Biomedical subjects

J Faubert

Publications and source records attributed to J Faubert.

12 recordsLinked to original sources

PubMed1995

Colour induced stereopsis in images with achromatic information and only one other colour.

Based on a model of transverse chromatic aberration (TCA) of the eye and perceptual effects previously discussed by the same author, it was predicted that colour induced binocular depth perception (chromostereopsis) is possible from images containing red or blue along with achromatic information (grey and black); that the direction of the perceived depth from certain colours should differ for the same TCA conditions; and that a greater depth perception should be possible in a blue/achromatic image than a red/achromatic image. Results of TCA manipulations support all three predictions and demonstrate that colour induced depth effects can be perceived in images having only achromatic information and a single non-achromatic colour. It is argued that optical factors, image characteristics and perceptual factors all play an important role in colour depth effects under natural viewing conditions. Further, the possibility is raised that colour and texture patterns which produce colour depth effects have evolved within certain species for camouflage or other behavioural purposes.

Color Perception↗

PubMed1995

The influence of two spatially distinct primers and attribute priming on motion induction.

In a series of experiments, we demonstrate the effects of two spatially distinct primers on motion induction (MI) and the influence of attribute characteristics on the resulting collision site. MI means that a primer such as a spot produces a motion sensation in a subsequently presented geometrical pattern such as a line or a rectangle. This pattern will appear to grow out of the spot. In the present paper we report that when two different locations of the visual field are activated simultaneously by presenting two spots prior to a bar between these spots, there is a motion sensation of two bars growing away from the spots and colliding in the centre (split priming effect). Attribute characteristics can have profound effects on this illusion. When two differently coloured isoluminant spots are presented and the subsequent bar is composed of either one of these colours, the induced motion is away from the spot of identical colour. We call this effect attribute priming. Manipulating the delay between the spot presentations (SOA) showed that timing had a strong effect on split priming, but very little on attribute priming. For split priming experiments with dichoptic presentations, we show that at shorter SOAs there is a dominant effect of the primer which is presented to the same eye as the bar, as opposed to the usual dominance of the later primer. For longer SOAs, however, the temporal sequence of the primers also plays a role in motion induction. Further, we report that geometrical arrangements can strongly influence the direction of perceived motion when more than a single primer is used. Generally, in motion induction with two primers, unlike what is found with a single primer, there appears to be a dominance of low-level effects such as geometry, attributes, and eye of presentation. For dichoptic presentations, however, this can be overcome for longer SOAs. The differences between the single and split priming paradigms are discussed in terms of the differential contribution of bottom-up and top-down processes.

Female↗

PubMed1995

Processing speed in the motion-induction effect.

The motion-induction effect, where an illusory motion is perceived within a bar when it is shown next to a spot presented slightly earlier, was studied with respect to the idea that it is based on differential processing speeds between the two ends of the bar. First, by using just a bar with a luminance gradient, the existence of a motion illusion (gradient motion) within such a bar was demonstrated, presumably due to the different processing speeds of differential luminances. When such a bar was used in the motion-induction effect, it was shown to modulate, for short delays, the strength of the effect up or down, according to the direction of the gradient with respect to the position of the spot. When the same bar was used in the double-motion-induction effect (split priming), in which motion is usually away from the later spot, it totally determined the perceived direction of illusory motion, independently of gradient direction with respect to the later spot or the time between the two spots. These results demonstrate, on the one hand, that differential local processing speed is a likely mechanism to underlie the motion-induction effect. On the other hand, they also suggest the involvement of other more global (and perhaps top-down) processes.

Analysis of Variance↗

PubMed1995

A model for global symmetry detection in dense images.

In this paper, a model is proposed for bilateral symmetry detection in images consisting of dense arrangements of local features. The model is elaborated on the basis of a psychophysical experiment showing that grouping precedes and facilitates symmetry detection. The proposed computational model consists of three stages: a grouping stage, a symmetry-detection stage, and a symmetry-subsumption stage. Reliance upon a preliminary grouping stage enables a significant reduction of the computational load for detecting symmetry. An implementation of the model is described, and results are presented, showing a good agreement of the model performance with human symmetry perception.

Computer Simulation↗

PubMed1994

Seeing depth in colour: more than just what meets the eyes.

Novel binocular depth illusions obtained from two-dimensional colour images are presented. It is demonstrated that the magnitude of these illusions is based on transverse chromatic aberration (TCA), however, the depth obtained cannot be observed unless specific conditions are met even if the TCA is present. Some form of perceptual organization occurring at and/or beyond the binocular fusion site of the brain, is required for some of these effects to occur. An example of a paradoxical finding leading to this conclusion is the observation that under some conditions the same colour can be perceived on separate depth planes while spatially adjacent colours from opposing ends of the visible spectrum (i.e. red and blue or green) can be perceived on the same depth plane simultaneously within the same image. Further, results show that some form of reference plane is required by the brain to use the colour induced disparity, without which, depth cannot be perceived even if the disparity information is present. This phenomenon is spatially tuned for medium to high frequency components and is still detectable under isoluminant conditions which would support the notion that it requires information from the parvocellular pathway. Binocular lustre and rivaldepth are ruled out as being significant factors in the effect. It is argued that this phenomenon represents an instance of global interactive processes induced by TCA while previous studies on chromostereopsis have concentrated on local aspects. Results of the present study may explain why under certain situations depth can be perceived in coloured images and not under other circumstances where TCA is still present.

Color Perception↗

PubMed1994

Intraattribute and interattribute motion induction.

The phenomenon of motion induction occurs, for example, when a bar that is presented next to a spot, which itself was presented slightly earlier, is not correctly perceived to appear everywhere simultaneously, but seems to grow out of the spot. The spot is said to prime one end of the bar. Experiments have been designed to throw more light on the local and global aspects of this phenomenon, in particular to establish whether this illusory motion percept can be observed when the spot and the bar stimuli are defined with respect to the background by one of a variety of attributes, such as luminance, color, stereodepth (crossed and uncrossed), texture, and motion (start and stop). It was found that all attribute combinations supported motion induction readily, but that the strength of the perceived motion (as measured by magnitude estimation) varied and depended more on the attribute defining the bar than on the attribute of the spot. Luminance and color gave the most vivid effects, whereas motion and depth showed the least vivid effects. The influence of the amount of luminance and color contrast on the strength of the effect was also determined and it was found that these variables affected motion induction most at very low contrast levels close to detection threshold. It is concluded that the illusory motion in this effect depends only slightly on the particular visual attribute channel that carries the stimulus information. This is consistent with the contention that it is a high-level, attention-related effect, phenomenologically similar to polarized gamma movement.

Attention↗

PubMed1994

An investigation of prism adaptation latency.

PURPOSE: The latency of adaptation of the human vergence system to a change in convergence or divergence forced upon it by a prism was investigated in this pilot study. METHODS: Adaptation was stimulated by a 5-s period of binocular vision through a prism of 0 delta, -8 delta, or 8 delta. Immediately thereafter and for a further period of 45 s, lateral heterophoria was monitored subjectively by an automated version of Duane's screen and parallax test. Adaptation was calculated from the phoria 40 s after the end of binocular vision. Tests were performed at 0.4 m. In the first test session, there was screening during the binocular period to test for flaws in the screening method. In the second session, there was no screening during the binocular period and phoria measurement was started without changing the power of the prism. In the third session, the prism was restored to 0 delta after the period of binocular vision. Immediately after the last 5-s test, tests were repeated with 1 s of binocular vision. The subject had excellent visual acuity, stereoacuity, and stereolatency. RESULTS: The test functioned correctly and showed good repeatability. The greatest adaptation to 8 delta was 59%. This was obtained with only 1 s of binocular vision. There was adaptation to -8 delta with 5 s of binocular vision but it was obscured by adaptation to 0 delta or 8 delta which persisted from previous tests. DISCUSSION: The reason why the subject's latency was not found more precisely is explained. Additional evidence is presented in support of the finding that prism adaptation can take place within 1 s (perhaps less) of binocular vision, and the persistence and dominance of adaptation to base-out prism at near is pointed out. Guidelines are proposed for the experimental measurement of prism adaptation latency.

Adaptation, Ocular↗

PubMed1992

Stereolatency: a stereopsis test for everyday depth perception.

An instrument is described which tests for the use of stereopsis by measuring how long it takes for depth perception to begin (its latency) after a transition from monocular to binocular vision. Starting at 1 s, the duration of binocular vision is reduced progressively toward a limit of 16 ms (or increased to a limit of 4 s) until the latency of stereopsis is found. The preliminary period of monocular vision acts as a probe for suppression of either eye and allows time for accommodation and vergence to stabilize before binocular vision begins. Typical results are presented.

Depth Perception↗

PubMed1991

A definitive case of paradoxical stereopsis.

Author JF had perfect stereoacuity but was unaware that he did not make much use of stereopsis under everyday conditions of seeing, i.e., his stereopsis was paradoxical. We describe how this condition was diagnosed and everyday depth perception restored. The objective of this report is to demonstrate convincingly that paradoxical stereopsis exists as a clinical entity. A combination of signs and symptoms is presented as a syndrome for the diagnosis of paradoxical stereopsis.

Depth Perception↗

PubMed1989

The diagnostic use of the second oscillatory potential in clinical electroretinography.

Of all the electroretinogram (ERG) components (a-wave, b-wave, and oscillatory potentials) only one oscillatory potential, OP2, was found to be significantly correlated with the absolute intensity of the flash stimulus (i.e., the intensity of the stimulus irrespective of the state of retinal adaptation). Our finding was further confirmed in single cell recordings of lateral geniculate unit activity in rabbits in which peak time of OP2 was found to correlate better with the geniculate activity. For these reasons we have identified OP2 as the "intensity coding" oscillatory potential of the ERG. In order to investigate if this new feature could have some clinical significance, we examined photopic ERGs recorded from patients affected with various retinopathies. In most instances the peak time of OP2 paralleled that of the b-wave, that is, in the ERG with delayed b-wave the peak time of OP2 was also delayed, while in ERGs with normal b-wave peak time the peak time of OP2 was also normal. However, in some conditions (especially in cone-rod diseases) a delayed OP2 was found in ERGs with normal b-wave peak times.

Adult↗

PubMed1987

Active-passive paradigm in assessing CCTV-aided reading.

Six male and seven female English-speaking university students, with corrected vision of 20/20 (6/6), were trained using the closed circuit television system (CCTV) as a reading aid. This was an attempt to identify the essential factors in CCTV-aided reading. Active and passive observers were trained in pairs, with the passive observer being the yoked control of the active observer. The control participants were trained alone using the same reading material without the use of a CCTV. All participants used material from the Thurstone Reading Kit throughout the five training sessions and were given a pre- and postreading test to assess the increase in reading speed measured in words per minute (WPM). The active observers increased their reading speed to a greater extent than the passive observers, and the passive observers to a greater extent than the control participants. Suggestions are made pertaining to future experimentation and use of the CCTV.

Analysis of Variance↗

PubMed1986

Multi-flash campimetry as an indicator of visual field loss in glaucoma.

Multi-flash campimetry is a computer-implemented technique used for the rapid assessment of temporal resolving power across the visual field. On the basis of previous reports that glaucoma patients exhibit impaired temporal sensitivity earlier than conventional perimetric field loss, the question of whether multi-flash campimetry might also be sensitive to the visual consequences of glaucoma was explored. Of the 27 patients that were tested, only one had normal temporal resolving power in both eyes, suggesting that multi-flash campimetry might be a useful aid in determining whether a patient should be diagnosed and treated as a glaucoma suspect or as a confirmed glaucoma patient.

Adult↗