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PubMed1986

Perceptual ambiguity and stability of reversible figures.

In this work, the results of two experiments on ambiguous patterns are reported, which have been obtained by presenting a series of stimuli designed, in both cases, to reduce gradually the ambiguity of the patterns. Such reduction has been performed by respectively increasing or decreasing the amount of graphic details in the experiments. Data of both experiments show a lengthening of mean reversal time. The increase in the stability of one percept can be regarded as associated with the increasing difficulties encountered by an observer in organizing and restating the alternative "hypothesis" through the perceptual mechanisms. The loss of balance in the durations of percepts is discussed in terms of their different informational contents. Finally, in Exp. 1 an analysis is made to evaluate to what extent an addition of perceptual cues, designed to reinforce a three-dimensional interpretation of a pattern, can influence its figure-ground alternation.

Adult↗
PubMed1991

Occlusion and the perception of coherent motion.

Recent work (Shimojo, Silverman & Nakayama, 1989; Vision Research, 29, 619-626) suggests that the visual system must discriminate between extrinsic boundaries (boundaries created by front occluding surfaces) and intrinsic boundaries (real object boundaries) in order to recognize objects and that this would importantly affect the way it solves the so-called "aperture problem" in motion. With the aid of a series of demonstrations (plus two formal experiments) we (1) propose a new explanation for the fact that edge line terminators in a "barber pole" display are perceived as intrinsic; (2) show that inner line terminators in a plaid pattern (i.e. those resulting from the intersection of the two sets of bars) specify coherent or incoherent motion depending on the availability of occlusion-related depth information; and (3) suggest a unitary scheme integrating the two current alternative solutions to the aperture problem, the one based on line terminators and the one based on a velocity space combination of orthogonal components.

Depth Perception↗
PubMed1992

The perception of shrinking in apparent motion.

Apparent motion was produced using two triangular patterns of different sizes, each exposed for 100 msec, with a 50-msec interstimulus interval and 200-msec recycle interval. The triangles were aligned either on center or on the midpoints of the bases. In Experiment 1, filled, outline, and three-dot triangles were viewed over four backgrounds: a blank illuminated field, and texture gradients constructed from horizontal lines, perspective lines, or a combination of these (full texture). In Experiment 2, outline and dot triangles were presented in one of three orientations: base down, base right, and base up over a blank background. Subjects made two forced-choice responses: apparent size was categorized as shrinking or not shrinking, and apparent motion was categorized as motion in depth or motion in a fixed frontal plane. The type of alignment was the major determiner of responses. When the midpoints of the base were aligned, the predominant response described a shrinking object in a fixed position in depth. When the centers were aligned, the predominant response described an object of constant size moving in depth.

Adult↗
PubMed1998

Recovery of 3D volume from 2-tone images of novel objects.

In 2-tone images (e.g., Dallenbach's cow), only two levels of brightness are used to convey image structure-dark object regions and shadows are turned to black and light regions are light regions are turned white. Despite a lack of shading, hue and texture information, many 2-tone images of familiar objects and scenes are accurately interpreted, even by naive observers. Objects frequently appear fully volumetric and are distinct from their shadows. If perceptual interpretation of 2-tone images is accomplished via bottom-up processes on the basis of geometrical structure projected to the image (e.g., volumetric parts, contour and junction information) novel objects should appear volumetric as readily as their familiar counterparts. We demonstrate that accurate volumetric representations are rarely extracted from 2-tone images of novel objects, even when these objects are constructed from volumetric primitives such as generalized cones (Marr, D., Nishihara, H.K., 1978. Proceedings of the Royal Society London 200, 269-294; Biederman, I. 1985. Computer Vision, Graphics, and Image Processing 32, 29-73), or from the rearranged components of a familiar object which is itself recognizable as a 2-tone image. Even familiar volumes such as canonical bricks and cylinders require scenes with redundant structure (e.g., rows of cylinders) or explicit lighting (a lamp in the image) for recovery of global volumetric shape. We conclude that 2-tone image perception is not mediated by bottom-up extraction of geometrical features such as junctions or volumetric parts, but may rely on previously stored representations in memory and a model of the illumination of the scene. The success of this top-down strategy implies it is available for general object recognition in natural scenes.

Adult↗
PubMed1981

False perspectives.

Textbook accounts of perception frequently contain grossly wrong perspective drawings and/or statements about perspective which misrepresent the available monocular information about distance and size, as well as giving the unfortunate impression that perspective alone elicits only a weak impression of recession. An early example of such a false perspective is the 'Ames window', which is not a correct rendering of a slanted window.

Depth Perception↗
PubMed1988

Perceptual alternations in stereokinesis.

When a flat ellipse is slowly rotated in the frontoparallel plane, two different 3-D percepts can be obtained: (i) a rigid circular disc tilting back and forth in 3-D space, and (ii) an elongated egg, slanted into 3-D space, whose end parts seem to be located at different distances from the observer and describe a circular trajectory with respect to the frontal plane. Under prolonged observation, the two 3-D percepts alternate in time, separated by brief intervals in which either the rotation of a rigid flat ellipse in the frontal plane or an amoeba-like distortion of a 2-D shape can be perceived. An experiment is reported in which the sequence of perceptual alternations was investigated. Results show that the 3-D disc is mostly preceded by impressions of elastic amoeba-like deformations, whereas the 3-D egg is mostly preceded by the percept of a rotating flat ellipse. Direct transitions from egg to disc are not as frequent as transitions from disc to egg. Results are discussed in relation to Braunstein and Andersen's hypothesis that phenomena of this sort might result from the stimulation of automatic mechanisms for perceiving size change (changing-size neutral channels).

Attention↗
PubMed1989

Viewing distance affects stereoscopic tilt created with spatial frequency disparity.

A controversy exists as to whether stereoscopic tilt created with interocular differences in spatial frequency is based on perception of spatial frequency disparity or positional disparity. To determine which hypothesis is correct, we investigated the influence of viewing distance on perceived tilt. Tilt was induced by having observers view, at three viewing distances, dichoptic spatial frequency grating patterns differing in frequency by 25%. By appropriate physical scaling of the size of the patterns, their spatial frequency and angular width remained unchanged as distance varied. Under such conditions, the spatial frequency disparity hypothesis predicts no effect of distance, whereas the positional disparity hypothesis predicts a significant effect of distance (due to stereoscopic depth constancy) on the magnitude of tilt. The results showed that perceived tilt does covary with distance, a result consistent with only the positional disparity hypothesis.

Adolescent↗
PubMed2002

Buildup and decay of a three-dimensional rotational aftereffect obtained with a three-dimensional figure.

Gaps in past literature have raised questions regarding the kinds of stimuli that can lead to three-dimensional (3-D) rotation aftereffects. Further, the characteristics of the buildup and decay of such aftereffects are not clear. In the present experiments, rotation aftereffects were generated by projections of cube-like stimuli whose dynamic perspective motions gave rise to the perception of rotation in unambiguous directions; test stimuli consisted of similar cubes whose rotation directions were ambiguous. In experiment 1, the duration of the adaptation stimulus was varied and it was found that the 3-D rotation aftereffect develops with a time constant of approximately 26 s. In experiment 2, the duration between adaptation and testing was varied. It was found that the 3-D rotation aftereffect has a decay constant of about 9 s, similar to that observed with 2-D motion aftereffects. Experiment 3 showed that the rotation aftereffects were not simple depth aftereffects. To account for these aftereffects and related data, a modification of an existing neural-network model is suggested.

Adult↗
PubMedDate not supplied

[Perceptive and praxic impairments in traumatic brain injury patients: significance in activities of daily living].

OBJECTIVE: The main goal of the present study is to describe the implication of different perceptive and praxic deficits in activities of daily living (ADL). We also review some assessing scales and intervention techniques used in the training of these mentioned alterations after a traumatic brain injury (TBI). DEVELOPMENT: There are three kinds of processes important for a correct ideation, motor planification and execution of a learned purposeful movement (praxis). Some of these processes are related to the visual basic perception of the objects (colour, form, size, etc). Others are related to the most complex perception functions (depth, location, recognition,etc). The third group of functions are those integrating the sensory information (discrimination right left side, corporal scheme disorders, etc). If ADL performance needs a suitable functioning of different structures of the nervous central system, then assessment and treatment will be necessary to obtain the best independence level of TBI patients. This is the main objective in Occupational Therapy. ADL may be separated in two different groups: basic abilities (feeding, grooming, dressing, etc) and instrumental abilities (meal preparation, managing money, shopping, using public transportation, etc). CONCLUSION: Impairments in both praxic and perceptive processes constitute one of the most prevalent deficits in TBI patients. From a clinical point of view, these deficits may lack the capacity to carry an independent life.

Activities of Daily Living↗
PubMed1997

Motion detection deficits in infantile esotropia without nystagmus.

PURPOSE: To investigate whether adults with infantile strabismus but without latent nystagmus have abnormalities of horizontal motion detection. METHODS: Eleven adult subjects with infantile esotropia but without latent nystagmus and 15 control subjects were required to detect the onset of motion and drift direction of a sinusoidal, spatial frequency grating that moved with linearly increasing velocity. The grating was presented monocularly in paracentral vision at an eccentricity of 16.5 degrees with a field size of 18 degrees. The contrast of the grating was just above contrast threshold for visibility. RESULTS: The mean velocity threshold for detection of motion was raised significantly in the patient group compared with the control group. Nine of the 11 subjects with infantile esotropia demonstrated directional asymmetry for the detection of motion. Thresholds were elevated more often when the grating was moving nasally in the squinting eye and temporally in the nonsquinting eye, and raised thresholds were more prevalent in the squinting eye. CONCLUSIONS: The findings indicate that in infantile esotropia, the presence of motion perception deficits are not always associated with the development of latent nystagmus. The predominance of nasally directed motion deficits in the squinting eye and temporally directed motion deficits in the nonsquinting eye was unexpected and may have been caused by abnormal development of cortical motion processing.

Adult↗
PubMed2003

Reversed-phi perception with motion-defined motion stimuli.

Perception of reversed-phi with motion-defined motion (MDM) stimuli was examined while varying various parameters including eccentricity. For peripheral viewing, reversed-phi was observed at all displacements between 30 degrees and 135 degrees. The perception most prominent at 90 degrees, but was disrupted by dichoptic presentation. These results suggest operations of an energy-based motion system similar to the first-order motion system for luminance motion, which most likely resides at a relatively early level (cf. [Vision Res. 33 (1993) 533]). For central viewing, reversed motion was observed only for larger displacements. The perceived motion at smaller displacements was predominantly in the forward direction. Transition between the two modes occurred around 90 degrees displacement. In addition, this motion perception was not disrupted by dichoptic presentation. This indicated the operation of a polarity independent matching-based motion system residing at a higher-level. Thus, the results indicate the involvement of at least two separate mechanisms for MDM detection, and that there is a dominance shift between the two systems according to the eccentricity.

Contrast Sensitivity↗
PubMed1997

Occlusion constraints and stereoscopic slant.

In binocular vision horizontal magnification of one retinal image leads to a percept of three-dimensional slant around a vertical axis. It is demonstrated that the perception of slant is diminished when an occlusion interpretation is possible. A frontoparallel plane located in the immediate vicinity of a slanted surface in a location which allows a perception of occlusion reduces the magnitude of perceived slant significantly. When the same plane is placed on the other side, the slant perception is normal because there is no alternative occlusion interpretation. The results indicate that a common border between the occluder and a slanted surface is not a necessary condition for the reduction effect. If the edges are displaced and the edge of the slanted surface is placed in a location in which it could be occluded, the effect still appears.

Depth Perception↗
PubMed1980

How does adaptation to disparity affect the perception of reversible figures?

The finding of Virsu in 1975 that prior adaptation to a version of the Schröder staircase with disparity tends to produce the opposite perspective interpretation of a subsequently presented zero-disparity stereogram was confirmed. A similar pattern of results was found when the test field was monocularly rather than binocularly presented, even after an interval of 30-60 seconds. The origin of the effect is discussed in the light of this new evidence.

Depth Perception↗
PubMed2003

Mirror-reflecting a picture of an object: what happens to the shape percept?

Although a two-dimensional picture never fully specifies the actual layout of the depicted three-dimensional scene, one is still able to make a three-dimensional interpretation. When a picture is mirror-reflected, the range of plausible scenes possibly corresponding with the depicted scene has not changed with respect to the original depiction. We were curious to find out whether the inherent picture ambiguities would be solved the same way or differently. Participants performed local attitude settings on three sets of pictures: (1) original pictures, (2) left-right mirrored pictures, and (3) up-down mirrored pictures. Pairwise comparison of the pictorial reliefs of the depicted object, reconstructed from the raw settings, revealed dissimilarities. The differences, however, could be drastically diminished by conducting an affine transformation correction taking into account not only the depths, but also the picture plane coordinates. The inherent ambiguities seemed thus to be solved differently between conditions. By factoring out different solutions to the ambiguities, the pictorial reliefs were found to be equivalent.

Adult↗
PubMed2000

Temporal dependencies in resolving monocular and binocular cue conflict in slant perception.

Observers viewed large dichoptic patterns undergoing smooth temporal modulations or step changes in simulated slant or inclination under various conditions of disparity-perspective cue conflict and concordance. After presentation of each test surface, subjects adjusted a comparison surface to match the perceived slant or inclination of the test surface. Addition of conflicting perspective to disparity affected slant and inclination perception more for brief than for long presentations. Perspective had more influence for smooth temporal changes than for step changes in slant or inclination and for surfaces presented in isolation rather than with a zero disparity frame. These results indicate that conflicting perspective information plays a dominant role in determining the temporal properties of perceived slant and inclination.

Cues↗
PubMed1977

Stereoscopic localization with the eyes asymmetrically converged.

When the eyes fixate a point in the median plane of the head, the physical surface to which the stereoscopic disparities of "nearer" and "farther" points are referred is the so-called frontal plane horopter. When, however, a point is fixated with the eyes in asymmetrical convergence, the horopter is now rotated with respect to the objective normal plane--that is, the surface that is normal to the direction of gaze. This rotation is believed to pose a problem with veridical stereoscopic localization. It has, therefore, been postulated that in asymmetrical convergence some physiological compensation takes place in order to preserve accurate stereoscopic perception. Some experimental evidence apparently supports this view. The logic of the arguments and the experiments are critically assessed with the conclusion that there is little evidence of any visual modification, nor would any seem likely. A reinterpretation of the stereoscopic axes of reference does occur, but this can be accounted for purely by innervational factors, the visual aspects remaining unchanged.

Afterimage↗
PubMed1996

Visual object recognition.

Visual object recognition is of fundamental importance to most animals. The diversity of tasks that any biological recognition system must solve suggests that object recognition is not a single, general purpose process. In this review, we consider evidence from the fields of psychology, neuropsychology, and neurophysiology, all of which supports the idea that there are multiple systems for recognition. Data from normal adults, infants, animals, and brain damaged patients reveal a major distinction between the classification of objects at a basic category level and the identification of individual objects from a homogeneous object class. An additional distinction between object representations used for visual perception and those used for visually guided movements provides further support for a multiplicity of visual recognition systems. Recent evidence from psychophysical and neurophysiological studies indicates that one system may represent objects by combinations of multiple views, or aspects, and another may represent objects by structural primitives and their spatial interrelationships.

Adult↗
PubMed2001

Runway width effects in the visual approach to landing.

The effect of changes in runway width on the perception of glide-slope has been the subject of extensive investigations. Despite considerable research, an explanation of this effect has been elusive. A mathematical model for glide-slope perception was published recently based on the premise that a desirable goal of perception is to form a perception with minimal uncertainty. One of the qualitative predictions of that model was that changes in the aspect ratio of the runway would affect the perceived glide-slope. In this article, the predictions of the model are quantified and are shown to be in close agreement with experimental results in the literature.

Aerospace Medicine↗