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At least 1,657 records · Page 92Linked to original sources
PubMed2006

Effects of depth, eccentricity and size of additional static stimulus on visually induced self-motion perception.

Static visual stimulus presented behind a moving pattern inhibits vection. On the other hand, the same static stimulus facilitates vection if it is located in front of the moving pattern. In this study, the effects of depth, eccentricity and size of the additional static stimulus on inhibition and facilitation of horizontal linear vection were investigated. Results indicated that both inhibition and facilitation become conspicuous with increasing the size of the static stimulus. Furthermore, the inhibition caused by the static foreground is dominated by the central stimulus, while the facilitation caused by the static background is more effective in the peripheral stimulus.

Depth Perception↗
PubMedDate not supplied

Computational gestalts and perception thresholds.

In 1923, Max Wertheimer proposed a research programme and method in visual perception. He conjectured the existence of a small set of geometric grouping laws governing the perceptual synthesis of phenomenal objects, or "gestalt" from the atomic retina input. In this paper, we review this set of geometric grouping laws, using the works of Metzger, Kanizsa and their schools. In continuation, we explain why the Gestalt theory research programme can be translated into a Computer Vision programme. This translation is not straightforward, since Gestalt theory never addressed two fundamental matters: image sampling and image information measurements. Using these advances, we shall show that gestalt grouping laws can be translated into quantitative laws allowing the automatic computation of gestalts in digital images. From the psychophysical viewpoint, a main issue is raised: the computer vision gestalt detection methods deliver predictable perception thresholds. Thus, we are set in a position where we can build artificial images and check whether some kind of agreement can be found between the computationally predicted thresholds and the psychophysical ones. We describe and discuss two preliminary sets of experiments, where we compared the gestalt detection performance of several subjects with the predictable detection curve. In our opinion, the results of this experimental comparison support the idea of a much more systematic interaction between computational predictions in Computer Vision and psychophysical experiments.

Animals↗
PubMed2000

Vision and cognition in Alzheimer's disease.

Alzheimer's disease (AD) is known to affect visual pathways, but potential concomitant effects on vision and cognitive performance are not well understood. We studied 43 individuals with AD of mild severity and 22 individuals without dementia on a battery of tests designed to measure multiple aspects of basic and higher-order visual perception and cognition. All subjects performed on the same visual and cognitive test batteries. The results showed no differences between groups on tests of static visual acuity, stereoacuity, dynamic visual acuity or motion direction discrimination. However, individuals with AD performed significantly worse on tests of static spatial contrast sensitivity, visual attention, shape-from-motion, color, visuospatial construction and visual memory. Correlation analyses showed strong relationships between visual and cognitive scores. The findings show that AD affects several aspects of vision and are compatible with the hypothesis that visual dysfunction in AD may contribute to performance decrements in other cognitive domains. The pattern of involvement indicates that AD affects multiple visual neural pathways and regions. It is possible that better understanding of vision-related dysfunction could aid diagnosis and interventions to improve functional capacity in patients with dementia.

Aged↗
PubMed1983

An examiner-induced visual vigilance apparatus.

A film/record instrument is described that could measure attending behavior, distractibility, hyperkinetic movement, and selected aspects of visual perception by recording the subject's response to significant visual stimuli.

Attention↗
PubMed1995

Extension of a recent therapy for dyslexia.

Recently, peculiarities of visual perception were found in dyslexic patients. Therefore, we investigated visual acuity, reading and spelling capabilities, as well as peripheral letter recognition in 54 children with reading and/or spelling problems. Subsequently, the children and their parents trained at home for approximately 0.5 h daily during 2-3 months. Training consisted of reading through a small aperture and of visuomotor coordination tasks. The mean results obtained in a reading test for all patients improved significantly after the training, but less so than those recorded in previous studies on children suffering from dyslexia. Although the therapy clearly improved performance, it was less specific than previously claimed.

Adolescent↗
PubMed2002

Motion perception of saccade-induced retinal translation.

Active visual perception relies on the ability to interpret correctly retinal motion signals induced either by moving objects viewed with static eyes or by stationary objects viewed with moving eyes. A motionless environment is not normally perceived as moving during saccadic eye movements. It is commonly believed that this phenomenon involves central oculomotor signals that inhibit intrasaccadic visual motion processing. The keystone of this extraretinal theory relies on experimental reports showing that physically stationary scenes displayed only during saccades, thus producing high retinal velocities, are never perceived as moving but appear as static blurred images. We, however, provide evidence that stimuli optimized for high-speed motion detection elicit clear motion perception against saccade direction, thus making the search for extraretinal suppression superfluous. The data indicate that visual motion is the main cue used by observers to perform the task independently of other perceptual factors covarying with intrasaccadic stimulation. By using stimuli of different durations, we show that the probability of perceiving the stimulus as static, rather than moving, increases when the intrasaccadic stimulation is preceded or followed by a significant extrasaccadic stimulation. We suggest that intrasaccadic motion perception is accomplished by motion-selective magnocellular neurons through temporal integration of rapidly increasing retinal velocities. The functional mechanism that usually prevents this intrasaccadic activity from being perceived seems to rely on temporal masking effects induced by the static retinal images present before and/or after the saccade.

Humans↗
PubMed1998

Perception of motion and qEEG activity in human adults.

This study was designed to relate visual perception of motion to cortical activity, by evaluation of the association of quantified electroencephalogram (qEEG) parameters with a video film projection. The EEG was recorded from 14 sites according to the International 10-20 system and a common average reference was used. Forty right-handed volunteers (mean age = 24 years) were examined. The video film consisted of 20 s sequences showing still shots and moving shots with human movements or object movements. The EEG was then subjected to spectral analysis; the spectral powers for the theta, alpha and beta bands were calculated for 14 s epochs and compared with sequences of the video film. All analyses were based on logarithmically transformed absolute spectral power values. The power values of each frequency band were analysed in a 3-way repeated measure ANOVA (Hemisphere x Electrode x Sequence). The results were represented by EEG cartography. Significant decreases in the alpha 1, beta 1 and beta 2 power values of EEG in centro-parietal regions of both hemispheres were shown during perception of human motion sequences. This suggests participation of the sensorimotor cortex during visual observation of human motion.

Adolescent↗
PubMed2004

The perception of doubly curved surfaces from anisotropic textures.

Most existing computational models of the visual perception of three-dimensional shape from texture are based on assumed constraints about how texture is distributed on visible surfaces. The research described in the present article was designed to investigate how violations of these assumptions influence human perception. Observers were presented with images of smoothly curved surfaces depicted with different types of texture, whose distribution of surface markings could be both anisotropic and inhomogeneous. Observers judged the pattern of ordinal depth on each object by marking local maxima and minima along designated scan lines. They also judged the apparent magnitudes of relative depth between designated probe points on the surface. The results revealed a high degree of accuracy and reliability in all conditions, except for a systematic underestimation of the overall magnitude of surface relief. These findings suggest that human perception of three-dimensional shape from texture is much more robust than would be reasonable to expect based on current computational models of this phenomenon.

Anisotropy↗
PubMed2002

Perception onset time during fixations in free viewing.

In this study, we investigated when visual perception begins in fixations. During picture viewing, the picture was degraded at the beginning of selected saccades and changed back to the original after varying intervals. Participants manually responded whenever they detected changes. The change-backs were undetected when they occurred <6 msec after the end of the saccade, marked by the peak of the overshoot in dual Purkinje image eyetracker data, and detection reached asymptote 32 msec after that marker. Eye velocity at the change-back time also affected detection likelihood. Apparently, perception begins around the time at which the eyes stop rotating at the end of a saccade, giving a psychological justification for measuring fixation durations from then. This also specifies the deadline for gaze-contingent display changes to occur without detectable image motion. Investigators using the dual Purkinje image eyetracker should consider the peak of the overshoot as the fixation onset time and measure intrafixational presentation times from then.

Fixation, Ocular↗
PubMed2002

Cognitive processing in the primary visual cortex: from perception to memory.

The primary visual cortex is the first cortical area of the visual system that receives information from the external visual world. Based on the receptive field characteristics of the neurons in this area, it has been assumed that the primary visual cortex is a pure sensory area extracting basic elements of the visual scene. This information is then subsequently further processed upstream in the higher-order visual areas and provides us with perception and storage of the visual environment. However, recent findings show that such neural implementations are observed in the primary visual cortex. These neural correlates are expressed by the modulated activity of the late response of a neuron to a stimulus, and most likely depend on recurrent interactions between several areas of the visual system. This favors the concept of a distributed nature of visual processing in perceptual organization.

Animals↗
PubMed1994

EXPLAN--a programming language for complex visual stimuli presentation.

Visual cognition research requires the flexible use of structured spatial patterns, characterized by various space and time parameters, which may be administered as visual stimuli. Dealing with this kind of study, we developed a special-purpose programming language and implemented a compiler to build executable programs. The language allows the presentation of stimuli, their space coordinates, persistency values, sequence, kinematic parameters, space-time proximity with other visual stimuli, determination of their modification according to external interaction, generation of subliminal stimuli, monitoring of different subject reactions, automatic reporting of stimulus presentation, and reaction monitoring. Such a language has been successfully utilized in a visual perception research.

Algorithms↗
PubMed2003

Occipito-parietal cortex activation during visuo-spatial imagery in early blind humans.

Using positron emission tomography, regional cerebral blood flow was studied in five early blind and five control volunteers during visuo-spatial imagery. Subjects were instructed to generate a mental representation of verbally provided bidimensional patterns that were placed in a grid and to assess pattern symmetry in relation to a grid axis. This condition was contrasted with a verbal memory task. Cerebral activation in both groups was similar during the visuo-spatial imagery task. It involved the precuneus (BA 7), superior parietal lobule (BA 7), and occipital gyrus (BA 19). These results are in accordance with previous studies conducted in sighted subjects that indicated that the same occipito-parietal areas are involved in visual perception as well as in mental imagery dealing with spatial components. The dorsal pathway seems to be involved in visuo-spatial imagery in early blind subjects, indicating that this pathway undergoes development in the absence of vision.

Adult↗
PubMed1995

Types and tokens unscathed: a reply to Whittlesea, Dorken, and Podrouzek (1995) and Whittlesea and Podrouzek (1995).

N. G. Kanwisher (1987; J. Park & N. G. Kanwisher, 1994) has explained repetition blindness in terms of a distinction in visual perception between type activation and token individuation; repeated items are successfully recognized (matched to stored types) but are less likely than unrepeated items to become individuated as separate perceptual tokens. Whittlesea and colleagues (B. W. A. Whittlesea, M. D. Dorken, & K. W. Podrouzek, 1995; B. W. A. Whittlesea & K. W. Podrouzek, 1995) argued that repetition blindness does not reflect different processing of repeated and unrepeated items but is better explained as the result of a combination of separate but nondistinctive processing of repeated items and postlist report biases. However, we argue that none of the results reported by Whittlesea and colleagues are inconsistent with the token-individuation hypothesis.

Blindness↗
PubMed1997

Cortical dynamics of lateral inhibition: metacontrast masking.

The dynamic properties of a neural network model of visual perception, called the boundary contour system, explain characteristics of metacontrast visual masking. Computer simulations of the model, with a single set of parameters, demonstrate that it accounts for 9 key properties of metacontrast masking: Metacontrast masking is strongest at positive stimulus onset asynchronies (SOAs); decreasing target luminance changes the shape of the masking curve; increasing target duration weakens masking; masking effects weaken with spatial separation; increasing mask duration leads to stronger masking at shorter SOAs; masking strength depends on the amount and distribution of contour in the mask; a second mask can disinhibit the masking of the target; such disinhibition depends on the SOA of the 2 masks; and such disinhibition depends on the spatial separation of the 2 masks. No other theory provides a unified explanation of these data sets. Additionally, the model suggests a new analysis of data related to the SOA law and makes several testable predictions.

Attention↗
PubMed2004

Reliability and validity of scores on the Slosson Visual Perceptual Skill Screener.

Psychometric characteristics of the Slosson Visual Perceptual Skill Screener were studied using three independent samples of 595 boys and 578 girls ages 5 to 10 years. Adequate characteristics for item selection were indicated by item difficulty (Mdn=.49) and item discrimination (Mdn=.59). Interitem consistency of .93 (n1 = 1,045) was recorded for the total scale, while 30-day test-retest reliability was .80 (n2=55). Convergent validity was estimated using the Bender Visual-Motor Gestalt test (Koppitz developmental scoring system; r = .43, n3=47), the Developmental Test of Visuo-motor Integration, Visual Perception subtest (r =.62, n3 =73), and the Test of Visual Perceptual Skills-Revised-Nonmotor Total Score (r = .63, n2 =59).

Discrimination, Psychological↗
PubMed1993

Rehabilitation technology: exploitation of R&D and current technologies.

This paper summarizes the contributions made by all of the authors in Session (1) of the 1st TIDE Congress and highlights the potentialities for exploitation of research and developments in areas of technology relevant to aspects of interfaces relating to visual perception, facilitation of human communication, prosthetics and orthotics, robotics and signal processing. Means by which promising technological developments can be expedited for more widespread availability are discussed together with aspects of consumer involvement and teamwork through consortia of relevant expertise.

Adult↗
PubMed2000

Seeing, sensing, and scrutinizing.

Large changes in a scene often become difficult to notice if made during an eye movement, image flicker, movie cut, or other such disturbance. It is argued here that this change blindness can serve as a useful tool to explore various aspects of vision. This argument centers around the proposal that focused attention is needed for the explicit perception of change. Given this, the study of change perception can provide a useful way to determine the nature of visual attention, and to cast new light on the way that it is - and is not - involved in visual perception. To illustrate the power of this approach, this paper surveys its use in exploring three different aspects of vision. The first concerns the general nature of seeing. To explain why change blindness can be easily induced in experiments but apparently not in everyday life, it is proposed that perception involves a virtual representation, where object representations do not accumulate, but are formed as needed. An architecture containing both attentional and nonattentional streams is proposed as a way to implement this scheme. The second aspect concerns the ability of observers to detect change even when they have no visual experience of it. This sensing is found to take on at least two forms: detection without visual experience (but still with conscious awareness), and detection without any awareness at all. It is proposed that these are both due to the operation of a nonattentional visual stream. The final aspect considered is the nature of visual attention itself - the mechanisms involved when scrutinizing items. Experiments using controlled stimuli show the existence of various limits on visual search for change. It is shown that these limits provide a powerful means to map out the attentional mechanisms involved.

Attention↗