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PubMed1982

The Charles Bonnet syndrome in perspective.

The clinical condition occurring in the elderly with preserved intellectual functions, characterized by vivid, elaborate and dynamic recurrent visual pseudo-hallucinatory phenomena of a pleasant or neutral nature and often associated with ocular pathology - the Charles Bonnet Syndrome - is described. Its main clinical features are illustrated and the aetiology discussed. Two new cases are presented and areas for future research are outlined.

Age Factors↗

PubMed1991

Children's communicative abilities revisited: verbal versus perceptual disambiguating strategies in referential communication.

The way seven- and nine-year-old French children and adults interpret ambiguous object descriptions was studied in four experiments. All four experiments followed the same basic procedure and consisted of two phases. During the training phase, plastic blocks varying in shape, colour and size were associated with fruit names. Some objects were always given just one label, the generic term, whereas others could be given either the generic or the specific label. For example, large circles were referred to as 'pommes' ('apples'), and large blue circles as 'reinettes' (a kind of apple). Still others were given no name at all. During the test phase of the experiment, subjects were presented with referentially ambiguous messages and asked to point to a given object (Experiments 1, 2 and 3) or to draw an object (Experiment 4). The messages consisted of either a generic term preceded by an article (e.g. '(la pomme'-'the apple') or a verbal description based on a colour adjective (e.g. 'le bleu'-'the blue one'). When presented with the generic term, older children and adults persistently chose the object for which no specific term had been given during the training phase. When presented with an adjective description, they chose the referent for which no fruit name had been given during the training phase. Younger children's choices depended on whether the article preceding the description was definite or indefinite. These results are consistent with the Gricean approach to referential communication proposed by Jackson & Jacobs (1982) and with an extended version of the principle of contrast proposed by Clark (1987, 1988).

Child↗

PubMed1988

Effects of serial lesions of telencephalic components of the visual system in pigeons.

A serial-lesion technique was used to investigate interactions in visual processing between telencephalic components of the pigeon visual system. Pigeons were trained to discriminate pairs of stimuli that differed in color, intensity or pattern. After mastering the discrimination tasks, they were assigned to one of three groups. The first group (WI-EII) received lesions of the visual Wulst and were retested. After the discrimination tasks were again mastered, a second set of lesions was made, this time in the ectostriatum. The birds were tested once again after the second surgery. The second group (EI-WII), underwent the same sequence of events except that the order of the lesions was reversed. In the third group (E + W), lesions of both the visual Wulst and ectostriatum were made in a single operation, followed by retesting. The performance after the first lesion of the subjects in each of the two-stage lesion groups was typical of performance after such lesions; i.e. the birds with visual-Wulst lesions showed little or no impairment on any of the tasks, whereas the pigeons with ectostriatum lesions showed considerable deficits in intensity and pattern discrimination, which diminished after prolonged retraining. In contrast, the pigeons in the one-stage group (E + W) showed profound deficits that appeared to be permanent. The performance after the second operation of the WI-EII group was the same as that of pigeons with lesions of ectostriatum alone; i.e. destruction of ectostriatum first or second resulted in the same duration of impairment. The performance of the EI-WII group after its visual Wulst lesion, however, was similar to that observed in the E + W group. The results are interpreted as a reflection of parallel processing within the avian visual system; i.e. the presence of an intact tectofugal pathway may mask the effects of thalamofugal pathway interruption.

Animals↗

PubMedDate not supplied

Global integration of local color differences in transparency perception: An fMRI study.

In normal viewing, the visual system effortlessly assigns approximately constant attributes of color and shape to perceived objects. A fundamental component of this process is the compensation for illuminant variations and intervening media to recover reflectance properties of natural surfaces. We exploited the phenomenon of transparency perception to explore the cortical regions implicated in such processes, using fMRI. By manipulating the coherence of local color differences around a region in an image, we interfered with their global perceptual integration and thereby modified whether the region appeared transparent or not. We found the major cortical activation due to global integration of local color differences to be in the anterior part of the parahippocampal gyrus. Regions differentially activated by chromatic versus achromatic geometric patterns showed no significant differential response related to the coherence/incoherence of local color differences. The results link the integration of local color differences in the extraction of a transparent layer with sites activated by object-related properties of an image.

Adult↗

PubMedDate not supplied

V4 lesions in macaques affect both single- and multiple-viewpoint shape discriminations.

The role of cortical area V4 in complex shape discriminations was studied by testing the effects of V4 lesions in macaques on the ability to visually discriminate between images of three-dimensional (3D) objects from different viewpoints. Stimuli were presented in pairs in the lower left or lower right visual field quadrants about 4 deg from the fovea, and the monkeys judged on each trial whether the two views were of the same or of different objects. Object similarity was varied to determine a threshold shape difference. V4 lesions caused profound, retinotopic, and apparently permanent disruptions of discrimination, regardless of whether the images represented single or multiple viewpoints. In V4 lesioned portions of the visual field, monkeys could discriminate objects only when they differed much more grossly in shape than was true in control locations. These effects of the lesion were virtually identical for discriminations that had been learned before lesions were placed and for those learned afterwards. As in previous studies, V4 lesions elevated contrast thresholds by approximately a factor of two, but control observations showed that this was not the basis of the disruption of shape discrimination. Manipulation of cues to shape showed that in control locations, monkeys maintained excellent shape discrimination despite a variety of stimulus alterations, whereas in V4 lesioned areas their performance was easily disrupted. This finding suggests that V4 may support visual shape discriminations by facilitating the use of multiple visual cues. However, the fact that single-viewpoint and multiple-viewpoint discriminations were similarly affected indicates that the disruption was not specific to 3D shape discrimination, but may apply to a variety of subtle discriminations.

Animals↗

PubMed2006

An empirical test of formal equivalence between Emmert's law and the size-distance invariance hypothesis.

Emmert's law and the size-distance invariance hypothesis have been said to be formally equivalent, provided that Emmert's law means that the perceived size of an afterimage is proportional to the perceived distance of the projected surface of the afterimage. However, there have been very few studies that have attempted to verify this formal equivalence empirically. We measured both the perceived size and distance of afterimages and real objects with the same proximal size. Nineteen participants projected afterimages of 1 deg in visual angle on the wall located at distances of 1 to 23 meters from the participants. They also observed real objects, disc-shaped and made from a sheet of Styrofoam board, with the same proximal size as that of the afterimages, which were located at the same physical distances as those of the wall on which the afterimages were projected. Each participant reproduced the apparent sizes of the afterimages and real objects using the reproduction method and estimated the apparent distances using the magnitude estimation method. When the mean apparent sizes of the afterimages and real objects, represented as a function of apparent distance, were fitted to a linear function, the slopes for the afterimages and real objects did not differ significantly. These results are interpreted as evidence for the formal equivalence of Emmert's law and the size-distance invariance hypothesis.

Adolescent↗

PubMed1986

Shape constancy and polar perspective.

The distortion of polar perspective depends on the depth of the tridimensional shape and on the observation distance. In four experiments using 54 undergraduates as subjects, we found that a compensation process which takes depth and observation distance into account corrects for such distortions. Compensation was demonstrated in experiments in which deceptive information on depth and on observation distance was provided. The result was distortions of the perceived shapes that would be expected if compensation were based on the deceptive information.

Cues↗

PubMed1989

Movement and visual attention: the spotlight metaphor breaks down.

The interfering effects of distractor letters are known to diminish with increasing distance from the target letter (Eriksen & Eriksen, 1974). This result is held to support spotlight models in which visual attention can only be assigned to contiguous regions of the visual field. However, the result is also consistent with the rival claim that attention is assigned to perceptual groups. Four experiments show that grouping of target and distractors by common motion can have more influence than their proximity. Distant distractor letters that move with a target letter produce more interference than static distractors that are nearer the target. Near distractors are equally ineffective if the target is static while they move. These results imply that attention is directed to perceptual groups whose components may be spatially dispersed. The spotlight metaphor seems inappropriate for visual attention in a dynamic environment.

Adult↗

PubMed1991

Visual curve tracing properties.

Subjects decided whether 2 dots were on the same curve or 2 different curves, and the curvature of the curves or the proximity of other (distractor) curves to the target curve was varied. Response time increased as the arc length of the curve connecting the 2 dots increased, suggesting that the curve was traced to perform the task. Tracing rate was faster for low- than high-curvature contours and was increasingly slower as distractor contours were increasingly proximal to the traced curve. Proximity results were predicted by a model in which response time depends on the ratio of the distance between the dots and the distance between adjacent lines. Curve tracing operations used to integrate information along contours are sensitive to several properties of the contours. The implications of the sensitivity of tracing operations to these curve properties are discussed.

Adult↗

PubMed1993

Recovering three-dimensional shape from perspective translations and orthographic rotations.

Perceived orientation in depth and 3D shape was investigated for perspective projections of translations and orthographic projections of rotations of 3D dihedral angles. The principal findings were that (a) perceived orientation in depth depends on the sign of the velocity gradient, even in the case of orthographic projections; (b) the relationship between perceived orientation and the sign of the velocity gradient is greater for shallower gradients in orthographic projections of rotations, consistent with previous findings for perspective translations; (c) the magnitudes of simulated dihedral angles were underestimated (relative depth overestimated) for orthographic projections of rotations but were overestimated for perspective projections of translations; and (d) the judged magnitude of the dihedral angle depends on the velocity ratio and on image compression; it cannot be predicted from the velocity ratio or the velocity gradient alone.

Adult↗

PubMed1993

Stimulus-driven attentional capture and attentional control settings.

Jonides and Yantis (1988) found that abrupt-onset singletons capture attention in visual search when onset is orthogonal to the target's defining and reported attributes and that color and brightness singletons do not. They concluded that abrupt onset may be unique in capturing visual attention. Folk, Remington, and Johnston (1992) challenge this conclusion and argue that (a) the occurrence of attentional capture is contingent on the adoption of an appropriate attentional control setting by the observer and (b) properties other than onset (in particular, color) can capture attention involuntarily. In this article, each of these claims is critically evaluated, and it is argued that the results reported by Folk et al., though important, do not definitively corroborate either one. The available evidence concerning stimulus-driven attentional capture is summarized, and 3 empirical generalizations that characterize the evidence are advanced.

Attention↗

PubMed1976

Detection errors on the word the: evidence for reading units larger than letters.

In four experiments, subjects read 100-word passages and circled instances of the letter t. Subjects missed a disproportionate number of ts in the word the. Evidence was provided against explanations of this effect involving the location and pronunciation of the t in the and against an explanation in terms of the redundancy of the. Rather the high frequency of the appeared to be critical, and it was suggested that high-frequency words are read in terms of units larger than the letter.

Auditory Perception↗

PubMed1976

An investigation of the relationship between viewing condition and preference for true and modified linear perspective and adults.

In four experiments, pictures varying in degree of perspective convergence from linear to parallel were observed under the following conditions: at an arbitrary station point, at correct station points, and with unconstrained view. Adults were asked to rank the pictures from the most to least natural and realistic-looking picture or from the most to the least accurate drawing. Subjects nearly always chose the parallel perspective pictures as most preferred and the linear perspective pictures as least preferred. Intermediate degrees of convergence were ranked accordingly. Results were interpreted in light of an argument for a pictorial station poing independent of the correct center of projection for a picture. Since this station point was calculated to be at a distance at least 10 times as great as the object is large, its assumption for pictorial viewing was termed the "Zoom "effect.

Adult↗

PubMed1994

Intact visual imagery and impaired visual perception in a patient with visual agnosia.

Although it is now well accepted that visual mental imagery and visual perception share common underlying mechanisms, there are several reports in which they are dissociated. Evidence for the separability of these processes is provided by a patient, C.K., who has a profound visual object recognition deficit attributable to an impairment in grouping or segmenting visual images. Despite this perceptual deficit, C.K. was able to draw objects in considerable detail from memory, and his knowledge of the visual appearance of objects was preserved on a variety of mental imagery tasks. Together with previous cases, these findings confirm the double dissociation between object recognition and perception. Interestingly, C.K. could also recognize newly constructed objects in his internal imagery. To accommodate these results, we propose a model in which imagery and perception are strongly associated but are also functionally specialized.

Adult↗

PubMed1996

Effects of changing viewing conditions on the perceived structure of smoothly curved surfaces.

Observers' perceptions of a male and a female torso were investigated using monocular and stereoscopic images under varying conditions of illumination. Observers judged the shapes of these torsos by adjusting a gauge figure to estimate the local slant and tilt at numerous probe points arranged in a lattice over the torso's surface. The results revealed that the judged surfaces in the monocular and stereoscopic conditions were related by an affine stretching transformation in depth that accounted for approximately 95% of the between-condition variance. There was also a strong affine component between the judgments obtained for the different illumination directions, although a further analysis of the residuals indicated that changing the direction of illumination influenced perceived structure in a piecewise manner.

Contrast Sensitivity↗

PubMed2000

Distortions in definite distance and shape perception as measured by reaching without and with haptic feedback.

Psychophysical studies reveal distortions in perception of distance and shape. Are reaches calibrated to eliminate distortions? Participants reached to the front, side, or back of a target sphere. In Experiment 1, feedforward reaches yielded distortion and outward drift. In Experiment 2, haptic feedback corrected distortions and instability. In Experiment 3, feedforward reaches with only haptic experience of targets replicated the shape distortions but drifted inward. This showed that outward drift in Experiment 1 was visually driven. In Experiment 4, visually guided reaches were accurate when participants used binocular vision but when they used monocular vision, reaches were distorted. Haptic feedback corrected inaccuracy and instability of distance but did not correct monocular shape distortions. Dynamic binocular vision is representative and accurate and merits further study.

Adult↗

PubMed1979

Spare the rod and spoil the icon.

Short-term visual storage was investigated with a successive field paradigm, so that correct performance depended upon combining visual information from two targets that were never on simultaneously. In the first two experiments, the stimuli consisted of two slides, each containing a 10' red dot on a gray surround, and separated by an interstimulus interval (ISI) from 20 to 400 msec. Subjects had to determine if the dots were vertically or horizontally aligned. In Experiment 1, the stimuli had either no contrast for the rods or no luminance contrast for the cones or high contrast. At short ISIs the cone contrast determined performance, whereas at long ISIs the rod contrast determined performance. In fact, when the dots were invisible to the rods, the task was impossible for long ISIs. In Experiment 2, performance was compared for zero log rod contrast and for small departures from zero. Even a small departure from zero log rod contrast resulted in above-chance performance. In Experiment 3, the stimuli were luminous rectangles and the task was to decide whether or not a 4' spatial gap was present between the two successively presented rectangles. Wavelength, luminance, and ISI were varied under both photopic and scotopic adapting conditions. The result was that the rods performed the task for ISIs of 150 msec or longer under scotopic conditions and under the photopic conditions that we were able to test. The results of the experiments taken together are consistent with the hypothesis that the cone icon is short, whereas the rod icon is robust and long lasting.

Color Perception↗

PubMed1980

Optic array determinants of apparent distance and size in pictures.

This study explored whether special mechanisms are operative in picture perception to correct for the distortion that occurs when pictures are viewed from the wrong station point. Five photographs were taken of a layout composed of two same-sized dolls positioned at different distances on a flat untextured ground. Perspective differences existed between the photographs as a function of varying the distance of the camera to the layout. Each picture was viewed from five station points along the normal by 12 adults who estimated the relative magnitude of the depth interval between the dolls and judged whether the rear doll was objectively smaller, larger, or the same size as the front doll. No evidence was found of a compensation mechanism operating to stabilize the pictured layout. Distance and size judgments were affected in a manner that was consistent with what would occur if an actual layout and the conditions under which it were viewed were similarly transformed.

Adult↗