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PubMed2001

Distance perception within near visual space.

We investigated the perception of distance of visual targets with constant size and luminance presented between 20 and 120 cm from subjects' eyes. When retinal disparity cues were present, the subjects could reproduce very accurately the distance of a seen reference in this area. When only extraretinal information was available, distance perception was still correct for distances of 40 cm or less. However, distances beyond 60 cm were underestimated. When forced to evaluate the distance between a reference and themselves, e.g. when evaluating the absolute distance or half the distance or twice the distance of a reference, subjects used an egocentric plane of reference located on average 10.4 cm in front of their eyes. Measurements of binocular eye movements indicated a clear relationship between vergence angle and target distance. The egocentric plane of reference at 10.4 cm also corresponds to the maximum achievable vergence. These results suggest that ocular convergence can be used as a reliable cue for distance within the arm's reaching space.

Adult↗
PubMed1989

From visual sensations to the seen-now and the seen-from-here.

If, as Gibson last proposed, we become aware (in the introspective attitude during visual perceiving) of the seen-now and the seen-from-here as such, there must occur not only outer awareness of these surfaces, but also a kind of inner awareness of certain of their experiential effects upon us. From Gibson's perspective, this conclusion would seem to mean that his ecological account must return, in this context, to visual sensations; or preferably it must include visual-perspectival appearances proceeding normally unnoticed in the stream of consciousness during visual perception of the environment.

Attention↗
PubMed2003

Reality monitoring and visual hallucinations in Parkinson's disease.

Between 8 and 40% of Parkinson disease (PD) patients will have visual hallucinations (VHs) during the course of their illness. Although cognitive impairment has been identified as a risk factor for hallucinations, more specific neuropsychological deficits underlying such phenomena have not been established. Research in psychopathology has converged to suggest that hallucinations are associated with confusion between internal representations of events and real events (i.e. impaired-source monitoring). We evaluated three groups: 17 Parkinson's patients with visual hallucinations, 20 Parkinson's patients without hallucinations and 20 age-matched controls, using tests of visual imagery, visual perception and memory, including tests of source monitoring and recollective experience. The study revealed that Parkinson's patients with hallucinations appear to have intact visual imagery processes and spatial perception. However, there were impairments in object perception and recognition memory, and poor recollection of the encoding episode in comparison to both non-hallucinating Parkinson's patients and healthy controls. Errors were especially likely to occur when encoding and retrieval cues were in different modalities. The findings raise the possibility that visual hallucinations in Parkinson's patients could stem from a combination of faulty perceptual processing of environmental stimuli, and less detailed recollection of experience combined with intact image generation.

Aged↗
PubMed1991

Visual motion commands for pursuit eye movements in the cerebellum.

Eye movements that follow a target (pursuit eye movements) facilitate high acuity visual perception of moving targets by transforming visual motion inputs into motor commands that match eye motion to target motion. The performance of pursuit eye movements requires the cerebellar flocculus, which processes both visual motion and oculomotor signals. Electrophysiological recordings from floccular Purkinje cells have allowed the identification of their firing patterns during generation of the image velocity and image acceleration signals used for pursuit. Analysis with a method based on a behavioral model converted the time-varying spike trains of floccular Purkinje cells into a description of the firing rate contributed by three visual motion signals and one oculomotor input. The flocculus encodes all the signals needed to guide pursuit.

Animals↗
PubMed1988

Imagery limitations in totally congenitally blind subjects.

Research on totally blind subjects performing tasks that involve visual imagery has often shown that they do not behave differently from matched sighted subjects, even when their blindness is congenital. If visual imagery is based on visual perception, such tasks may not required visual imagery. In the present article visual images are considered as representations maintaining some properties of visible objects and constructed on the basis of information from various sources. Owing to the absence of visual experience, the limitations of such representations are explored in a series of experiments requiring memorization of single nouns, pairs of nouns, or triplets of nouns associated with a cue noun. Recall by blind subjects was impaired when multiple interactive images (with noun pairs and triplets) are formed. The poorer recall of blind subjects reflected also loss of order information. Recall was better for both groups with locative noun cues and high-imagery targets.

Adolescent↗
PubMed2005

What is the signal in noise?

Visual perception is limited by both the strength of the neural signals, and by the noise in the visual nervous system; however, little is known about what aspects of the input noise the human visual system is sensitive to, i.e., what is the signal in noise? In order to investigate this question we asked observers to discriminate differences in the strength of one-dimensional white noise. We measured their response consistency and classification images and compared the results with an ideal energy detector. Our results and modelling show that discrimination of noise is limited by the observer's template (i.e., the weighted combination of energy in each stimulus component) plus higher order nonlinearities (systematic noise), and by sources of random internal noise. We found that systematic noise is present only near detection threshold. Surprisingly, we found that the human template is "adaptive"-- its shape depends on the spatial frequency band of the noise-suggesting that sensitivity to spatial noise is not simply determined via passive filtering.

Discrimination, Psychological↗
PubMed2005

Synchrony dynamics in monkey V1 predict success in visual detection.

Behavioral measures such as expectancy and attention have been associated with the strength of synchronous neural activity. On this basis, it is hypothesized that synchronous activity affects our ability to detect and recognize visual objects. To investigate the role of synchronous activity in visual perception, we studied the magnitude and precision of correlated activity, before and after stimulus presentation within the visual cortex (V1), in relation to a monkey's performance in a figure-ground discrimination task. We show that during the period of stimulus presentation a transition in synchronized activity occurs that is characterized by a reduction of the correlation peak height and width. Before stimulus onset, broad peak correlations are observed that change towards thin peak correlations after stimulus onset, due to a specific decrease of low-frequency components. The magnitude of the transition in correlated activity is larger, i.e. a stronger desynchronization occurs, when the animal perceives the stimulus correctly than when the animal fails to detect the stimulus. These results therefore show that a transition in synchronous firing is important for the detection of sensory stimuli. We hypothesize that the transition in synchrony reflects a change from loose and global neuronal interactions towards a finer temporal and spatial scale of neuronal interactions, and that such a change in neuronal interactions is required for figure-ground discrimination.

Animals↗
PubMed1999

Do deaf people see better? Texture segmentation and visual search compensate in adult but not in juvenile subjects.

The research concerning the visual perception in deaf subjects has led to contradictory results: Deaf subjects have been reported to show enhanced visual perceptual skills compared to hearing subjects (Neville & Lawson, 1987). On the other hand, there are indications that acoustic deprivation may produce an inferiority in all sensory modalities (Myklebust, 1964). These contradictions may be due to methodological differences: The investigators selected different conditions (e.g. attentive/nonattentive) and various samples of deaf subjects (e.g., different age, language, and aetiology groups). In our study, we tested a large sample of deaf subjects with texture segmentation and visual search conditions, which allowed us to differentiate between visual processing with and without attentional load. All deaf subjects had profound hearing loss within the first year of life. Our results suggest that the visual processing capacity of deaf children and adolescents does not exceed that of age- and gender-matched hearing subjects. Rather, deaf school children show deficits in visual processing in conditions with and without attentional load. Age (6 to 20 years), language used (oral, sign, oral + sign), and aetiology for deafness (genetic, maternal rubella, perinatal, infection in the first year of life, unknown) did not consistently influence the results. The deficits in visual processing were partially compensated for in adult deaf subjects. The performances of deaf and hearing adults in trials that could be solved preattentively did not differ statistically significantly, but in attention-dependent trials the deaf subjects were more efficient than the hearing controls. We conclude that visual compensation for deafness is limited to attention-dependent tasks and does not develop until adulthood.

Adolescent↗
PubMed1984

Three gradients and the perception of flat and curved surfaces.

Researchers of visual perception have long been interested in the perceived slant of a surface and in the gradients that purportedly specify it. Slant is the angle between the line of sight and the tangent to the planar surface at any point, also called the surface normal. Gradients are the sources of information that grade, or change, with visual angle as one looks from one's feet upward to the horizon. The present article explores three gradients--perspective, compression, and density--and the phenomenal impression of flat and curved surfaces. The perspective gradient is measured at right angles to the axis of tilt at any point in the optic array; that is, when looking down a hallway at the tiles of a floor receding in the distance, perspective is measured by the x-axis width of each tile projected on the image plane orthogonal to the line of sight. The compression gradient is the ratio of y/x axis measures on the projected plane. The density gradient is measured by the number of tiles per unit solid visual angle. For flat surfaces and many others, perspective and compression gradients decrease with distance, and the density gradient increases. We discuss the manner in which these gradients change for various types of surfaces. Each gradient is founded on a different assumption about textures on the surfaces around us. In Experiment 1, viewers assessed the three-dimensional character of projections of flat and curved surfaces receding in the distance. They made pairwise judgments of preference and of dissimilarity among eight stimuli in each of four sets. The presence of each gradient was manipulated orthogonally such that each stimulus had zero, one, two, or three gradients appropriate for either a flat surface or a curved surface. Judgments were made were made for surfaces with both regularly shaped and irregularly shaped textures scattered on them. All viewer assessment were then scaled in one dimension. Multiple correlation and regression on the scale values revealed that greater than 98% of the variance in scale values was accounted for by the gradients. For the flat surfaces a mean of 65% of the variance was accounted for by the perspective gradient, 28% by the density gradient, and 6% by the compression gradient. For curved surfaces, on the other hand, a mean of 96% of the variance was accounted for by the compression gradient, and less than 2% by either the perspective gradient or the density gradient.(ABSTRACT TRUNCATED AT 400 WORDS)

Depth Perception↗
PubMed1977

Visual acuity and contrast sensitivity in multiple sclerosis--hidden visual loss: an auxiliary diagnostic test.

In 48 patients with multiple sclerosis sine-wave gratings were used to test visual sensitivity for coarse, medium, and fine detail rather than measuring visual acuity for fine detail only, as in conventional clinical tests. In 20/48 patients the test revealed a visual defect of neural origin, qualitatively different from that caused by refractive error. In 11 of these 20 patients, visual sensitivity to detail of medium coarseness was markedly degraded, even though sensitivity to both coarse and fine detail was unimpaired. In 3 of these 20 patients visual sensitivity to coarse detail was selectively degraded. These visual defects could not be detected by the Snellen test, yet the patient might experience visual problems in everyday life and also experience distorted visual perception. Possible neural bases for these visual impairments are discussed. Since 8 of the 14 patients with selective loss showed no clinical evidence of visual involvement, the test can aid the earlier diagnosis of multiple sclerosis.

Adolescent↗
PubMed2006

Psychophysiological diagnostics of functional states in sports medicine.

OBJECTIVES: The methods of psychological diagnostics ignored the integral criteria of functional states which are used in athletes. That is the reason why the elaboration of psychophysiological diagnostics criteria for higher qualification athletes was performed. BACKGROUND: The diagnostics of psychophysiological functions obtains information on individual and typological characteristics of higher neural system, peculiarities of forming and improvement of special experience and indicators of fatigue and hypertension of athletes. METHODS: The 26 higher qualification athletes (men), members of National team (Judo) were examined. The psychophysiological and neurodynamic functions such as functional mobility of nervous processes, strength of nervous processes, time perception, attention volume, operational thinking coefficient and short-term memory volume were registered by a computer system. The differential scale of functional states of psychophysiological functions in athletes was elaborated. RESULTS: The results of the investigation showed that 10 subjects had high parameters of functional states of psychophysiological functions and 14 have the intermediate level. The functional states of psychophysiological functions in higher qualification athletes characterize the functional system responsible for the results of sport. The optimization of the perception and information processing using the short-term memory reflects the psychophysiological compensatory mechanisms of a decline of visual perception and information processing capability in athletes. CONCLUSIONS: The psychophysiological states in higher qualification athletes are characterized by the functional system responsible for the result of sport (Tab. 7, Ref. 13).

Humans↗
PubMed1987

On the recognition of order and disorder.

We compare several algorithms for the recognition of ordered and disordered images. As image sources we use waves of the Belousov-Zhabotinskii reaction coupled to convective motion in a petri dish. This device allows reversibly the generation of periodic (ordered) and aperiodic (disordered) patterns. The best match between the parametric description and the observations is obtained by an "autodifference function". This function is computed by summing up intensity differences over all pairs of picture elements having a given distance on the picture plane. Then, the minimum of this function is determined upon variation of the distance. This algorithm is not only efficient for the recognition of order and disorder in "machine vision", but also plausible in biological visual perception.

Form Perception↗