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T V Papathomas

Publications and source records attributed to T V Papathomas.

8 recordsLinked to original sources
PubMed1992

Sensitivity to colour- and to orientation-carried motion respectively improves and deteriorates under equiluminant background conditions.

This study presents two distinct effects produced by manipulation of the background illumination on the directional sensitivity to colour- and orientation-carried motion. The two motion percepts were produced with two of a class of stimuli extensively used by the first and last authors in apparent-motion studies. The stimuli were designed to produce motion perception by virtue of spatiotemporal matching of (a) colour with orientation systematically mismatched (Colour across Orientation, CxO) and of (b) orientation with colour systematically mismatched (OxC). An increase in background illumination from dark to the equiluminance point (relative to the luminance of the discrete stimulus microelements) entails a significant increase and decrease of directional performances with CxO and OxC stimuli, respectively. It is proposed that these anti-symmetrical background effects have distinct neurophysiological origins. For CxO stimuli, improvement of directional performances at the equiluminant point is presumably due to the inactivation of the inhibitory effect of the luminance-motion pathway on the chromatic-motion pathway. The opposite effect obtained with OxC stimuli, previously referred to as the veto effect (Gorea and Papathomas, 1988 Invest. Ophthal. Vis. Sci. Suppl., 29, 265), is supposed to be entailed by the inactivation of the luminance-oriented mechanism, the only motion sensitive mechanism activated by this stimulus configuration.

Color Perception↗
PubMed1991

Two carriers for motion perception: color and luminance.

Starting with the experiments of Ramachandran and Gregory (Nature, 275, 55-56, 1978), several psychophysical studies in apparent motion (AM) have established that the perception of motion is significantly impaired at equiluminance. Still debated, however, is whether color alone can resolve ambiguities in AM. We report here on several psychophysical experiments, the quantitative results of which indicate that color does play a substantial role in AM. These findings seem to support recently proposed neurophysiological frameworks according to which there exist significant interactions among the neuronal pathways mediating the perception of basic visual attributes such as color, motion, form and depth.

Color Perception↗
PubMed1991

Texture segregation by chromatic and achromatic visual pathways: an analogy with motion processing.

We present results to show that texture segregation can be obtained through the so-called coherent spatial grouping of local shape (orientation) and of local color under both nonequiluminant and equiluminant conditions. Color grouping entails texture segregation independent of orientation grouping, while the reverse is not true under equiluminant conditions. The experiments permit the isolation of chromatic- and luminance-oriented mechanisms, as well as of chromatic nonoriented mechanisms, all of which contribute to texture discrimination. As a general rule, the present results (including the asymmetry between color and orientation grouping) are similar to those obtained by us in a series of motion-perception experiments. This similarity suggests that the perceptual rules governing spatial grouping are analogous (if not identical) to those governing spatiotemporal grouping. As in the case of directional discrimination, texture-discrimination performances may be accounted for by the activation of higher-order units receiving inputs from subunits, all of which display similar tuning properties within a multidimensional space.

Color Perception↗
PubMed1990

Ambiguity in 3-D patterns induced by lighting assumptions.

A bistable pattern is shown with white and black bars with horizontal and vertical orientations which produce an impression of thin slabs stacked up in depth either toward or away from the observer. It is postulated that the ambiguity is induced by the observer's assumption of the direction of the light source.

Attention↗
PubMed1989

Stereoscopic illusion based on the proximity principle.

A class of ambiguous random-dot stereograms were created that share the following interesting property: Although the binocular disparity forms a periodic 'sawtooth' waveform as a function of row number (the disparity is constant for a given row), these stimuli yield a monotonically increasing depth percept along the rows. The random-dot pattern of each row is periodic along the horizontal direction for the purpose of producing an ambiguous depth percept. It is this ambiguity that makes it possible for the periodic stimulus to give rise to a monotonic percept. This monotonic percept is substantially enhanced when the rows are shown in temporal sequence instead of all being displayed together. Experiments are reported which indicate that this illusion is due to the proximity, or pulling, effect in stereopsis.

Depth Perception↗
PubMed1989

Motion processing by chromatic and achromatic visual pathways.

We describe a family of stimuli consisting of colored bars of different orientations, which, when presented in rapid succession, may elicit unambiguous motion perception. These stimuli permitted the isolation of directional spatiotemporal information extracted from oriented luminance clues, from nonoriented chromatic-plus-luminance clues, or, when the stimuli were presented under equiluminant conditions, from pure chromatic clues. As a general rule, matching of orientation induces weaker motion-detection performances than does matching of color. When the orientation clues are in competition with the chromatic ones, motion perception based on the former is always overridden by motion perception based on the latter. We indirectly isolated an oriented chromatic mechanism that also contributes to motion perception. We finally showed that, under equiluminant conditions, matching of orientation across different colors is inefficient in eliciting motion perception, either because motion information is extracted poorly across different chromatic channels or because such channels show little orientational selectivity. Because motion strength determined by each of the manipulated attributes follows different functions with the displacement (or velocity) of the stimuli, we propose the existence of three underlying mechanisms, a luminance mechanism, a chromatic-plus-luminance mechanism, and a pure chromatic mechanism, each of which provides motion information.

Attention↗