Spatio-temporal contrast sensitivity, coherent motion, and visible persistence in developmental dyslexia. (1/121)

Three experiments measured spatio-temporal contrast sensitivity, coherent motion, and visible persistence in a single group of children with developmental dyslexia and a matched control group. The findings were consistent with a transient channel disorder in the dyslexic group which showed a reduction in contrast sensitivity at low spatial frequencies, a significant reduction in sensitivity for coherent motion, and a significantly longer duration of visible persistence. The results were also examined by classifying the dyslexic group into dyseidetic, dysphonetic, and mixed (dysphoneidetic) subgroups. There were no differences between the control and dyseidetic groups in contrast sensitivity, in coherent motion and in visible persistence. In comparison to the control group, the mixed (dysphoneidetic) dyslexic subgroup was found to have a significant reduction in contrast sensitivity at low spatial frequencies, a significant reduction in sensitivity for coherent motion, and a significantly longer duration of visible persistence. In comparison to the control group, the dysphonetic group only showed a reduction in contrast sensitivity at low spatial frequencies. Comparisons between the dyseidetic, dysphonetic and mixed dyslexic subgroups showed that there were no substantive differences in contrast sensitivity, coherent motion, and visible persistence. The results support the proposal and findings by Borsting et al. (Borsting E, Ridder WH, Dudeck K, Kelley C, Matsui L, Motoyama J. Vis Res 1996;36:1047-1053) that a transient channel disorder may only be present in a dysphoneidetic dyslexic subgroup. Psychometric assessment revealed that all the children with dyslexia appear to have a concurrent disorder in phonological coding, temporal order processing, and short-term memory.  (+info)

Enhanced motion aftereffect for complex motions. (2/121)

We measured the magnitude of the motion after effect (MAE) elicited by gratings viewed through four spatial apertures symmetrically positioned around fixation. The gratings were identical except for their orientations, which were varied to form patterns of global motion corresponding to radiation, rotation or translation. MAE magnitude was estimated by three methods: the duration of the MAE; the contrast required to null the MAE and the threshold elevation for detecting an abrupt jump. All three techniques showed that MAEs for radiation and rotation were greater than those for translation. The greater adaptability of radiation and rotation over translation also was observed in areas of the display where no adapting stimulus had been presented. We also found that adaptation to motion in one direction had equal effects on sensitivity to motion in the same and opposite directions.  (+info)

Colour at edges and colour spreading in McCollough effects. (3/121)

Broerse and O'Shea [(1995) Vision Research, 35, 207-226] proposed that the subjective colours in McCollough effects (MEs) consist of two components: edge colours appearing along the edges of contours, and spread colours radiating from edge colours into adjacent uncontoured regions of test patterns. This proposal was examined in five experiments. First, we demonstrated that fine coloured lines located immediately adjacent to the edges of otherwise achromatic square-wave gratings (i.e. colour-fringed gratings) are sufficient to induce MEs comparable in strength to MEs induced with desaturated versions of traditional uniformly-coloured gratings (Experiments 1 & 2). We then quantified edge and spread colours while varying light/dark duty cycles (white-bar width) in gratings with colour-fringed edges (Experiment 3), uniformly-coloured gratings (Experiment 4), and in achromatic gratings tinged with ME colours after adaptation to colour-fringed gratings (Experiment 5). Whereas the perceived magnitude of edge colours remained constant in all cases, spread colours remained constant only for uniformly-coloured gratings. For both MEs and gratings with colour-fringed edges, spread colours decreased as a function of increasing duty cycle, confirming that conventional MEs may be simulated by gratings with colour-fringed edges. We propose that edge colours arise as a consequence of neural operations correcting for the eye's chromatic aberration, while spread colours reveal a neural filling-in process operating to achieve colour constancy. In seeking to implement these suggestions, we present a putative framework based on the receptive-field properties of single cells described in contemporary neurophysiological investigations of colour.  (+info)

Afterimages, grating induction and illusory phantoms. (4/121)

Under some conditions (dark or light inspection areas) illusory gratings often appear to be in-phase with the inducing gratings and under others (gray inspection area) illusory gratings often appear to be out-of-phase with the inducing gratings. McCourt reported that point-by-point brightness matches reveal only out-of-phase illusory gratings, no matter what the luminance of the inspection area (McCourt, M. E. (1994). Vision Research, 34, 1609-1617). Since the technique used might have led to afterimages which mimic out-of-phase illusory gratings, the present series of experiments was undertaken to determine how such afterimages might bias illusory grating judgments. Afterimages were induced during fixation with brief flashes of inducing gratings within the inspection area (Experiment 1), or by vertical shifts in the entire stimulus which exposed the retina to real gratings prior to judgments within the inspection area (Experiment 2). Experiment 2 was replicated with drifting inducing gratings (Experiment 3). The subjects were asked to indicate whether illusory gratings appeared in- or out-of-phase. The results of all three experiments reveal that out-of-phase illusory gratings predominate, and that afterimages can only bias judgments with stationary displays. It is suggested that grating induction is perceived when subjects attend to local contrast differences, while phantom visibility is facilitated when attention is captured by the more global aspects of the stimulus.  (+info)

A diagnostic sign in migraine? (5/121)

At the bedside it was noted that, after ocular fundoscopy, patients with migraine complained more often of an after-image than did non-migraineurs. This phenomenon was then investigated in consecutive patients attending a general neurology outpatient clinic. The relative risk for the diagnosis of migraine in patients reporting an after-image was 2.91 (95% confidence interval 1.96 to 4.34), and the sensitivity, specificity and positive predictive value of this observation for the diagnosis of migraine were 0.63, 0.75 and 0.55 respectively. After-images were equally likely to be reported by migraineurs with and without aura, and by patients with migraine equivalents. The after-image phenomenon probably reflects the heightened sensitivity to visual stimuli of patients with migraine. Although a diagnosis of migraine is primarily established by the patient's history, the presence of an after-image following ocular fundoscopy may support this diagnosis.  (+info)

Spatial aspects of object formation revealed by a new illusion, shine-through. (6/121)

When a vernier stimulus is presented for a short time and followed by a grating comprising five straight lines, the vernier remains invisible but may bequeath its offset to the grating (feature inheritance). For more than seven grating elements, the vernier is rendered visible as a shine-through element. However, shine-through depends strongly on the spatio-temporal layout of the grating. Here, we show that spatially inhomogeneous gratings diminish shine-through and vernier discrimination. Even subtle deviations, in the range of a few minutes of arc, matter. However, longer presentation times of the vernier regenerate shine-through. Feature inheritance and shine-through may become a useful tool in investigating such different topics as time course of information processing, feature binding, attention, and masking.  (+info)

Shine-through: temporal aspects. (7/121)

If a vernier stimulus precedes a grating for a very short time, the vernier either remains invisible, but may bequeath some of its properties to the grating (feature inheritance), or might shine through keeping its features - depending on the number of grating elements [Herzog, M. H. & Koch, C., 2001. Seeing properties of an invisible element: feature inheritance and shine-through. Proceedings of the National Academy of Science USA 98, 4271-4275]. Feature inheritance and shine-through represent two different states of feature binding [Herzog, M. H., Koch, C., & Fahle, M., Switching binding states. Visual Cognition (in press)], whereas shine-through depends in subtle ways on the spatial layout of the grating [Herzog, M. H., Fahle, M., & Koch, C., (2001). Spatial aspects of object formation revealed by a new illusion, shine-through Vision Research]. Here, we show that also temporal parameters of the grating influence shine-through. For example, a delayed presentation of certain grating elements can deteriorate performance dramatically.  (+info)

Are corresponding points fixed? (8/121)

Several investigators have claimed that the retinal coordinates of corresponding points shift with vergence eye movements. Two kinds of shifts have been reported. First, global shifts that increase with retinal eccentricity; such shifts would cause a flattening of the horopter at all viewing distances and would facilitate fusion of flat surfaces. Second, local shifts that are centered on the fovea; such shifts would cause a dimple in the horopter near fixation and would facilitate fusion of points fixated at extreme viewing distances. Nearly all of the empirical evidence supporting shifts of corresponding points comes from horopter measurements and from comparisons of subjective and objective fixation disparity. In both cases, the experimenter must infer the retinal coordinates of corresponding points from external measurements. We describe four factors that could affect this inference: (1) changes in the projection from object to image points that accompany eye rotation and accommodation, (2) fixation errors during the experimental measurements, (3) non-uniform retinal stretching, and (4) changes in the perceived direction of a monocular point when presented adjacent to a binocular point. We conducted two experiments that eliminated or compensated for these potential errors. In the first experiment, observers aligned dichoptic test lines using an apparatus and procedure that eliminated all but the third error. In the second experiment, observers judged the alignment of dichoptic afterimages, and this technique eliminates all the errors. The results from both experiments show that the retinal coordinates of corresponding points do not change with vergence eye movements. We conclude that corresponding points are in fixed retinal positions for observers with normal retinal correspondence.  (+info)