Psychology

Previous research has demonstrated that the short-range process in apparent movement, as studied with random-dot cinematograms, exhibits global cooperativity; that is, computations performed by local elements interact nonlinearly and are pooled. Other research using displays containing extended contours has implicated the short-range process, but has never demonstrated global cooperativity. In the first of four experiments, it was shown that under certain conditions of presentation, a short-range motion percept exhibiting apparent global cooperativity can be obtained when collections of randomly located contours are rotated -about-the center of a display, despite the fact that the displacement of peripheral contours falls outside the normal limit of the short-range process. Experiments 2–4 were conducted to provide further evidence that the observed motion is short-range (i.e., it canbe disrupted by illuminating the interstimulus interval or with dichoptic viewing) and that the percept is globally cooperative (i.e., masking the center of the display, where separations between corresponding elements across frames are smallest, results in a decline in the frequency of reports of the short-range percept). Control observations suggest that the effect produced with masks was not due to a decrease in the number of elements in the display. The argument that the display exhibits a short-range process with global cooperativity is further developed.

A modified version of the Ternus display was used to assess the relative effects of element position cues on reports of group and end-to-end movement. In this display, two rows of stimulus elements are joined by connecting lines. In one version of the display, the connecting lines remain stationary across frames, facilitating the interpretation that the associated stimulus elements also remain stationary. In another version of the display, one end of the connecting lines shifts horizontally from frame to fame, facilitating the interpretation that the associated stimulus elements have also shifted. The experiment showed that when the connecting lines remain stationary, reports of end-to-end movement increase, regardless of the interstimulus interval (ISI) at which the frames alternate. When the connecting lines shift, reports of group movement increase, regardless of ISI. Theoretical interpretations of the results involving both relatively low-level motion signals and higher order perceptual influences are considered.

Southeast Missouri State University, Cape Girardeau, Missouri 63 701 The maintenance of constant and coherent percepts of three-dimensional objects, even in the midst of visual noise, is important to our ability to navigate the environment. In the present experiments, observers viewed computer-animated simulations of three-dimensional spheres rotating around the vertical axis in depth. In the first experiment, the addition of noise in the form of randomly moving display elements reduced subjects’ judgments of depth and the accuracy of their rotation-direction judgments, although the phenomenal appearance of three-dimensional structure was maintained throughout. In the second experiment, a change from frame to frame in the orientation of the vectors that composed the simulation reduced, but did not destroy, perceived depth and rotation-direction accuracy. The effect of a change in orientation between vectors of successive frames of a simulation depended upon the length of those vectors. It is argued thatdynamic perspective— the information provided by movement and perspective together—is a significant factor in the maintenance of threedimensional object constancy.

The structure-from-motion theorem asserts that three projective views of four elements in motion are sufficient to specify the structure of the object to which the elements belong. Seven experiments were conducted to determine whether the perceptual recovery of structure in human observers is favorable to an interpretation of the theorem as aperformance theory. In six experiments, subjects either discriminated simulations of objects in rotation around the y-axis versus randomly perturbed counterparts or tried to identify them. As the rotation angle between frames of the simulations increased, performance accuracy decreased. Furthermore, performance accuracy did not depend upon the type of projection (parallel versus polar) or upon the coplanarity/ noncoplanarity of the stimulus elements. A seventh experiment showed that the subjective appearance of structure breaks down when the angle of rotation between successive frames exceeds about 35.5°. Because the subjects' performance in some conditions exceeded expectations based upon constraints imposed by the structure-from-motion theorem, it is suggested that additional algorithms or heuristic rules might need to be considered when interpreting human recovery of structure in such displays.

In three experiments, difference thresholds (dLs) and points of subjective equality (PSEs) for three-dimensional (3-D) rotation simulations were examined. In the first experiment, observers compared pairs of simulated spheres that rotated in polar projection and that differed in their structure (points plotted in the volume vs. on the surface), axis of rotation (vertical,y, vs. horizontal, x), and magnitude of rotation (200—700). DLs were lowest (7%) when points were on the surface and when at least one sphere rotated around they-axis and varied with changes in the independent variables. PSEs were closest to objective equality when points were on the surface of both spheres and when both spheres rotated about thex-axis. In the second experiment, subjects provided direct estimates of the rotations of the same spheres. Results suggested a reasonable agreement between PSEs for the indirect scaling and direct-estimate procedures. The third experiment varied sphere diameter (and therefore mean linear velocity of stimulus elements) and showed that although rotation judgments are biased by mean linear velocity, they are not likely to be made solely on the basis of that information. These and past results suggest a model whereby recovery of structure is conducted by low-level motion-detecting mechanisms, whereas rotation (and other) judgments are based on a higher level representation.

Two experiments investigated the importance of figurai relationships among stimulus elements in the determination of the kind of motion perceived in bistable apparentovement displays. Stimulus frames containing the words MITE and ITEM were constructed in such a way that the letters I, T, and E appeared in corresponding locations while the M was in disparate locations between frames. When the frames were alternated with a 20-msec ISI, observers maintained local figurai identity in their percepts of apparent movement. However, when the frames were alternated with an 80-msec ISI, observers reported apparent movement of the entire groups of letters and thereby ignored figurai relationships in their percepts of apparent movement. In a second experiment, it was found that figurai nonidentity between elements of successive frames always resulted in apparent movement of a perceptual group or “blob” regardless of ISI. It is suggested that short-range apparent-movement percepts rely strongly upon figurai identity among corresponding stimulus elements, whereas long-range apparent-movement percepts do not.

In a dynamic visual display which consisted of two alternating frames, each with three line elements, the individual elements which possessed a high degree of spatial correspondence from one frame to the next were paired perceptually and moved about locally. Any remaining elements moved independently and followed different movement rules. When there was no high degree of spatial correspondence of any elements across frames, or when the interval of time between frames was long, the elements behaved as a perceptual unit and moved about as a group. The local pairings and movements appeared to be mediated by a process with different functional characteristics from those of the process which mediated the wholistic movement of the elements as a group.

We cite two different perceptual-correspondence principles whose emphasis can help to disambiguate the otherwise ambiguous Ternus display in apparent movement (a display that can alternately be seen in one of two possible configurations). One of these principles is spatial correspondence, which emphasizes the maintenance of similar stimulus elements in given locations over time. The other principle is relation correspondence, which emphasizes the maintenance of the inter-organization of stimulus elements across frames. Each of four experiments reported here made use of a different stimulus feature (eg element color or element texture) to emphasize these different correspondence principles in Ternus displays. As predicted, an emphasis upon spatial correspondence resulted in an increase in reports of element movement compared to a featureless control condition. Emphasis on relation correspondence resulted in an increase in reports of group movement. Extensions and limitations of the u...

A display was devised for the purpose of studying the information afforded by kinetic optical occlusion (the progressive erasure and replacement of static elements within a display). A microcomputer generated a series of equally spaced light bars on a dark background. The first bar on the left was suddenly blanked and, after a pause of variable duration (an interblank interval, or IBI), was replaced. As the first bar was replaced, the second bar in the series was blanked, and so on, until each bar in the pattern had been blanked and replaced. Depending upon the duration of the IBI, this display gave rise to one of two alternative percepts: the observer either saw movement of a dark shadow ‘in front of’ the pattern of bars (with IBIs ≲ 50 ms) or he saw right-to-left stroboscopic movement of successive bars (with IBIs ≳ 85 ms). At some intermediate IBI (the transition IBI) the display was bistable. A two-bar variant of the original display was also studied and found to be bistable und...

Using a bistable apparent-motion display, Odic and Pratt (2008, Perception37 1790–1804) have recently presented data that they interpret as being inconsistent with what they call “the two-process theory”. Instead, they argue, their data can be explained by the differential-activation theory along with a process they identify as “temporal summation of contrast”. It is argued here that Odic and Pratt misinterpreted the two-process distinction and used a display that was too unusual to be adequately addressed by it. Further, their use of the differential-activation theory and, in particular, the temporal summation of contrast, seems problematic. It is concluded that there is little in their data and theoretical interpretation to justify rejection of the two-process approach.