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PubMed2000

How does the brain sustain a visual percept?

Perception involves the processing of sensory stimuli and their translation into conscious experience. A novel percept can, once synthesized, be maintained or discarded from awareness. We used event-related functional magnetic resonance imaging to separate the neural responses associated with the maintenance of a percept, produced by single-image, random-dot stereograms, from the response evoked at the onset of the percept. The latter was associated with distributed bilateral activation in the posterior thalamus and regions in the occipito-temporal, parietal and frontal cortices. In contrast, sustained perception was associated with activation of the pre-frontal cortex and hippocampus. This observation suggests that sustaining a visual percept involves neuroanatomical systems which are implicated in memory function and which are distinct from those engaged during perceptual synthesis.

Brain↗
PubMed1993

The limitation of visual perception in restorative dentistry.

Accurate judgement of size, depth and angle is required in the practice of restorative dentistry. Most assessments are made simply by visual examination, and these judgements are therefore often subjective; the limitation of visual perception renders such judgements inaccurate and subject to variation. The use of standardized objects to allow size or angle judgement by direct comparison improves accuracy, and their routine use may improve the quality of restorative treatment.

Dental Cavity Preparation↗
PubMed2004

Visual perception of extent and the geometry of visual space.

The question of how perceived extents are related to the corresponding physical extents is a very old question that has not been satisfactorily answered. The common model is that perceived extent is proportional to the product of image size and perceived distance. We describe an experiment that shows that perceived extents are substantially larger than this model predicts. We propose a model that accounts for our results and a large set of other results. The principal assumption of the model is that, in the computation of perceived extent, the visual angle signal undergoes a magnifying transform. Extent is often perceived more accurately than the common model predicts, so the computation is adaptive. The model implies that, although the perception of location and the perception of extent are related, they not related by Euclidean geometry, nor by any metric geometry. Nevertheless, it is possible to describe the perception of location and extent using a simple model.

Humans↗
PubMed2004

Distortions in the visual perception of shape.

It is known that visual illusions lead to a distorted perception of the length and orientation of lines, but it is not clear how these illusions affect the appreciation of the shape of closed forms. In this study two experiments were performed to characterize distortions in the visual perception of the shape of quadrilaterals and the extent to which these distortions were similar to the distortions of haptically sensed shapes. In the first experiment human subjects were presented with two quadrilaterals side by side on a computer monitor. One was a reference shape; the other was rotated and distorted relative to the first. The subjects used the computer mouse to adjust the corners of the distorted quadrilateral to match the shape of the target quadrilateral. They made consistent errors on this task: the adjusted quadrilateral was about 2% wider and about 2% shorter than the veridical shape. Furthermore, subjects adjusted the inner angles of the quadrilateral to make them closer to 90 degrees . The first type of error was also present in a second experiment in which, in a two-alternative forced-choice paradigm, subjects viewed a reference shape and were asked to indicate which of two transiently presented quadrilaterals was closest to the target shape. The width/height errors and the inner angle errors were comparable to those described previously when subjects felt the outline of a quadrilateral and then drew its reproduction in the absence of vision, suggesting that the distortion occurs in the process of remembering the shape.

Adult↗
PubMed1981

Visual perception of verticality and horizontality among elderly fallers.

Patients who had suffered repeated falls or a single fall which led to hospitalization were tested for errors in visual perception of the verticality or horizontality of a rod presented on a video screen. Fallers had significantly greater average error scores than nonfallers. Fallers were more likely to be elderly or hemiplegic, and these conditions were also associated with elevated error scores. However, when age or hemiplegia was removed as a factor in the analysis, perceptual errors were still significantly greater in fallers than in nonfallers.

Accidents, Home↗
PubMed2004

Pictures, preparations, and living processes: the production of immediate visual perception (anschauung) in the late-19th-century physiology.

This paper addresses the visual culture of the late-19th-century experimental physiology. Taking this case of Johann Nopomuk Czermak (1828-1873) as a key example, it argues that images played a crucial role in acquiring experimental physiological skills. Czermak, Emil Du Bois-Reymond (1818-1896) and other late-19th-century physiologists sought to present the achievements and perspective of their discipline by way of "immediate visual perception (unmittelbare Anschauung)." However, the images they produced and presented for this purpose were strongly mediated. By means of specifically designed instruments, such as the "cardioscope," the "contraction telegraph," and the "frog pistol," and specifically constructed rooms, so-called "spectatoriums," physiologists trained and controlled experiments on their own. Studying the material culture of physiological image production reveals that technological resources such as telegraphy, photography, and even railways contributed to making physiological facts anschaulich. At the same time, it shows that the more traditional image techniques of anatomy played an important role in physiological lecture halls, especially when it came to displaying the details of vivisection experiments to the public. Thus, the images of late 19th century physiology stood half-way between machines and organisms, between books and instruments.

History, 19th Century↗
PubMed1995

Use of filters to treat visual-perception problem creates adherents and sceptics.

Patients who think they have a visual-perception dysfunction known as scotopic sensitivity-Irlen syndrome have trouble reading and may experience almost-constant headaches. Some find they are helped by coloured filters developed by a California researcher, Helen Irlen, who published a book on the subject called Reading by the Colors. Although Irlen has been criticized for not publishing scientific proof of the validity of her theories, her techniques have found some support, including some within the medical community.

Adult↗
PubMedDate not supplied

Monkey perirhinal cortex is critical for visual memory, but not for visual perception: reexamination of the behavioural evidence from monkeys.

Overdependence on discrimination learning paradigms to assess the function of perirhinal cortex has complicated understanding of the cognitive role of this structure. Impairments in discrimination learning can result from at least two distinct causes: (a) failure to accurately apprehend and represent the relevant stimuli, or (b) failure to form and remember associations between stimulus representations and reward. Thus, the results of discrimination learning experiments do not readily differentiate deficits in perception from deficits in learning and memory. Here I describe studies that do dissociate learning and memory from perception and show that perirhinal cortex damage impairs learning and/or memory, but not perception. Reanalysis and reconsideration of other published data call into further question the hypothesis that the monkey perirhinal cortex plays a critical role in visual perception.

Animals↗
PubMedDate not supplied

[The psychophysiological basis of the compensatory development of visual perception in vision disorders].

At comparison of the data obtained in psychophysical and electrophysiological studies, disturbances were revealed of neurophysiological mechanisms of visual perception in children with the diseases of retinocortical tract. For compensation of the disturbances and development of perception a method is suggested of specific activation of neuronal formations based on their fundamental property--plasticity. The method consists in using rhythmical repeated stimulation of eyes with images which widely vary by the brightness, contrast, colour, form, orientation and spatial position in the visual field. Experimental verification of the method was conducted by a special program of correction studies with children having weak and residual sight. After 2-3 years of studies a significant increase was observed of probability of correct recognition of images by one parameter or combination of two parameters, increase of perception volume and improvement of its other properties. It is supposed that activation of sensory information processes lowers the level of deprivation and mobilizes reserve mechanisms of compensation in the projection and associative areas of the cerebral cortex.

Adaptation, Physiological↗
PubMed2000

Visual perception of dynamic properties: cue heuristics versus direct-perceptual competence.

Constructivist and Gibsonian approaches disagree over the possibility of direct perceptual use of advanced information. A trenchant instance concerns visual perception of underlying dynamic properties as specified by kinematic patterns of events. For the paradigmatic task of discrimination of relative mass in observed collisions, 2 mathematical models are developed, 1 model representing a direct, invariant-based approach, and 1 representing a cue-heuristic approach. The models enable a critical experimental design with distinct predictions concerning performance data and confidence ratings. Although pretraining results were mixed, the invariant-based model was empirically confirmed after a minimal amount of training: Competence entails the use of advanced kinematic information in a direct-perceptual ("sensory") mode of apprehension, in contrast to beginners' use of simpler cues in an inferential ("cognitive") mode.

Biomechanical Phenomena↗
PubMed1984

Are theories of perception necessary? A review of Gibson's The Ecological Approach to Visual Perception.

Representational theories of perception postulate an isolated and autonomous "subject" set apart from its real environment, and then go on to invoke processes of mental representation, construction, or hypothesizing to explain how perception can nevertheless take place. Although James Gibson's most conspicuous contribution has been to challenge representational theory, his ultimate concern was the cognitivism which now prevails in psychology. He was convinced that the so-called cognitive revolution merely perpetuates, and even promotes, many of psychology's oldest mistakes. This review article considers Gibson's final statement of his "ecological" alternative to cognitivism (Gibson, 1979). It is intended not as a complete account of Gibson's alternative, however, but primarily as an appreciation of his critical contribution. Gibson's sustained attempt to counter representational theory served not only to reveal the variety of arguments used in support of this theory, but also to expose the questionable metaphysical assumptions upon which they rest. In concentrating upon Gibson's criticisms of representational theory, therefore, this paper aims to emphasize the point of his alternative scheme and to explain some of the important concerns shared by Gibson's ecological approach and operant psychology.

Animals↗
PubMed2000

Visual perception and stimulus orientation in cattle.

The pupil in the eye of adult cattle is oval under contraction with the long axis nearly horizontal. Based on simple optophysical facts it is hypothesised that visual perception in such eyes is different for stimuli with vertically-separated details rather than stimuli with horizontally-separated details. This hypothesis was tested with three adult dairy bulls using an operant conditioning technique. The bulls had to discriminate a solid white line from broken white lines with decreasing interspaces. They solved this task better when the stimuli were presented vertically rather than horizontally. This result is discussed in terms of visual acuity and related to the topographical anatomy of the eye, particularly the pupil.

Animals↗
PubMed2006

Understanding the neural correlates of visual perception: a survey of our toolbox.

One of the central goals of neuroscience is to elucidate the link between behavior and neuronal activity at both single neuron level and the level of neuronal ensembles. New analytical approaches and techniques constantly emerge that correlate neural activity with perception and behavior with ever finer temporal and spatial scales. In this article I review some of these approaches employed in studies of visual perception in which perceptual reports were required from subjects. I divide these methods into 'correlational' methods and 'perturbation' methods; the former measures normal neural activity and correlates it with behavior, the latter perturbs normal neural activity to cause behavioral alterations. I compare such approaches applied to primates with those applied to human subjects, and discuss how these approaches could be combined and made more powerful in the future.

Algorithms↗
PubMed2004

Visual perception in Parkinson disease dementia and dementia with Lewy bodies.

OBJECTIVE: To quantify visual discrimination, space-motion, and object-form perception in patients with Parkinson disease dementia (PDD), dementia with Lewy bodies (DLB), and Alzheimer disease (AD). METHODS: The authors used a cross-sectional study to compare three demented groups matched for overall dementia severity (PDD: n = 24; DLB: n = 20; AD: n = 23) and two age-, sex-, and education-matched control groups (PD: n = 24, normal controls [NC]: n = 25). RESULTS: Visual perception was globally more impaired in PDD than in nondemented controls (NC, PD), but was not different from DLB. Compared to AD, PDD patients tended to perform worse in all perceptual scores. Visual perception of patients with PDD/DLB and visual hallucinations was significantly worse than in patients without hallucinations. CONCLUSIONS: Parkinson disease dementia (PDD) is associated with profound visuoperceptual impairments similar to dementia with Lewy bodies (DLB) but different from Alzheimer disease. These findings are consistent with previous neuroimaging studies reporting hypoactivity in cortical areas involved in visual processing in PDD and DLB.

Activities of Daily Living↗