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PubMed1987

Field processes in stereovision. A description of stereopsis appropriate to ophthalmology and visual perception.

There is, as yet, no satisfactory theory of stereopsis, despite the fact that our overt knowledge of "solid seeing" is now about 150 years old, and that contributions to our understanding come today from many fields: ophthalmology, psychology, psychophysics, neurophysiology, computer modelling, and optical-TV display technology. We review herein, and demonstrate for the reader whenever possible, certain key perceptual properties of the stereoscopic event of which any general theory must take account: vector stereoscopy and the neural grid, depth in empty visual fields, the relationship between stereoscopic and cognitive contours, stereoscopic contour formation in the presence of blur (thus, at low levels of central visual acuity), the phenomenon of cortical locking and of neural grid evocation in the presence of either peripheral or central rivalry, certain unusual ranges of figural mismatch and the concept of the horopter in relation to modern single cell electroneurophysiology in animals and to the constancy of visual directions. Some comments are also made on the concept of disparity processing by single cortical neurons, together with a short discussion of the implications of certain views of the genetics of stereovision for the perception of novel random texture sine-wave stereograms. We conclude that any theory pertinent to ophthalmology and visual science must combine the global concepts of cortical integration, the neural lock and the neural grid, herein introduced, with the more classical concepts of particulate or local binocular cortical correspondence. Certain preliminary steps in this direction are presented.

Cognition↗
PubMed2003

Gamma-band phase clustering and photosensitivity: is there an underlying mechanism common to photosensitive epilepsy and visual perception?

Photosensitive epilepsy (PSE) is the most common form of human reflex epilepsy, appearing in up to 10% of epileptic children. It also offers a highly reproducible model to investigate whether changes in neuronal activity preceding the transition to an epileptic photoparoxysmal response (PPR) may be detected. We studied 10 patients with idiopathic PSE (eight female, mean age 26 years, range 9-51 years) using magnetoencephalography. In addition, we also studied the responses of five normal controls (mean age 24 years, age range 9-35 years) and three non-photosensitive epileptic patients (mean age 10 years, range 8-11 years). Spectral analysis of the MEG signals recorded during intermittent photic stimulation revealed relevant information in the phase spectrum. To quantify this effect, we introduced a second order response feature of the stimulus-triggered visual response preceding the PPR: the phase clustering index, which measures how close the phases of successive periods are grouped for each frequency component for all periods of the stimuli applied. We recorded a total of 86 PPRs, including several absence seizures, in nine of the 10 patients. We found that an enhancement of phase synchrony in the gamma-band (30-120 Hz), harmonically related to the frequency of stimulation, preceded the stimulation trials that evolved into PPRs, and differed significantly from that encountered in trials not followed by PPR or in control subjects. This novel finding leads us to postulate that a pathological deviation of normally occurring synchronization of gamma oscillations, underlying perceptional processes, mediates the epileptic transition in PSE.

Adolescent↗
PubMed1999

Gravity affects the preferred vertical and horizontal in visual perception of orientation.

The aim of this study was to evaluate the influence of gravity on the representation and storage of visual orientation information. On earth, measurements of response time and variability for a task of aligning remembered visual stimuli showed a distinct preference for horizontally and vertically oriented stimuli when the body and gravitational axes were aligned. This preference was markedly decreased or disappeared when the body axis was tilted with respect to gravity but was maintained for tests performed in microgravity. We conclude that subjects acquire and store visual orientation in a multi-modal reference frame that combines proprioceptive and gravitational information when both are available.

Adult↗
PubMed1988

The role of the right hemisphere in form perception and visual gnosis organization.

Peculiarities of series of picture interpretations and Rorschach test results in patients with unilateral benign hemispheric tumours are discussed. It is concluded that visual perception in the right hemisphere has hierarchic structure, i.e., each successive area from the occipital lobe towards the frontal having a more complicated function. Visual engrams are distributed over the right hemisphere in a manner similar to the way the visual information is recorded in holographic systems. In any impairment of the right hemisphere a tendency towards whole but unclear vision arises. The preservation of lower levels of visual perception provides for clear vision only of small parts of the image. Thus, confabulatory phenomena arises, which are specific for right hemispheric lesions.

Adult↗
PubMed1994

The visual perception of three-dimensional shape from self-motion and object-motion.

To evaluate the influence of egomotion on the three-dimensional visual processing of structure-from-motion (SFM), we compared the visual discrimination between planar and spherical surfaces during subject-translation, object-translation, or rotation of the object in depth. Performance was the best for object-rotation, intermediate for subject-translation, and the poorest for object-translation--and thus increased with the quality of retinal image stabilization achieved in the different conditions. This suggests that the major role of self-motion information was to stabilize retinal images. In view of previous results, we propose that the interactions between self-motion information and SFM are reduced to functional complementarity, in the sense that self-motion can lift visual ambiguities but does not improve the sensitivity of SFM processes.

Adult↗
PubMed1986

Evidence for early extraction of emergent properties in visual perception: a replication.

In conditions precluding focused attention, illusory conjunctions of triangles were more frequent when the emergent characteristic of closure was present in the display together with the component parts than when it was not. This replicates Treisman and Paterson's 1984 result, which is the strongest evidence available in favor of the notion of an early extraction of emergent properties.

Adult↗
PubMed1998

[Visual perceptions in ophthalmic migraine].

The number of patients with ophthalmic migraine is steadily increasing. The symptoms of their attacks are dominated by typical visual aura, followed by major or minor headache. The authors present the experience of three physicians who observed the symptoms on themselves. The first one has suffered from the disease already for fifty years, the second and third one 25 years each. In the discussion the authors deal with the differential diagnosis of the disease and its possible treatment.

Aged↗
PubMed1994

Visual perception of intentional motion.

A series of experiments were performed to investigate how motion sequences provide information about the intentional structure of moving figures or actors. Observers had to detect simulations of biologically meaningful motion within a set of moving letters. In the first two experiments a factorial design was used, with type of instruction as a between-subject factor and six movement parameters (number of items, speed and directness of target and distractors, and 'relentlessness' of target movement) as within-subject factor; in the final two experiments, the visibility of the goal towards which the target moved and the use of a tracking movement to distinguish the target were varied. In such displays search time increases with increasing number of stimuli. It was found that (a) the more direct the motion, the more likely it was to be interpreted as intentional; (b) intentional motion was much easier to detect when the target moved faster than the distractors than when it moved more slowly; (c) recognition of intentionality was impaired but not abolished if the goal towards which the target was moving was invisible; and (d) participants did not report intentional movement when the target was distinguished by brightness rather than the manner in which it moved. We argue that the perception of intentionality is strongly related to observers' use of conceptual knowledge, which in turn is activated by particular combinations of features. This supports a process model, in which intentionality is seen as the result of a conceptual integration of objective visual features.

Animals↗
PubMed1975

Interaction between two letters in visual perception.

Letters were exposed by tachistoscope for 17 ms to adult subjects. In Experiment I, single letters were exposed. The proportion of errors differed strongly for particular letters. In Experiment II pairs of letters were exposed. The first letter of a pair was recognized more easily than when exposed alone in Experiment I. In Experiment III pairs of letters were exposed but the subject's task was to recognize only one letter of the pair. Both, first and second letters of the pair were then recognized better than in Experiment I. The results were interpreted in terms of facilitatory and inhibitory effects occurring on different levels of the visual system.

Adult↗
PubMed1997

Visual perception of motor anticipation in cursive handwriting: influence of spatial and movement information on the prediction of forthcoming letters.

The execution of a graphemic sequence is constrained by spatial demands that result in fluctuations of letter shape and movement time. When producing two letters (ll, le, or ln) the movement time and the letter shape of the first letter depend on the execution constraints of the second one. The motor system thus anticipates the production of the forthcoming graphemic sequence during the production of the first letter. An experiment is reported the aim of which was to examine whether the visual system could exploit this anticipatory information to predict the identity of the letter following the l. Different ls belonging to ll, le, and ln were presented on a screen. Subjects had to predict to which couple of letters (ll, le, or ln) the presented l belonged to, by using information on the shape of the l and/or the movement that produced it. Results showed that the percentages of correct responses were higher in the conditions where the stimulus provided kinematic information than in the condition in which only spatial information was available. The ability to predict the forthcoming letter seems to be mediated by implicit knowledge on motor anticipation rules.

Adult↗
PubMed2006

Isolating the impact of visual perception on dyslexics' reading ability.

A large body of data suggests that phonological deficits play an important causal role in dyslexics' reading difficulties. The functional role of visual impairments is still highly debated. Many recent studies have shown clear visual deficits in large subgroups of dyslexics. However, the relationship between these deficits and visual routines required for reading is not clear. To assess the direct contribution of visual factors to dyslexics' slower and less accurate reading, we composed a task that was similar to single word reading in its basic visual characteristics, but had none of the other (phonological, morphological, semantic, etc.) aspects of reading. Young adult dyslexics, with average or above general cognitive abilities, and controls matched for age and cognitive skills participated in the study. We measured both SOA and contrast thresholds for identifying unfamiliar letters. Letters were chosen from an alphabet graphically similar to Hebrew and English (a subset of Georgian letters), but unfamiliar to the subjects. Effects of decreasing letter size, increasing letter crowding (by adding a flanker letter on each side) and adding white noise, were measured. Dyslexics performed as well as controls under all test conditions, and had similar effect sizes. We thus conclude that, despite the data showing that dyslexics have marked difficulties with single word reading, the cause of these difficulties is not a visual processing deficit.

Adult↗
PubMed2000

Evidence for multistability in the visual perception of pigeons.

Perceptual multistability refers to cases where perception alternates between two or more interpretations of an unchanging sensory stimulus. In a first experiment we trained eight pigeons to discriminate horizontal and vertical apparent motion stimuli and then presented a multistable display. In five cases their behavior showed alternations similar to human experiments. In a second experiment we varied the aspect ratio of the display in order to support the hypothesis of a percept-driven nature of the switching behavior. The pecking rates and mean phase durations varied as predicted. This is the first evidence of visual multistability in animals confronted with classical ambiguous figures. The data suggest a stochastic mechanism.

Adaptation, Physiological↗
PubMed1982

Visual perception of relative mass in dynamic events.

Two experiments are reported which examine how the dynamic property of relative mass in collision events is specified by kinematic properties of a visual motion display. In most cases observers are accurate at detecting the 'heavier' of two objects, although they do not take advantage of the completely general optic information that is available. Instead, they rely on limited information that breaks down at extreme values of elasticity and relative initial velocity. In addition, observers appear to utilize different information for relative mass with different types of collisions. It is suggested that reliance on such limited information may be appropriate for perceivers operating in the restricted context of a terrestrial environment.

Humans↗
PubMed2005

Visual perception and interception of falling objects: a review of evidence for an internal model of gravity.

Prevailing views on how we time the interception of a moving object assume that the visual inputs are informationally sufficient to estimate the time-to-contact from the object's kinematics. However, there are limitations in the visual system that raise questions about the general validity of these theories. Most notably, vision is poorly sensitive to arbitrary accelerations. How then does the brain deal with the motion of objects accelerated by Earth's gravity? Here we review evidence in favor of the view that the brain makes the best estimate about target motion based on visually measured kinematics and an a priori guess about the causes of motion. According to this theory, a predictive model is used to extrapolate time-to-contact from the expected kinetics in the Earth's gravitational field.

Adaptation, Physiological↗
PubMed2002

Brain Areas Active during Visual Perception of Biological Motion.

Theories of vision posit that form and motion are represented by neural mechanisms segregated into functionally and anatomically distinct pathways. Using point-light animations of biological motion, we examine the extent to which form and motion pathways are mutually involved in perceiving figures depicted by the spatio-temporal integration of local motion components. Previous work discloses that viewing biological motion selectively activates a region on the posterior superior temporal sulcus (STSp). Here we report that the occipital and fusiform face areas (OFA and FFA) also contain neural signals capable of differentiating biological from nonbiological motion. EBA and LOC, although involved in perception of human form, do not contain neural signals selective for biological motion. Our results suggest that a network of distributed neural areas in the form and motion pathways underlie the perception of biological motion.

Brain Mapping↗
PubMed2002

Visual perception of planar orientation: dominance of static depth cues over motion cues.

We measured the ability to report the tilt (direction of maximal slope) of a plane under monocular viewing conditions, from static depth cues (square grid patterns) and motion parallax (small rotations of the plane about a frontoparallel axis). These two cues were presented separately, or simultaneously. In the latter case they specified tilts that were either collinear (coherent case) or orthogonal (conflict case). The field of view was small (8 degrees) or large (60 degrees). In small field, for motion parallax, the reported tilt depends strongly on the orientation of the plane relative to the rotation axis, being totally ambiguous when tilt is collinear with the rotation axis. In contrast, in large field, the reported tilt depends little on this variable, and is accurately specified by motion cues. In both cases static cues strongly dominated the tilt reports. Hence static grid patterns constitute robust tilt cues, which can dominate contradictory tilt indications from motion parallax, and should be considered as essential for the visual orientation during locomotion, or the immersion in virtual reality environments.

Adult↗