia.

The effect of flankers on three tasks in central, peripheral, and amblyopic vision.

Abstract: Using identical stimuli and methods, we assessed the effects of flankers on three different tasks, orientation discrimination, contrast discrimination, and detection, in central, peripheral, and amblyopic vision. The goal was to understand the factors that limit performance of a task in the presence of flankers in each of these visual systems. The results demonstrate that: (1) For unflanked targets, the losses in peripheral and amblyopic vision (relative to the normal fovea) are ordered, with the loss of unflanked contrast discrimination thresholds considerably smaller than those for either detection or orientation discrimination. (2) For flanked targets, in normal foveal vision and anisometropic amblyopia, the critical distance is more or less proportional to the target size, whereas in peripheral and strabismic amblyopic vision, the critical distance shows much less (or no) dependence on target size. (3) For the normal fovea, and anisometropic amblyopia, when the target is large (>≈0.2 deg) the amount of threshold elevation induced by flankers is low, increasing when the target is very small. On the other hand, for the periphery and the amblyopic eyes of most strabismic amblyopes, the elevation is large over the range of sizes tested. (4) In peripheral and strabismic amblyopic vision, remote flankers elevate orientation discrimination and contrast discrimination thresholds but not detection thresholds. Our results show clearly that the effects of flanks depend on both the task and the type of visual system. We conclude that in normal foveal vision and anisometropic amblyopia, the effects of flankers largely reflects a reduction in visibility and may be explained by masking. On the other hand, in peripheral vision and strabismic amblyopia, the effects of flankers on orientation discrimination and to a lesser extent contrast discrimination cannot be explained by simple masking and are due to crowding.

ICAT: A Computational Model for the Adaptive Control of Fixation Durations

Abstract: Eye movements depend on cognitive processes related to visual information processing Much has been learned about the spatial selection of fixation locations, while the principles governing the temporal control (fixation durations) are less clear Here we review current theories for the control of fixation durations in tasks like visual search, scanning, scene perception, and reading and propose a new model for the control of fixation durations We distinguish two local principles from one global principle of control First, an autonomous saccade timer initiates saccades after random time intervals (Local-I) Second, foveal inhibition permits immediate prolongation of fixation durations by ongoing processing (Local-II) Third, saccade timing is adaptive, so that the mean timer value depends on task requirements and fixation history (Global) We demonstrate by numerical simulations that our model qualitatively reproduces patterns of mean fixation durations and fixation duration distributions observed in typical experiments When combined with assumptions of saccade-target selection and oculomotor control, the model accounts for both temporal and spatial aspects of eye-movement control in two versions of a visual search task We conclude that the model provides a promising framework for the control of fixation durations in saccadic tasks Psychonomic Bulletin & Review