The relationship between visual persistence and event perception in bistable motion display

Illusory size determines the perception of ambiguous apparent motion

The visual system constructs perceptions based on ambiguous information. For motion perception, the correspondence problem arises, i.e., the question of which object went where. We asked at which level of processing correspondence is solved – lower levels based on information that is directly available in the retinal input or higher levels based on information that has been abstracted beyond the input directly available at the retina? We used a Ponzo-like illusion to manipulate the perceived size and separations of elements in an ambiguous apparent motion display. Specifically, we presented Ternus displays – for which the type of motion that is perceived depends on how correspondence is resolved – at apparently different distances from the viewer using pictorial depth cues. We found that the perception of motion depended on the apparent depth of the displays, indicating that correspondence processes utilize information that is produced at higher-level processes.

How voluntary spatial attention influences feature biases in object correspondence

Our visual system is able to establish associations between corresponding images across space and time and to maintain the identity of objects, even though the information our retina receives is ambiguous. It has been shown that lower level factors-as, for example, spatiotemporal proximity-can affect this correspondence problem. In addition, higher level factors-as, for example, semantic knowledge-can influence correspondence, suggesting that correspondence might also be solved at a higher object-based level of processing, which could be mediated by attention. To test this hypothesis, we instructed participants to voluntarily direct their attention to individual elements in the Ternus display. In this ambiguous apparent motion display, three elements are aligned next to each other and shifted by one position from one frame to the next. This shift can be either perceived as all elements moving together (group motion) or as one element jumping across the others (element motion). We created a competitive Ternus display, in which the color of the elements was manipulated in such a way that the percept was biased toward element motion for one color and toward group motion for another color. If correspondence can be established at an object-based level, attending toward one of the biased elements should increase the likelihood that this element determines the correspondence solution and thereby that the biased motion is perceived. Our results were in line with this hypothesis providing support for an object-based correspondence process that is based on a one-to-one mapping of the most similar elements mediated via attention.

Competition between color and luminance in motion correspondence

The visual system needs to solve the correspondence problem (i.e., which elements belong together across space and time) to allow stable representations of objects. It has been shown that spatiotemporal and feature information can influence this correspondence process, but it is unclear how these factors interact with each other, especially when they are more or less prominent due to changes in contrast magnitude. We investigated this question using a variation of the Ternus display, an ambiguous apparent motion display, in which three elements can either be perceived as moving together (group motion) or as one element jumping across the others (element motion). In the first experiment, we biased the percept by presenting some of the elements with the same feature (isoluminant color or luminance), such that they were either compatible with group motion or with element motion (simple feature biases). To change the strength of the feature bias, we manipulated the contrast magnitude of the feature. In three more experiments we introduced competitive displays, in which some of the elements showed a color/luminance based element bias of varying contrast magnitude, while other elements showed a luminance/color based group bias of varying contrast magnitude (competing feature bias). We found that for a simple feature bias the contrast magnitude did not affect the strength of the bias. For competing feature biases, however, the contrast magnitude did influence correspondence, as the bias strength increased with contrast. The implications of our results for current motion and feature-based theories of correspondence are discussed.

Conceptualizations of Short-Range and Long-Range Processes in Apparent Movement

The two-process distinction in apparent movement posits the existence of competitive `short-range' and `long-range' processes. It was proposed to account for the fact that the visual system seems to generate different qualitative percepts under different spatiotemporal conditions of stimulation. It has been shown to be in accord with empirical data from random-dot cinematogram experiments and bistable-percept experiments, as well as with subjective experience. Although it has been developed by some into a model of motion perception, the two-process distinction is perhaps best conceptualized as a metatheoretical perspective rather than a theory. That is, the two-process distinction has guided the development of a number of theories, all of which share the notion of a basic processing dichotomy. The present paper elaborates these ideas and addresses criticisms of the two-process distinction, arguing that they inappropriately test the processes against fixed `criteria'. It is claimed here that, like all complex perceptual processes, those associated with the two-process distinction cannot be easily isolated by manipulations of individual stimulus parameters in the search for criterion behavior. The nature of perceptual theories is discussed in this context, and the notion of modes of perceiving is used as a conceptualization for the two-process distinction. Consistency between the two-process distinction and other theoretical conceptualizations is shown. Conclusions are drawn and suggestions are made for the future of the two-process distinction.

The Evolution of Explanations of a Perceptual Phenomenon: A Case History Using the Ternus Effect

The Ternus effect involves a multi-element stimulus that can lead to either of two different percepts of apparent movement depending upon a variety of stimulus conditions. Since Ternus's 1926 discussion of this phenomenon, many researchers have attempted to explain it. We examine the history of explanations of the Ternus effect and show that they have evolved to contemporary theoretical positions that are very similar to Ternus's own ideas. Additionally, we describe a new experiment showing that theoretical positions that emphasize element grouping and element identity within groups can predict the effects of certain stimulus manipulations on the Ternus effect.

Evidence for scene-based motion correspondence

To maintain stable object representations as our eyes or the objects themselves move, the visual system must determine how newly sampled information relates to existing object representations. To solve this correspondence problem, the visual system uses not only spatiotemporal information (e.g., the spatial and temporal proximity between elements), but also feature information (e.g., the similarity in size or luminance between elements). Here we asked whether motion correspondence relies solely on image-based feature information, or whether it is influenced by scene-based information (e.g., the perceived sizes of surfaces or the perceived illumination conditions). We manipulated scene-based information separately from image-based information in the Ternus display, an ambiguous apparent-motion display, and found that scene-based information influences how motion correspondence is resolved, indicating that theories of motion correspondence that are based on "scene-blind" mechanisms are insufficient.

A century of Gestalt psychology in visual perception: I. Perceptual grouping and figure-ground organization

In 1912, Max Wertheimer published his paper on phi motion, widely recognized as the start of Gestalt psychology. Because of its continued relevance in modern psychology, this centennial anniversary is an excellent opportunity to take stock of what Gestalt psychology has offered and how it has changed since its inception. We first introduce the key findings and ideas in the Berlin school of Gestalt psychology, and then briefly sketch its development, rise, and fall. Next, we discuss its empirical and conceptual problems, and indicate how they are addressed in contemporary research on perceptual grouping and figure-ground organization. In particular, we review the principles of grouping, both classical (e.g., proximity, similarity, common fate, good continuation, closure, symmetry, parallelism) and new (e.g., synchrony, common region, element and uniform connectedness), and their role in contour integration and completion. We then review classic and new image-based principles of figure-ground organization, how it is influenced by past experience and attention, and how it relates to shape and depth perception. After an integrated review of the neural mechanisms involved in contour grouping, border ownership, and figure-ground perception, we conclude by evaluating what modern vision science has offered compared to traditional Gestalt psychology, whether we can speak of a Gestalt revival, and where the remaining limitations and challenges lie. A better integration of this research tradition with the rest of vision science requires further progress regarding the conceptual and theoretical foundations of the Gestalt approach, which is the focus of a second review article.

Spatio-temporal priority revisited: the role of feature identity and similarity for object correspondence in apparent motion

We live in a dynamic environment in which objects change location over time. To maintain stable object representations the visual system must determine how newly sampled information relates to existing object representations, the correspondence problem. Spatiotemporal information is clearly an important factor that the visual system takes into account when solving the correspondence problem, but is feature information irrelevant as some theories suggest? The Ternus display provides a context in which to investigate solutions to the correspondence problem. Two sets of three horizontally aligned disks, shifted by one position, were presented in alternation. Depending on how correspondence is resolved, these displays are perceived either as one disk "jumping" from one end of the row to the other (element motion) or as a set of three disks shifting back and forth together (group motion). We manipulated a feature (e.g., color) of the disks such that, if features were taken into account by the correspondence process, it would bias the resolution of correspondence toward one version or the other. Features determined correspondence, whether they were luminance-defined or not. Moreover, correspondence could be established on the basis of similarity, when features were not identical between alternations. Finally, the stronger the feature information supported a certain correspondence solution the more it dominated spatiotemporal information.

Attention modulates spatio-temporal grouping

Dynamic stimuli are ubiquitous in natural viewing conditions implying that grouping operations need to operate, not only in space, but also jointly in space and time. Moreover, in natural viewing, attention plays an important role in controlling how resources are allocated. We investigated how attention interacts with spatio-temporal perceptual grouping by using a bistable stimulus, called the Ternus-Pikler display. Ternus-Pikler displays can give rise to two different motion percepts, called Element Motion (EM) and Group Motion (GM), the former dominating at short Inter-Stimulus Intervals (ISIs) and the latter at long ISIs. Our results indicate that GM grouping requires more attentional resources than EM grouping. Different theoretical accounts of perceptual grouping and attention are discussed and evaluated in the light of the current results.

Ternus effect: two processes or differential activation? Comments on Odic and Pratt's 2008 paper

Using a bistable apparent-motion display, Odic and Pratt (2008, Perception 37 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.

Differential-Activation Theory Can Account for the Ternus Display: Rejoinder to Petersik

The Ternus display is a bistable apparent-motion display that has captivated researchers for decades. Recently, Odic and Pratt (2008, Perception37 1790–1804) provided evidence against the well-known two-process theory of Braddick and Adlard [1978, Visual Psychophysics and Physiology (New York: Academic Press) pp 417–426] and provided an explanatory framework using the differential-activation theory of Gilroy et al (2001, Perception & Psychophysics63 847–861). A comment by Petersik (this issue) challenges the methodology and theoretical implications of Odic and Pratt, and claims that the two-process distinction still has a role to play in the Ternus display. In this rejoinder, we examine the main points made by Petersik and expand on the differential-activation theory and its applicability to the Ternus display.

Laterality and pattern persistence in bistable motion perception

Bistable motion perception refers to two competing perceptions that can result when frames consisting of three elements are displaced laterally by one element. At short inter-frame intervals, the dominant percept is that the end elements in the display are moving; at long inter-frame intervals, perception is of all the elements moving coherently to the right or left. This research shows that coherent motion is more likely to be perceived when presentations are parafoveal than foveal and when they are to the right visual field than the left visual field. These results support the idea that visual pattern persistence is shorter in the parafovea than in the fovea, and shorter in the right than in the left visual field.

Can semantic knowledge influence motion correspondence?

Semantic factors are presumed to have little influence on motion perception. Two experiments examined the effects of an object's semantic identity on motion correspondence using the Ternus paradigm. Motion correspondence was not influenced by whether the object depicted is typically moving or stationary, but it was influenced by the way(s) in which an object's components typically move relative to one another: perceived correspondence differed depending on whether the motion tokens constituted the feet of a person walking or the wheels of a car. Apparently, semantic knowledge can influence motion correspondence, although such influence is weak and may be restricted to certain types of semantic information. The adaptive significance of such restricted influences is considered.

Discrimination of an orientation difference in dynamic textures

We investigated whether the response of a motion sensor was related to the specificity of sensory information (orientation and direction of motion) used to compute motion energy. This was done in two ways. First, we assessed whether orientation discrimination of a target line, which segregated by an orientation difference from a textured background, was improved with two-frame apparent motion stimulation (as compared with static presentation). Second, we investigated whether the amount of improvement (in either orientation or direction of motion discrimination) depends on a particular combination of target orientation and direction of motion (either orthogonal or parallel). We found that the percentage of correct responses in the discrimination task (a) was higher for a moving target than for a static one; (b) was higher when the target was oriented more orthogonally to motion direction than background elements; (c) was little affected by background motion and (d) decreased with frame duration in the direction of motion task whereas it was largely unaffected by frame duration in the discrimination of orientation task. These results suggest that discrimination of moving texture boundaries is based on a motion sensor tuned to a particular combination of orientation and direction of motion, which is capable of signalling the orientation of a moving target more accurately than a static sensor.

Parallel search for conjunctions with stimuli in apparent motion

A series of experiments was conducted to determine whether apparent motion tends to follow the similarity rule (i.e. is attribute-specific) and to investigate the underlying mechanism. Stimulus duration thresholds were measured during a two-alternative forced-choice task in which observers detected either the location or the motion direction of target groups defined by the conjunction of size and orientation. Target element positions were randomly chosen within a nominally defined rectangular subregion of the display (target region). The target region was presented either statically (followed by a 250 ms duration mask) or dynamically, displaced by a small distance (18 min of arc) from frame to frame. In the motion display, the position of both target and background elements was changed randomly from frame to frame within the respective areas to abolish spatial correspondence over time. Stimulus duration thresholds were lower in the motion than in the static task, indicating that target detection in the dynamic condition does not rely on the explicit identification of target elements in each static frame. Increasing the distractor-to-target ratio was found to reduce detectability in the static, but not in the motion task. This indicates that the perceptual segregation of the target is effortless and parallel with motion but not with static displays. The pattern of results holds regardless of the task or search paradigm employed. The detectability in the motion condition can be improved by increasing the number of frames and/or by reducing the width of the target area. Furthermore, parallel search in the dynamic condition can be conducted with both short-range and long-range motion stimuli. Finally, apparent motion of conjunctions is insufficient on its own to support location decision and is disrupted by random visual noise. Overall, these findings show that (i) the mechanism underlying apparent motion is attribute-specific; (ii) the motion system mediates temporal integration of feature conjunctions before they are identified by the static system; and (iii) target detectability in these stimuli relies upon a nonattentive, cooperative, directionally selective motion mechanism that responds to high-level attributes (conjunction of size and orientation).

Visible persistence and form correspondence in Ternus apparent motion

Visual stimuli remain visible for some time after their physical offset (visible persistence). Visible persistence has been hypothesized to play an important role in determining the pattern of correspondence matching in the Ternus apparent-motion display. In this display, one or more elements reappears in overlapping locations at different times, whereas another element appears alternately to the right or the left of these elements. Usually either the elements are perceived to move coherently as a group (group motion), or one element may be perceived to hop over one or more other elements (element motion). According to the visible-persistence account of the perceptual organization of the Ternus display, element motion is seen when the temporal gap between elements in overlapping locations is small enough to be bridged by visible persistence; if it is not, group motion is seen. We conducted four experiments to test this visible-persistence account. In Experiments 1 and 2, a form correspondence cue (line length) was introduced to bias the visual system toward the element-motion interpretation, while visible persistence was either reduced or eliminated. The element-motion percept dominated despite the elimination of visible persistence. In Experiments 3 and 4, we found that Ternus elements presented without interruption, and thus presumably persisting over time, can be perceived in group motion. Together, the results indicate that visible persistence is neither necessary nor sufficient to account for the pattern of correspondence matches in the Ternus display.

Perceptual grouping in space and time: evidence from the Ternus display

We report three experiments investigating the effect of perceptual grouping on the appearance of a bistable apparent-motion (Ternus) display. Subjects viewed a Ternus display embedded in an array of context elements that could potentially group with the Ternus elements. In contrast to several previous findings, we found that grouping influenced apparent motion perception. In Experiment 1, apparent motion perception was significantly affected via grouping by shape similarity, even when the visible persistence of the elements was controlled. In Experiment 2, elements perceived as moving without context were perceived as stationary without context were perceived as moving when grouped with moving elements perceived as stationary without context were perceived as moving when grouped with moving context elements. We argue that grouping in the spatial and temporal domains interact to yield perceptual experience of apparent-motion displays.

Polarity matching in the Ternus configuration

Two experiments were conducted to examine the effect of changes in the sign of element contrast on perceptions of the Ternus apparent motion display. In the first experiment, the contrast polarity of all three elements in the display were alternated from the first frame of view to the second. At short durations, this increased perceptions (relative to a control condition) of simultaneity in the display, decreased perceptions of element motion, and did not significantly affect perceptions of group motion. At long durations, this manipulation did not affect performance. In a second experiment, patterns of element polarity were manipulated to favour perceptions of either element motion or of group motion relative to a control condition in which all elements had identical contrast polarity. At a long duration, this manipulation affected perceptions of the configuration; this manipulation did not affect the appearance of the display at a short duration. Together, these results are inconsistent with the predictions of Grossberg and Rudd's [Psychological Review, 99, 78-121 (1992)] motion oriented contrast filter. However, they are consistent with a model of motion correspondence processing that includes a polarity matching constraint.

Simultaneity in the Ternus configuration: psychophysical data and a computer model

The Ternus configuration is an apparent motion display which is typically described as being bistable; subjects usually describe seeing either element motion or group motion, depending upon temporal properties of the display. The results of an experiment are reported in which subjects are also permitted to report seeing four stationary display elements (simultaneity). It was found that simultaneity was produced when both frame durations and interstimulus intervals (ISIs) were brief. A weaker than expected effect of stimulus onset asynchrony (SOA) was revealed because this third type of judgment was obtained. Furthermore, statistical analyses indicated that SOA was not by itself the best predictor of judgment type. The interaction between duration and ISI was also an important predictor. This suggested that a complete account of the Ternus configuration requires two mechanisms: a visible persistence mechanism, governed by an SOA law, and a motion correspondence mechanism, governed by an ISI law. These two mechanisms were added to Dawson's (1991) [Psychological Review, 98, 569-603] autoassociative network for motion correspondence processing. The resulting model could generate each of the three interpretations of the Ternus configuration at appropriate combinations of frame duration and ISI.