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At least 667 records · Page 37Linked to original sources
PubMed2006

Preservation of Emmert's law in a visual form agnosic.

Size constancy was investigated in DF, a patient with visual form agnosia, using a technique based on Emmert's law of visual after-images. DF was first given a task in which she was asked to indicate the distance of a vertical surface and a task where she had to estimate the width of a series of squares (widths ranging from 5 cm to 35 cm) placed at varying distances and having a constant visual angle. In the distance estimation task, DF greatly overestimated the distance of the vertical surface placed in front of her. DF also had great difficulty performing the size estimation task. DF then performed a task in which she stared at a bright 5 cm square for a brief period of time at a distance of 30 cm followed by the presentation of a vertical surface which varied in distance and was asked to indicate the width of the after-image either verbally or manually. DF's after-images conformed to the size-distance relationship predicted by Emmert's law--as the distance of the vertical surface increased her perception of the size of the after-images also increased. These data demonstrate that although DF is rather impaired in tasks that require explicit estimates of size and distance, at some level, DF must have relatively intact size constancy mechanisms given that her estimates of the width of the after-image conform to Emmert's law. Thus, the processes underlying explicit judgements of size and distance appear to differ from those underlying the size and distance scaling of after-images.

Adult↗
PubMed1986

A study of systematic visual perseveration involving central mechanisms.

The subject of this investigation experiences vivid after-images which persist for tens of seconds following stimulation by light insufficiently intense to produce after-images in normal subjects. The after-images occur at all points in the visual field, and have spatial structure closely similar to that of the stimuli which elicit them, the only distortion being the absence of high (greater than or equal to 10 cycles X deg-1) spatial frequencies. The duration of the after image is dependent on the spatial structure and spectral content of the eliciting stimulus, being maximum for low, nonzero spatial frequencies (1-2 cycles X deg-1) and yellow or green wavelengths. The after-images appear 'khaki' or 'mustard-green' in colour, regardless of the spectral composition of the light stimulus, and those produced by flickering or moving targets do not themselves exhibit temporal fluctuations, although movement produces a streak along the path described by the moving object. After six weeks of treatment with the anticonvulsant drug, carbamazepine, the duration of the after-images fell by about 30%, but no further reduction occurred over the following three months. In spite of the prolonged after-images, the subject has normal sensitivity for detection of flicker and movement, but that for detection of a single flash is raised abnormally at low background illumination. Neither CT nor MRI brain scans revealed any abnormality. These responses are discussed in relation to those found in other subjects suffering visual perseveration or palinopsia, and we examine their possible causes.

Adult↗
PubMed1998

Blindsight and shape perception: deficit of visual consciousness or of visual function?

Two people with homonymous right hemianopias were tested on a number of measures of non-conscious and conscious perception of shape in the blind field. Experiment 1 examined preparatory manual adjustments in grasping objects. Both subjects performed well above chance not only in three-dimensional location but also in preforming the hand to the shape, size and orientation of objects. In Experiment 2 single upper-case letters were briefly exposed in the blind field, and subjects made forced choices among 6 alternatives in the sighted field. Performance improved over blocks of trials and was above chance, but not dramatically. In Experiment 3 single upper-case words were briefly presented in the blind field, and subjects chose which of two words exposed after in the intact field was semantically closer. In Experiment 4 subjects had to give the meaning of single ambiguous words (e.g. BANK) presented both visually in the intact field and auditorily. Each ambiguous word was preceded by a single upper-case word briefly presented in the blind field, biasing each meaning on different blocks of trials (e.g. MONEY and RIVER). In Experiment 3, although results were in the appropriate direction, they were not consistently well above chance. By contrast, in Experiment 4 both subjects were consistently semantically biased to a high degree by words in the blind field. Experiments 2, 3 and 4 taken together suggest that indirect techniques (priming) are more sensitive to showing effects of non-conscious perception than direct ones (forced-choice). More importantly the experiments indicate that not only orientation but curvature, structural descriptions of component strokes and spatial ordering of letters are registered non-consciously in the blind field. Experiment 5 examined after-images in the blind and sighted fields, showing veridical conscious perception of shape in the blind field provided it was accompanied by a shape in the sighted field which together formed a good Gestalt. Experiment 6 showed conscious perception of illusory contours spanning the hemifields induced by Kanizsa figures. The experiments suggest that aspects of shape are much better perceived in blindsight than previously thought, that this is independent of their use in motor control, that the main deficit in blindsight is one of consciousness, and that the loss of conscious vision in the blind field is far from total. The effects and their relationship to those in other neuropsychological deficits suggest an intimate link between perceptual consciousness, attention and object perception.

Afterimage↗
PubMed2006

Local and global motion after-effects are both enhanced in migraine, and the underlying mechanisms differ across cortical areas.

Visual after-effects are illusions that occur after prolonged viewing of visual displays (pattern adaptation). The motion after-effect (MAE), for example, is an illusory impression of motion that is seen after viewing moving displays. After-effects have been used extensively in basic vision research as well as in clinical settings, and have been reported to be enhanced in migraine. Pattern adaptation is a cortical phenomenon that reflects both cellular mechanisms acting within individual neurons and specific interactions between groups of neurons activated by the adapting display. A remarkable feature of the MAE is that its duration is only slightly reduced if a delay is inserted between the end of the adaptation and the test display ('storage'). The reduction is consistent with recovery of the cellular component, and the residual with network changes that are maintained during the delay. This study aimed (i) to assess explanations for prolonged MAEs in migraine by teasing apart the proposed cellular and network components of adaptation using storage; (ii) to determine the extent of cortical abnormality in migraine using local and global MAEs, which reflect adaptation at different stages of the visual system. Fifty migraine (22 with, 28 without aura) and 50 control participants adapted to motion before viewing a stationary or dynamic (random motion) test, which consequently appeared to move in the opposite direction (local and global MAEs, respectively). Half of the trials included a delay between the adapting and test displays. The results extend those reported previously, as both local and global MAEs lasted longer in migraine compared with the control group. Global MAEs survived delays almost completely for both groups, whereas local MAEs were reduced to a greater extent in migraine. There were no significant differences between migraine subgroups classified according to the presence or absence of visual aura. These results suggest that cellular recovery is slowed in migraine for early but not later visual cortical areas. Sustained network changes following adaptation are implicated across cortical areas. Differences between people with and without migraine on various measures of visual perception have been attributed to abnormal cortical processing in migraine, variously described by hyperexcitability, heightened responsiveness and/or a lack of intra-cortical inhibition. The results are not consistent with hyperexcitability resulting from a lack of inhibition in migraine, but are consistent with extended suppression of intra-cortical excitation. The implications of these results for alternative models of hyperexcitability are discussed.

Adaptation, Physiological↗
PubMed2005

Effects of Alzheimer's disease on visual enumeration.

Speeded enumeration of visual objects typically produces fast and accurate performance for up to 3 to 4 items (subitization) but slower and less accurate performance thereafter (counting). We investigated enumeration ability in patients with Alzheimer's disease (AD) and in age-matched controls. AD patients were slower overall than controls. The subitizing span was significantly reduced in AD patients compared with controls (2.3 vs 3.5 items) and counting rate was significantly slower (451 vs 349 ms/item). Error rates were similar in the two groups except at numerosity 3, when AD patients made errors but controls did not (consistent with their subitizing spans). Within the AD patient group, several aspects of performance correlated significantly with Mini-Mental State Examination scores. Together, the results provide a striking contrast with studies showing preservation of enumeration ability in normal aging.

Afterimage↗
PubMed1971

Sensory anomalies.

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Adaptation, Ocular↗