The influence of perceptual anchors and visual noise on the vertical-horizontal illusion

In an earlier study Chapanis and Mankin examined the vertical-horizontal illusion in a visually-rich environment. In that study they feit that perceptual anchors and visual noise may have influenced the Ss’ estimates of the real-world objects that were used as stimuli. The present experiment attempted to test in a controlled laboratory environment the questions raised in the previous experiment. In the present experiment Ss were asked to estimate the height of a vertical stimulus line by indicating its corresponding horizontal distance. The vertical stimulus line was presented in a background of visual noise and potential anchors. The results indicate that the anchors did influence estimates and that this influence was, in turn, affected by the visual noise.     

The moon tilt illusion.

Besides the familiar moon illusion [e.g. Hershenson, 1989 The Moon Illusion (Hillsdale, NJ: Lawrence Erlbaum Associates)], wherein the moon appears bigger when it is close to the horizon, there is a less known illusion which causes the moon's illuminated side to appear turned away from the direction of the sun. An experiment documenting the effect is described, and a possible explanation is put forward.     

Probe distractors can influence negative priming by perceptual grouping

Negative priming occurs when a response is slower to a target that was a previously ignored stimulus. According to a computational model of inhibition, negative priming results from relative activation between a target and a distractor in probe trials. Thus, the degree of competition in the probe trial should influence negative priming. The results from three experiments supported this hypothesis. Probe distractors grouped with a target by a rectangle produced a larger negative priming effect than did distractors isolated from the target. Moreover, neither prime-probe similarity nor perceptual interference could explain the grouping effect. The results suggest that perceptual mechanisms can influence negative priming via a race in the probe trial.     

Induced perceptual grouping

ABSTRACT— The term perceptual grouping is associated with classical principles such as similarity and proximity. This article reports induced perceptual grouping, a phenomenon that occurs when placement of a uniform set of items near a structured set induces grouping within the otherwise uniform set. For example, when items grouped pair-wise by similarity are placed near another set of unstructured items, an analogous pair-wise grouping links elements of the second set. Induced grouping affected reaction times in two different visual search tasks, with reaction times depending on whether the target properties were contained within a group or crossed group boundaries as defined by induced grouping due to similarity, proximity, or common fate. Induced grouping was reduced when grouping between the structured and unstructured sets was weakened by means of a common-region cue or decreased similarity. Induced grouping appears to reflect the computation of hierarchical structure in visual images.     

Top-down influences on perceptual grouping

Observers are faster to find and discriminate a target pair of adjacent, same-shaped objects that are within the same perceptual group (within group) versus within adjacent perceptual groups. Manipulating the probability of a within-group pair modulated this grouping effect such that it increased as the percentage of within-group trials (25%, 50%, or 75%) increased. This probability effect was stronger for extrinsic grouping factors (common region and connectedness) than for intrinsic factors (proximity and color similarity). Further experiments, however, suggested that this extrinsic-intrinsic difference was due to differences in overall speed with which the target pair was found. The results are interpreted as indicating that grouping is sensitive to top-down influences as long as there is sufficient time for feedback to operate.     

The effect of perceptual grouping on haptic numerosity perception

Attention, Perception, & Psychophysics - We used a haptic enumeration task to investigate whether enumeration can be facilitated by perceptual grouping in the haptic modality. Eight...     

Time course of perceptual grouping

An investigation was made of the time course of perceptual grouping that is based on two qualitatively different spatial relationships: proximity and alignment. An index of grouping capacity was used to assess the processing time required before a backward pattern mask interfered with grouping. Stimuli consisted of bistable arrays of disjunct dots that were followed by a mask. Grouping cues, either proximity or alignment, were randomly assigned to either the horizontal or vertical orientation, and subjects indicated whether the dots appeared grouped as a series of horizontal or vertical lines. Spatial metrics of the cues were systematically altered until they no longer served as a cue for grouping, thereby determining the grouping threshold. The stimulus onset asynchrony (SOA) of the mask, relative to the test stimulus, ranged from 33.3 to 150 msec. The SOA at which grouping thresholds first became elevated identified the point at which the mask first interfered with the grouping process, thereby identifying the processing time required for grouping by the specified cue. The processing time for grouping by proximity and alignment differed significantly, requiring means of 87.6 and 118.8 msec, respectively, for processing to be completed. These measurements serve to identify the processing time necessary for spatially integrating stimulus elements into unified forms, thereby delineating temporal constraints at this stage of visual processing.     

The role of objects in perceptual grouping.

Perceptual organization can be viewed as the selection of the best or "most reasonable" parse of a given scene. However, the principles that determine which interpretation is most reasonable have resisted most attempts to define them formally. This paper summarizes a formal theory of human perceptual organization, called minimal model theory, in which the best interpretation of a given scene is expressed as the formally minimal interpretation in a well-defined space of possible interpretations. We then focus specifically on the role of types of grouping units, in particular the difficult notion of "object". Although grouping is often thought of as the process of dividing the image into objects, most research in perceptual grouping actually focuses on simpler types of units, such as contours and surfaces. Minimal model theory characterizes grouping units at a logical level, demonstrating how formal assumptions about units induce the observer to place a certain preference ranking on interpretations. The theory is then applied to the more subtle problem of objects, culminating in a definition for objects that is formally rigorous but at the same time captures some of the flexibility of human intuitions about objects.     

The construction of perceptual grouping displays using GERT

To study perceptual grouping processes, vision scientists often use stimuli consisting of spatially separated local elements that, together, elicit the percept of a global structure. We developed a set of methods for constructing such displays and implemented them in an open-source MATLAB toolbox, GERT (Grouping Elements Rendering Toolbox). The main purpose of GERT is to embed a contour in a field of randomly positioned elements, while avoiding the introduction of a local density cue. However, GERT's modular implementation enables the user to create a far greater variety of perceptual grouping displays, using these same methods. A generic rendering engine grants access to a variety of element-drawing functions, including Gabors, Gaussians, letters, and polygons.     

Influence of past experience on perceptual grouping

We used primed matching to examine the microgenesis of perceptual organization for familiar (upright letters) and unfamiliar (inverted letters) visual configurations that varied in the connectedness between their line components. The configurations of upright letters were available for priming as early as 40 ms, irrespective of connectedness between their line components. The configurations of connected inverted-letter primes were also available this early, but the configurations of disconnected inverted letters were not available until later. These results show that past experience contributes to the early grouping of disconnected line segments into configurations. These findings suggest an interactive model of perceptual organization in which both image-based properties (e.g., connectedness) and input from object memories contribute to perceptual organization.     

Visual attention and perceptual grouping

Perceptual organization is thought to involve an analysis of bothtextural discontinuities andperceptual grouping. In earlier work, we found that textural discontinuities were detected normally even when visual attention was engaged elsewhere. Here we report how perceptual grouping is affected when visual attention is engaged by a concurrent visual task. To elicit perceptual grouping, we used the Gestalt demonstrations of grouping on the basis of proximity and of similarity. Four tasks were investigated, some requiring the observer to discriminate between horizontal and vertical grouping, and some requiring the observer to merely detect the presence or absence of grouping. Visual attention was engaged at the center of the display by a form identification task. The detection of a textural discontinuity served as a control task. Concurrent form identification conflicted with all four grouping tasks, resulting in a significant reduction of grouping performance in each case. No performance reduction was observed when either form identification or grouping discrimination was combined with the detection of a textural discontinuity. These results suggest that perceptual grouping and form identification compete for visual attention, whereas the detection of a textural discontinuity does not.     

The effect of structure and degree of tilt on the tilted room illusion

The effect of structure and angle of tilt on the magnitude of the tilted-room illusion and its relationship to the illusion produced by a tilted-line field were examined in two experiments. In Experiment L, nine groups of 13 Ss were tested under three conditions of room tilt in the frontal plane (22.5, 45, and 67.5 deg) and under three conditions of structure (empty room, room with a back wall of stripes, and room with furniture). The results indicate that, while magnitude and direction of the illusion vary with degree of room tilt, structure increases the magnitude of the illusion only at a 45-deg room tilt. In Experiment 2, a field of lines was presented through a circular reduction tube to three groups of 13 Ss under three conditions of line tilt (22.5, 45, and 67.5 deg). An illusion occurred that was much smaller in magnitude and functionally different for the three tilt conditions when compared with the tilted-room illusion.     

Cue summation enables perceptual grouping

In order to study how basic features interact in shape perception, subjects detected target figure presence and identified target figure shape in a combined 2-alternative forced-choice task. Target figures were embedded in Gabor random fields and were defined by feature contrast in spatial frequency, orientation, or both. Two figure classes were used: block figures, which could not be identified at low feature-contrast levels, and lozenge figures, which could be identified to a moderate degree. Results showed a substantial double-cue benefit for both figure classes in detection. In identification, however, a double-cue benefit was practically absent for blocks, whereas, for lozenges, cue summation proved to be much stronger than in detection. The finding that the way basic features interact is modulated by figure class and psychophysical task indicates involvement of 2 distinct perceptual mechanisms in shape perception. The 1st one highlights salient features and enables local grouping on an early feature-specific processing level. The 2nd one is higher level, serves form completion and grouping of salient contours into global shapes, and enables figure perception.     

Exposure-time and spatial-frequency effects in the tilt illusion

The exposure durations of a vertical test line and a tilted inducing grating were varied and the tilt illusion thus generated was found to change as a function of this variation. Significant direct effects (acute-angle expansion) and indirect effects (acute-angle contraction) were found to occur at times consistent with Andrew's estimate of the time course of inhibition in the visual system when the inducing grating had a spatial frequency of 10 cycles deg-1. However, a 2.71 cycles deg-1 grating gave significant effects at exposure durations of 10 as well as 1000 ms, while in a further experiment a 10.91 cycles deg-1 grating gave significant effects at 1000 ms only. These results seem to suggest that orientation interactions thought to be due to inhibition (direct effect) and disinhibition (indirect effect) may occur within both sustained and transient channels with concomitant differences in time constants.     

The relation between similarity grouping and perceptual constancy.

In two experiments on whether similarity grouping is based on the retinal or the perceived slopes of lines, observers judged the grouping of diagonal lines when a display of vertical and diagonal lines was presented upright and when it was slanted 75 deg toward the floor. The floorwise slanting of the display improved gouping, as would be expected if the spontaneous organization of a field into groups in strongly influenced by the projection-level representation of stimuli. It is proposed that similarity grouping occurs at the readout stage of stimulus processing and involves segregating a field into groups on the basis of extrafoveal stimulus differences that are responded to in parallel; or if sequentially, very quickly and independently of focal attention.     

Visual attention and perceptual grouping.

Perceptual organization is thought to involve an analysis of both textural discontinuities and perceptual grouping. In earlier work, we found that textural discontinuities were detected normally even when visual attention was engaged elsewhere. Here we report how perceptual grouping is affected when visual attention is engaged by a concurrent visual task. To elicit perceptual grouping, we used the Gestalt demonstrations of grouping on the basis of proximity and of similarity. Four tasks were investigated, some requiring the observer to discriminate between horizontal and vertical grouping, and some requiring the observer to merely detect the presence or absence of grouping. Visual attention was engaged at the center of the display by a form identification task. The detection of a textural discontinuity served as a control task. Concurrent form identification conflicted with all four grouping tasks, resulting in a significant reduction of grouping performance in each case. No performance reduction was observed when either form identification or grouping discrimination was combined with the detection of a textural discontinuity. These results suggest that perceptual grouping and form identification compete for visual attention, whereas the detection of a textural discontinuity does not.     

Late influences on perceptual grouping: illusory figures

Two experiments demonstrate that grouping can be strongly influenced by the presence of figures defined by illusory contours. Rectangular arrays were constructed in which a central column of figures could group either with those on one side, on the basis of perception of figures defined by illusory contours, or with those on the other side, on the basis of physically present inducing elements. In all displays, subjects grouped according to the illusory figures significantly more often than for control displays that contained the same inducing elements, but rearranged so that illusory contours were degraded or eliminated. A second experiment showed that in objectively defined grouping tasks, subjects grouped faster by illusory figures than by inducing elements. These results indicate that grouping can occur after illusory contours have been perceived.