Foveal

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

Effective Connectivity within Human Primary Visual Cortex Predicts Interindividual Diversity in Illusory Perception

Abstract: Visual perception depends strongly on spatial context. A classic example is the tilt illusion where the perceived orientation of a central stimulus differs from its physical orientation when surrounded by tilted spatial contexts. Here we show that such contextual modulation of orientation perception exhibits trait-like interindividual diversity that correlates with interindividual differences in effective connectivity within human primary visual cortex. We found that the degree to which spatial contexts induced illusory orientation perception, namely, the magnitude of the tilt illusion, varied across healthy human adults in a trait-like fashion independent of stimulus size or contrast. Parallel to contextual modulation of orientation perception, the presence of spatial contexts affected effective connectivity within human primary visual cortex between peripheral and foveal representations that responded to spatial context and central stimulus, respectively. Importantly, this effective connectivity from peripheral to foveal primary visual cortex correlated with interindividual differences in the magnitude of the tilt illusion. Moreover, this correlation with illusion perception was observed for effective connectivity under tilted contextual stimulation but not for that under iso-oriented contextual stimulation, suggesting that it reflected the impact of orientation-dependent intra-areal connections. Our findings revealed an interindividual correlation between intra-areal connectivity within primary visual cortex and contextual influence on orientation perception. This neurophysiological-perceptual link provides empirical evidence for theoretical proposals that intra-areal connections in early visual cortices are involved in contextual modulation of visual perception.

Drug treatments for eye movement disorders

Abstract: Advances in the treatment of eye movement disorders The modern rationale for the treatment of abnormal eye movements rests on current concepts of the neurobiology of ocular motility and vision.1 In order to see clearly the details in our visual world, images must be held quite still upon the retina, especially the central, foveal part, which has the highest density of photoreceptors. In order to read, which concerns detection of high spatial frequencies, image motion should ideally be less than about 5°/s.2 If image drift substantially exceeds this limit, visual acuity will decline and the illusion that the world is moving (oscillopsia) may be experienced. Normally, three main mechanisms hold gaze (the line of sight) steady, so that our view of the world is clear and stable.2 The first is the vestibulo-ocular reflex, by which the motion detectors of the inner ear initiate eye movements to compensate for head perturbations, such as occur during locomotion. The second mechanism is “visual fixation,” which has two components: the detection of retinal image drift and programming of corrective eye movements, and the suppression of unwanted eye movements that take the eye away from the target. The third mechanism depends on a neural network that makes it possible to hold the eyes at an eccentric position (for example, in lateral gaze). Malfunction of any of these three mechanisms may cause the eyes to drift away from the object of regard; corrective, quick phases (saccades) may then redirect the fovea towards the target. Thus a definition of nystagmus is repetitive to and fro involuntary eye movements that are initiated by slow drifts of the eye. It is important to differentiate pathological nystagmus from physiological nystagmus. During rotation of the head and body in space, both vestibular nystagmus and optokinetic nystagmus act to …

Extra-foveal Colour Metrics

Abstract: A series of monochromatic equal-brightness stimuli of subtense 40′ x 80′ was established through the spectrum by direct comparison with a neighbouring 530 mμ patch of retinal illumination 10-8 phot...

The photoreceptors in atypical achromatopsia.

Abstract: 1. The receptoral mechanisms underlying the vision of two atypical achromats of the complete variety were studied with standard psychophysical procedures. 2. Under scotopic conditions the spectral sensitivity of each achromat was well described by the CIE (Commission Internationale de l'Eclairage) scotopic sensitivity function and the recovery of sensitivity after a retinal bleach showed characteristic duplex behaviour with the time constant of recovery of the slower phase matching that of normal rod vision for both foveal and peripheral stimulation. 3. Their spectral sensitivity was measured under conditions of chromatic adaptation in order to reveal any residual middle or long wavelength cone activity. Only one photopic spectral responses was found and this was adequately described by the spectral sensitivity function of Stiles pi 3 mechanism of normal vision. 4. Increment threshold measurements as a function of background intensity revealed a double-branched function in the fovea. The lower branch was found to have the spectral sensitivity of the rods; the upper branch that of Stiles' pi 3 mechanism. Stiles-Crawford measurements of directional sensitivity confirmed that the branch with the rhodopsin action spectrum had the directional sensitivity of rods and that the branch with the action spectrum of pi 3 had the directional sensitivity of cones. 5. These was no evidence for hue discrimination under photopic conditions. Regions of apparently normal performance on hue discrimination tests on more careful examination could be explained by luminosity judgements mediated by short wavelength-absorbing receptors. 6. We reject the notion of there being rhodopsin-filled cones in the fovea of these subjects. The foveal and peripheral vision of each of these achromats can be adequately described in terms of the participation of only two types of receptor, namely normally functional rods under scotopic conditions and normally functioning short wavelength-absorbing cones under photopic conditions. They are therefore functional blue mono-cone monochromats, an explanation which was originally proposed by Blackwell & Blackwell (1957) over thirty years ago.

Texture segmentation as a function of eccentricity, spatial frequency and target size.

Abstract: Two experiments examined the role of fundamental spatial frequency, target area and retinal eccentricity in texture segmentation. In Experiment 1, a backward-masked target comprising lines oriented orthogonally to the surround was briefly presented at the fovea, and at eccentricities ranging from 2.55 to 7.63 deg. Reaction time and accuracy were better when targets were presented at non-foveal locations. In Experiment 2, eccentricity effects were examined when both spatial frequency and target area were varied. Accuracy was highest and RT fastest at near-peripheral, not foveal locations. The eccentricity corresponding to optimal performance was related inversely to spatial frequency. Results suggest that the near periphery is more adept than the fovea at the early processing which underlies rapid texture segmentation.