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PubMed1987

Putting order in the impossible.

The class of visual illusions called 'impossible figures' (illusory spatial interpretations of pictures) is analyzed in order to introduce an ordering into the great variety of such figures. Such an ordering facilitates reference, unifies terminology, and establishes a conceptual framework for further investigations of the subject, making easier the choice and systematic generation of various types of figures (for example, in systematic psychological experiments). First, the notion of 'impossible figure' is defined and certain other related classes of figures (so-called 'likely' and 'unlikely' figures) are distinguished. Second, the fundamental 'impossibility sources' are identified as elementary 'building blocks' of all impossible figures. Finally, two broad classes of impossible figures, multibars (or 'impossible polygons') and striped figures, are briefly described.

Attention↗
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↗
PubMed1988

Reference frame effects on shape perception in two versus three dimensions.

Three experiments are reported in which it is tested whether the Gestalt effect of configural orientation on shape perception operates on two-dimensional (2-D) or three-dimensional (3-D) representations of space. It is known that gravitationally defined squares and diamonds take longer to discriminate in diagonal arrays than in horizontal or vertical arrays. In the first experiment it is shown that this interference effect decreases dramatically in magnitude when pictorial depth information is added so that subjects perceive the target shapes in different depth planes. In the second experiment this difference is shown not to be due to relative size of the target shapes or to occlusion of a background plane. It is also shown, in the final experiment, that this difference is not due to linear perspective information or merely to perception of the target figures in a 3-D scene. The overall pattern of results supports the position that this configural reference frame effect arises primarily when the elements of the configuration are coplanar, and that the principal organization underlying it is the structure of the perceived 3-D environment rather than that of the 2-D image. In all three experiments, however, there is also a small interference effect in the noncoplanar 3-D conditions. This might be due either to some aspect of reference frame selection operating on the 2-D image representation or to the failure of subjects to see depth in the 3-D stimuli on some proportion of the trials.

Attention↗
PubMed1988

Perceptions of depth elicited by occluded and shearing motions of random dots.

A computer-controlled display of random dots was used to study perceptions of depth. In this display, a field of stationary random dots surrounded a rectangular area in which random dots moved with uniform velocity in a single direction. The boundaries of this rectangle did not move. When dot motion was perpendicular to the longer boundary of the rectangle (occluded motion), the rectangle seemed to be behind the stationary background surround. Motion parallel to the longer boundary of the rectangle (shearing motion) made it appear in front of the surround. The relative lengths of the sides of the rectangle determined which effect predominated. Thus, for motion perpendicular to the long axis of the rectangle the occlusion predominated and naive subjects reported that the central area seemed farther away than the surround. For shearing motion parallel to the long axis, the subjects reported that the rectangle was closer than the surround and the strength of both effects also depended on the length-to-width ratio of the rectangle. If there was occluded motion along the long axis, as the length-to-width ratio increased so did the likelihood that subjects would report seeing the rectangle behind the surround. Conversely, with shearing motion along the long axis, increasing the length-to-width ratio increased the likelihood that the rectangle would appear unambiguously in front of the surround. Some subjects integrated the two cues with the resulting perception being a rotating cylinder. The occlusion effect was stronger than the shearing effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Acceleration↗
PubMed1988

How contrast affects stereoacuity.

Stereoacuity and its dependence on contrast were measured at four spatial frequencies separated by 1 octave steps. Using a method of adjustment, observers adjusted the retinal disparity of an aperiodic narrow-band stimulus until it appeared in the depth plane defined by two flanking reference lines. Variations in contrast affected stereoacuity (the standard deviation of ten depth settings), with better performance observed at higher contrasts. Data were fit with straight lines (on a log-log plot), indicating a power-law dependence on contrast; the slope was steeper at lower spatial frequencies. These findings are consistent with the idea that disparity is computed from the responses of size-tuned mechanisms characterized by nonlinear contrast transfer functions. In a second experiment, the effects of interocular differences in contrast on stereoacuity were studied for two conditions. In the first condition, one eye always viewed a high-contrast target while the other eye viewed targets of successively lower contrast; in the second condition, one eye always saw a target of near-threshold contrast while the other eye saw targets of successively higher contrast. When the fixed contrast was high, stereoacuity deteriorated steadily as the interocular difference in contrast increased; the loss of stereoacuity was greatest at the lowest spatial frequency. When the fixed contrast was low, however, small increases in the contrast to one eye had no deleterious effect on stereoacuity. Once interocular contrast settings exceeded a certain difference, stereoscopic acuity began to deteriorate at lower spatial frequencies. These results address the issue of the stage of visual processing at which contrast exerts its influence on stereopsis.

Adult↗
PubMed1988

Alleys on an extensive apparent frontoparallel plane: a second experiment.

Small light points were presented, in the dark, around a point in the center which was fixed at a distance of about 3 m from the subject. In experiment 1, the subject adjusted the positions of points so that all were frontoparallel and in three horizontal series, each consisting of five points, with the middle series level with the eyes, to satisfy the following conditions: (i) the three series must appear straight and horizontally parallel; (ii) the points of each of the five triplets must appear equally separated vertically; (iii) the three points of each triplet must appear to move horizontally along straight and parallel paths; (iv) the three points of each triplet must appear to move horizontally with a constant vertical separation. The most distant points were about 0.51 rad to the left and right of center, and about 0.22 rad above and below. In experiment 2, with the configuration of points obtained in experiment 1, the subject assessed ratios of all perceptual distances between points and also from the subject to all points. From experiment 1 (three subjects used), Gaussian curvature K and a constant related to depth perception (sigma) were estimated under the assumption that the frontoparallel plane is a Riemannian plane of constant curvature K and that Luneburg's mapping functions between visual space and physical space hold. The analysis was made according to equations different from those used previously. The results of experiment 2 (two subjects used) were analyzed by a new computer program in which no preassumed mapping functions are necessary for the estimation of K. From both analyses it is clear that there is no need to assume any other value of K than 0 (Euclidean) to describe the geometry of the frontoparallel plane. This presents a striking contrast to the results from experiments on parallel and equidistance alleys running toward the subject on the horizontal plane.

Attention↗
PubMed1989

Structure from stereo by associative learning of the constraints.

A computational model of structure from stereo that develops smoothness constraints naturally by associative learning of a large number of example mappings from disparity data to surface depth data is proposed. Banks of disparity-selective graded response units at all spatial locations in the visual field were the input data. These cells responded to matches of luminance change at convergent, divergent, or zero offsets in the left and right 'retina' samples. Surfaces were created by means of a pseudo-Markov process. From these surfaces, shaded marked and ummarked surfaces were created, along with random-dot versions of the same surfaces. Learning of these example shaded and shaded marked surfaces allowed the system to solve stereo mappings both for the surfaces it had learned and for surfaces it had not learned but which had been created by the same pseudo-Markov process. Further, the model was able to solve some random-dot versions of the surfaces when the surfaces had been learned as shaded marked surfaces.

Association Learning↗
PubMed1990

The importance of velocity gradients in the perception of three-dimensional rigidity.

Sequential presentation of a number of random-dot patterns which when super-imposed yield an expanding flow field leads to the perception of a coherent motion towards the observer. The motion vectors in this type of flow field all radiate from the origin. This percept of a global coherent expanding flow results only when the local speeds (magnitude of the local motion vectors) are zero at the centre and increase linearly towards the periphery. If all the dots radiate outwards but have the same speed, a clear percept of three-dimensional nonrigidity arises.

Acceleration↗
PubMed1990

Emmert's law in the Ames room.

The Ames distorted room illusion, in which the perceived sizes of objects placed within the room differ from their objective sizes, has been used to support arguments for indirect perception. A study is reported in which Emmert's law of the apparent size of after-images was examined in relation to the Ames room's illusory alteration of apparent and actual distances. Size judgments of afterimages projected into the Ames room were compared with control conditions in which both actual and apparent afterimage projection distances were reproduced. Results indicate that Emmert's law may not provide a simple geometrical relationship between proximal image size and actual viewing distance, and that the processes involved in making afterimage size judgments are similar to those processes involved in making size judgments of 'real world' objects.

Adult↗
PubMed1990

Amodal representation of occluded surfaces: role of invisible stimuli in apparent motion correspondence.

A series of demonstrations were created where the perceived depth of targets was controlled by stereoscopic disparity. A closer object (a cloud) was made to jump back and forth horizontally, partially occluding a farther object (a full moon). The more distant moon appeared stationary even though the unoccluded portion of it, a crescent, changed position. Reversal of the relative depth of the moon and cloud gave a totally different percept: the crescent appeared to flip back and forth in the front depth plane. Thus, the otherwise-robust apparent motion of the moon crescents was completely abolished in the cloud-closer case alone. This motion-blocking effect is attributed to the 'amodal presence' of the occluded surface continuing behind the occluding surface. To measure the effect of this occluded 'invisible' surface quantitatively, a bistable apparent motion display was used (Ramachandran and Anstis 1983a): two small rectangular-shaped targets changed their positions back and forth between two frames, and the disparity of a large centrally positioned rectangle was varied. When the perceived depths supported the possibility of amodal completion behind the large rectangle, increased vertical motion of the targets was found, suggesting that the amodal presence of the targets behind the occluder had effectively changed the center position of the moving targets for purposes of motion correspondence. Amodal contours are literally 'invisible', yet it is hypothesized that they have a neural representation at sufficiently early stages of visual processing to alter the correspondence solving process for apparent motion.

Attention↗
PubMed1991

Evidence for independent processing of subjective contour brightness and sharpness.

Subjective contours have been of considerable interest because of their importance to theories and physiological models of form perception. In particular, they have recently been characterized as the result of magnocellular cortical processing. There is, however, a paucity of parametric data relating to basic psychophysical parameters in this field. Two experiments are reported in which the roles of subjective contour size, retinal eccentricity, and flicker rate in subjective contour salience were investigated. Eleven observers estimated subjective contour magnitude using an Ehrenstein configuration. Configurations ranging in size from 0.25 to 3 deg were presented to three retinal loci (fovea, 2 deg, and 4 deg) at flicker rates ranging from 5 to 15 Hz. Subjective contour brightness and distinctness were measured separately. Brightness was greatest at a subjective contour size of about 1.25 deg, at flicker rates of 5-7 Hz, and at 3 deg peripheral for all flicker rates and all but the smallest stimulus sizes. Distinctness decreased with eccentricity and flicker, but remained high at small diameters (thus implicating spatially sensitive mechanisms). Taken together, the results support a magnocellular processing of subjective contours with respect to brightness, but also suggest that there is a parvocellular contribution to subjective contour sharpness.

Adult↗
PubMed1991

Orientation disparity, deformation, and stereoscopic slant perception.

Koenderink and van Doorn's theory, that the basis of stereoscopic slant perception is the deformation component of the disparity, field, was tested for slant around a horizontal axis, which produces images with a vertical ramp of horizontal disparity (horizontal shear) characterised by a global orientation disparity at the vertical meridian. The disparity field in this case can be parsed into two components, deformation and curl, which each contribute half of the orientation disparity. This case was compared with similar random-dot stimuli in which the deformation component was doubled and the curl component eliminated or vice versa. All three types of stimuli had identical orientation disparity at the vertical meridian. A condition in which there was no such orientation disparity, but deformation was present, was also included. It was found that perceived slant was not related to the deformation present, as Koenderink and van Doorn's theory would predict, but was predictable from the orientation disparity at the vertical meridian per se.

Depth Perception↗
PubMed1991

Rapid discrimination of McCollough effects.

The McCollough effect is a colour aftereffect that is contingent on pattern orientation. Three experiments were conducted to establish whether such aftereffect colours could serve as a basis for discrimination in several rapid discrimination tasks. In the first experiment it was investigated whether aftereffect colours could act like a simple 'feature' in a visual search task involving a difficult orientation discrimination. Without McCollough adaptation, the time taken to detect a 'target' among 'distractors' increased substantially as the number of distractors increased. With adaptation, detection time was essentially independent of the number of distractors, indicating that the nature of the task changed from a difficult orientation discrimination to a simple discrimination based on differences in aftereffect colours. The second and third experiments employed a difficult four-alternative forced-choice procedure in which subjects were required to discriminate a monochromatic patch of square-wave grating oriented at 45 degrees from three others oriented at 135 degrees (and vice versa). The gratings were presented very briefly (67-333 ms) followed by a 500 ms mask. Subjects performed the task with and without McCollough adaptation. Performance was strikingly better after adaptation: colour aftereffects could be used to make the discrimination even at exposure durations as short as 67 ms. The third experiment demonstrated that this enhanced performance was indeed due to perceived colour differences (rather than a possible contrast difference). The results of the three experiments are discussed in relation to proposals about the locus of the McCollough effect.

Adaptation, Ocular↗
PubMed1991

The detectability of geometric structure in rapidly changing optical patterns.

Human vision is sensitive to the coherent structure and motion of simple dot patterns undergoing rapid random transformations, even when the component dots are widely separated spatially. A study is reported in which visual sensitivity to translations, rotations, expansions, pure shear, and additive combinations of these transformations was investigated. Observers discriminated between coherent (correlated) movements, in which all the component dots moved simultaneously in corresponding directions and distances, and incoherent (uncorrelated) movements, in which the movements of individual dots were statistically independent. In experiment 1 the accuracy of coherence discrimination was found to be similar for all four of the basic transformations and to increase linearly with the distance of the movements. The discriminability of coherent versus incoherent motion was also found to be similar to the detectability of any motion, suggesting that concurrent movements of individual dots are visually interrelated. In experiments 2 and 3 the visual independence of these four groups of transformations was tested by comparing the accuracy of coherence discrimination of each of the transformations presented alone with that when added to background motions produced by each of the four transformations. Coherence discriminations were less accurate when the target transformation was added to another background transformation, indicating that these transformations are not visually independent. Rotations and expansions, however, were visually independent. In experiment 3 qualitatively similar effects for patterns of several different sizes and dot densities were found. In general, an impressive visual sensitivity to globally coherent structure and motion under several different geometric transformations was observed in these experiments. A basic theoretical issue concerns the local visual mechanisms underlying this sensitivity.

Depth Perception↗
PubMed1991

The perception of mirror-reflected objects.

In what ways and under what conditions does an object appear to differ from its enantiomorph (its mirror reflection)? This 'mirror question' or its popular counterpart, "Why does a mirror reverse left and right but not up and down?" is frequently encountered, but an acceptable answer is not to be found in the literature. The question is approached as an experimental problem in visual psychophysics. A mirror optically reverses the axis perpendicular to its surface. What are the perceptual consequences of this stimulus transformation? This question is examined in four experiments by using stimuli of varying complexity and familiarity. Apparent reversals are demonstrated along right-left, front-back, top-bottom, and oblique axes, depending on the perceived asymmetries of the stimulus object. Perceived asymmetry is shown to depend both on structural asymmetries and on canonical axes and orientations defined by social convention. It is concluded that an object appears to differ from its enantiomorph by an apparent reversal along the axis of least perceived asymmetry. Implications for perceptual frames of reference and for the perception of symmetry are discussed.

Adult↗
PubMed1991

The ceiling of the Church of St Ignatius and the perception of concave surfaces.

This research describes two hitherto unobserved phenomena in the frescoes of the seventeenth century architect and painter, Andrea Pozzo, painted on the vaulted ceiling of the central nave of the Church of St Ignatius in Rome. The present research also reports the results of two experimental studies on the problem of the perception of shapes projected on concave surfaces. A quantitative evaluation of the phenomena perceived from various points of observation is made by means of stimuli projected at various angles on a semicylindrical surface. The validity of the assumption of invariance, and in particular of the projective invariant called the cross-ratio, is discussed within the framework of ecological theories on perception.

Architecture↗
PubMed1994

Test of balanced transparency.

It is implicitly or explicitly assumed in current transparency models that all the parts of a completely transparent surface have the same perceived degree of transparency. In general, the two experiments reported here have shown that this assumption is false. Consequently, any general transparency equation based on this assumption is unjustified. Separate transparency equations for the different parts of a transparent surface are instead justified. This indicates the need for a model of the overall judgment of transparency of these parts. In the second experiment the hypothesis that the judged degree of transparency of a whole transparent surface is a weighted average of the judged degrees of transparency of the different parts of this surface was tested. The results contradict this hypothesis and support the idea that the judgment of transparency of a whole surface and that of its parts depend on different stimulus conditions.

Color Perception↗