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At least 145 records · Page 8Linked to original sources
PubMed1987

Perception of random-dot symmetry and apparent movement at and near isoluminance.

There have been conflicting reports on whether apparent movement in random-dot kinematograms is abolished at isoluminance. The present results suggest that it is, provided that dynamic (uncorrelated) surrounds are used, and the subject has to report the shape of the target rather than the presence of movement in an isolated portion of the target. On the other hand, perception of random-dot symmetry is still possible at isoluminance. The reason for this difference appears to be the need for exact-position information in movement but not symmetry perception. Control experiments suggest that the effects are not due to artefacts such as chromatic aberration in the eye.

Adult↗
PubMed1987

Illusory contour orientation discrimination.

Just noticeable differences (JNDs) in orientation for real lines and illusory contours were compared. JNDs in orientation of an illusory contour and of a real line differ by less than a factor two. JNDs in orientation of an illusory contour showed meridional variations similar to those obtained for a real line. By scaling measurements illusory contours are equally visible at all orientations, so meridional variations in illusory orientation discrimination reflect an anisotropy in orientation processing mechanisms. JNDs in orientation measured at an oblique reference orientation improve with practice for an illusory contour as well as for a real line. However while the effect of practice transfers from an illusory to a real contour, the reverse is not true. These results suggest that there are two paths for processing orientation: one activated only by real lines, the other concerned with both real and illusory contours.

Adult↗
PubMed1996

The influence of target angular velocity on visual latency difference determined using the rotating Pulfrich effect.

Visual latency difference was determined directly in normal volunteers, using the rotating Pulfrich technique described by Nickalls [Vision Research, 26, 367-372 (1986)]. Subjects fixated a black vertical rod rotating clockwise on a horizontal turntable turning with constant angular velocity (16.6,33.3 or 44.7 revs/min) with a neutral density filter (OD 0.7 or 1.5) in front of the right eye. For all subjects the latency difference associated with the 1.5 OD filter was significantly greater (P < 0.001) with the rod rotating at 16.6 rev/min than at 33.3 revs/min. The existence of an inverse relationship between latency difference and angular velocity is hypothesized.

Adult↗
PubMed1990

Illusory contour orientation discrimination in the cat.

We present the first evidence that a non-human species (the cat) is able to discriminate the orientation of illusory contours. Following Vogels and Orban45, we used two types of illusory contours. In one type, the illusory contour was defined by a number of contour-inducing semicircles, of which the endpoints were separated by a gap. In the other pattern, the inducing semicircles were shifted in phase along their diameter and their endpoints were aligned along the contour. Just noticeable differences in orientation were measured (at the 73.5% correct level), using a Wetherill and Levitt49 staircase procedure. Values in the order of 11 degrees were obtained when using the first type of illusory contour. Just noticeable differences with the second type were in the order of 17 degrees. Reducing the salience of the illusory contour, whether by scrambling the contour, or by decreasing the number or the contrast of inducing semicircles, systematically increased discrimination thresholds.

Animals↗
PubMed1992

Optical disguising of orbital deformity with prism and cylinder lenses.

This paper describes the way in which prism and cylinder lenses may be used to disguise orbital dystopia when the affected eye is blind. The lenses used can correct the height of the eye, the opening of the eyelids or the rotation or slant of the eye. Four cases are presented to illustrated this technique, which may be used either instead of, or as an adjunct to orbital surgery. An investigation of these effects is described using a mannikin head fitted with an optician's trial frame, with measuring by the reflex metrograph. This revealed that an inferiorly displaced eye may be elevated by 4 to 5 mm, that the vertical eyelid opening may be widened (or narrowed) by just under 20% and that an adverse slant of the eye may be rotated in either direction by about 4 degrees.

Adolescent↗
PubMed1978

Visual agnosia in an artist.

This report describes a 73 year old left-handed male artist who presented with agnosic symptoms following an occipital cerebral vascular accident. Against a background of a memory disturbance, but otherwise essentially intact linguistic and cognitive capacities, the patient was generally unable to identify single objects on visual presentation, and displayed marked difficulty in interpreting complex objects, depicted scenes, and partially occluded figures. The patient's preserved ability to recognize geometric forms, to perceive optical illusions, and to copy designs, and objects with considerable accuracy suggested the clinical picture of visual agnosia. An examination was undertaken of the effects of this recognition disorder on the artist's capacity to draw. Despite an inability to recognize an object or scene, the patient retained various techniques (perspective, shadowing, designation of texture) which allowed him to copy the display in a veridical fashion. When displays were recognized, or when the patient was given only the name of the object and asked to draw it, he adopted a less slavish approach, characteristic of his pre-morbid artwork. While his post-morbid drawings bore a strong similarity to his earlier works, such features as insufficient differentiation of figure from ground, over-elaboration of detail, and areas of neglect revealed the debilitating effects of the recognition disorder. Moreover, an examination of the patient's strategies revealed numerous compensatory verbal and motor procedures which guided his drawing. Acknowledgments. We wish to acknowledge Dr. Prather Palmer, J.R.'s neurologist, for his cooperation and for making available his very thorough examination notes. Thanks also to Dr. Kent Stevens for providing the stereograms, and to Dr. Frank Benson and Dr. Edgar Zurif for their helpful comments on the manuscript.

Aged↗
PubMed1999

The Fröhlich effect: a consequence of the interaction of visual focal attention and metacontrast.

Usually we assume that the central nervous system preserves temporal sequences. Here we show that moving objects--in the context of behaviour often dangerous ones--are seen with a shorter latency than stationary (flashed) objects. In addition moving objects are deblurred. Two mechanisms contribute to this functional specialisation: cue-induced visual focal attention and metacontrast. Under unnatural conditions these mechanisms lead to an optical illusion first described by Fröhlich [Fröhlich, F. W. (1923). Uber die Messung der Empfindungszeit. Zeitschrift für Sinnesphysiologie, 54, 58-78].

Adult↗
PubMed1989

Binocular depth from surfaces versus volumes.

Subjects were asked to compare the relative depths of two binocular targets embedded in different random dot stereogram backgrounds. The disparities of the background points were either randomized, corresponding to a scattering of points within a volume, or arranged according to a sawtooth (triangle-wave) disparity profile (i.e., a set of slanted planar surfaces separated by sharp depth discontinuities). When the targets were embedded in the random volume, their depths were perceived in accordance with their relative disparities. But when the target points were embedded in the sawtooth surfaces their depths were systematically misperceived in a manner predicted by the incorrect depth interpretation of the background points. Rather than seeing a sawtooth pattern, the background points resembled a staircase in depth, and the targets, which appeared embedded in different steps, were misjudged in depth accordingly. The effect suggests a distinction between the depth processing of isolated binocular features and those associated with continuous surfaces.

Attention↗
PubMed1989

Illusory conjunctions inside and outside the focus of attention.

This article addresses 2 questions that arise from the finding that visual scenes are first parsed into visual features: (a) the accumulation of location information about objects during their recognition and (b) the mechanism for the binding of the visual features. The first 2 experiments demonstrated that when 2 colored letters were presented outside the initial focus of attention, illusory conjunctions between the color of one letter and the shape of the other were formed only if the letters were less than 1 degree apart. Separation greater than 2 degrees resulted in fewer conjunction errors than expected by chance. Experiments 3 and 4 showed that inside the spread of attention, illusory conjunctions between the 2 letters can occur regardless of the distance between them. In addition, these experiments demonstrated that the span of attention can expand or shrink like a spotlight. The results suggest that features inside the focus of attention are integrated by an expandable focal attention mechanism that conjoins all features that appear inside its focus. Visual features outside the focus of attention may be registered with coarse location information prior to their integration. Alternatively, a quick and imprecise shift of attention to the periphery may lead to illusory conjunctions among adjacent stimuli.

Adult↗
PubMed1989

Visual misalignment in arc and chevron figures.

Five experiments investigated an apparent misalignment effect in 90 degrees arc figures. Preliminary observations showed that the effect occurs also in chevron figures, in an afterimage of the arc figure, and haptically in arc- and chevron-shaped objects. The experiments showed that the effect is greater with 3 radial lines than with 2, absent without them, and present in a figure consisting of only 3 radial lines. The effect with arc figures was consistently greater than that with chevron figures, a difference found not to be due to an apex marking the midpoint of the latter, and it was of intermediate size in figures with 1 arc boundary and 1 chevron boundary. The misalignment was also greater in narrow, elongated figures. The issues singled out for discussion are the effect of context on the misalignment effect with 3 radial lines, a possible explanation in terms of perceptual compromise, the difference in the effect between arcs and chevrons, and the relationship between this illusion and the Morinaga illusion.

Adult↗