Selective adaptation with reversible figures: Don’t change that channel

An adaptation-test paradigm was used in two experiments examining processes underlying the perceived reversals of a rotating Necker cube. Adaptation and test cubes were either the same or different with respect to their visual fields of presentation (Experiment 1) or their sizes (Experiment 2). Results of both experiments indicated that, following subjects’ adaptation to a different cube, reversal rate of the test cube did not differ from that obtained without prior adaptation experience. In contrast, reversal rate of the test cube was elevated following adaptation to the same cube. Additional findings of Experiment 1 were that a test cube presented to the same visual field as the adaptation cube yielded a higher reversal rate than did a simultaneously presented cube in the opposite visual field. Also, the reversal rate of one cube was not influenced by the simultaneous presentation of a second cube. Results of both experiments were interpreted in terms of the fatigue and recovery of multiple, largely independent, localized neural channels. Thus, the results tie reversible-figure illusions to other visual phenomena thought to involve similar fatigue processes within localized visual channels (e.g., tilt, motion, and size aftereffects).     

Depth capture by generic-view motion

The present study investigated the spatial interaction between the generic view and accidental view. First, the generic-view principle was applied to the three-dimensional motion of a wire-frame cube. Second, the effects of inner rotating dots and generic/accidental views of inner bars on an accidental view of a cube were investigated to show the spatial interaction between generic and accidental views. This investigation revealed that the generic view of a rotating cube generated a clearer three-dimensional perception than the accidental view, which is consistent with the generic-view principle. Both the unambiguous three-dimensional cue of inner dots and the generic view of inner bars "captured" the depth perception of the accidental view of the rotating cube. This finding suggests that the generic-view principle not only works locally, but then propagates globally. Further, it indicates how the visual system implements the generic-view principle to incorporate more complicated scenes.     

EEG gamma-band response during the perception of Necker cube reversals

In two former studies our research group reported frontal gamma-band enhancement during multistable visual perception and reversal rate dependent differences in the gamma-band. In these studies, a dynamic reversible figure was used which was based on the phenomenon of apparent motion. The aim of this study was to examine whether the results obtained with a dynamic motion paradigm can be replicated with the static Necker cube. The results demonstrate a general frontal gamma-band enhancement and higher induced gamma activity for subjects with a relatively high reversal rate in comparison to subjects with a relatively low reversal rate. This pattern of results fits well to the findings obtained with the dynamic motion paradigm. Therefore, the important role of frontal gamma activity for figure reversals has received further evidence. The results support the involvement of attentional top-down processing of figure reversal that is not directly related to binding processes.     

Dynamic object perception by pigeons.

Three experiments examined pigeon discrimination of computer-generated three-dimensional (3-D) projections of cube and pyramid objects. Four pigeons were tested using a go/no-go procedure involving static and dynamically rotating presentations of these stimuli. Transfer tests with different types of rotational and featural transformations suggested the pigeons may have used a 3-D representation of the objects as their primary means of performing the discrimination. The comparative implications for object and motion perception in animals are considered.     

Limits on integrating motion information across saccades

In two experiments, we investigated whether people could detect changes in the rotary motion of a cube. A rendering of a cube rotating at a constant angular velocity was presented on a video monitor and, at a key point in the trial, a cross was presented to one side of the cube as a cue for a saccade. On some trials, a change in the rotation occurred either about 100 msec before the saccade or during the saccade; on other trials, there was no change. The change consisted of moving the cube to a new position in the "rotation sequence," after which it continued to rotate at the same angular velocity as before. There was also a control on all trials to ensure that change detection was not due to the detection of low-level motion. Although detection of the change was well above chance when it occurred during the fixation, it was at chance when it occurred during the saccade, except in the case of one participant (who was in both experiments). This chance performance also occurred in Experiment 2 for (1) a slower rotation speed and (2) an axis of rotation that made the rotation planar. The participant who had above chance performance (and as good as that when the change occurred during a fixation) reported using a "strategy" that did not track the path of the cube. It thus appears that there is no natural way in which the visualsystem tracks this rotary motion, and that detection of change requires some sort of recoding. This finding raises the question of whether good performance in other, apparently similar, motion-detection tasks is a result of similar recoding.     

As the cube turns: Evidence for two processes in the perception of a dynamic reversible figure

The reported reversals of a rotating Necker cube, which changes direction of rotation when a perspective reversal occurs, were examined under a number of conditions. These permitted comparisons of reversal rates within viewing periods, across successive viewing periods within an experimental session, and across successive weekly sessions. In addition, observers viewed either one or two rotating cubes simultaneously within each of the various viewing periods. Clear evidence for a learning effect was obtained in the form of significant savings across successive viewing periods and sessions. At the same time, results from the multiple-cube conditions and from the pattern of reversals within individual viewing periods appeared to be more consistent with a process of neural fatigue. A two-stage model of reversible-figure perception is proposed which is characterized by (1) fatiguing with extended viewing of the two sets of neural channels that underlie the two percepts of the reversible figure, and (2) learning, which helps to establish the organization of the cortical channels as well as moderate channel activity via such processes as attention and strategy.     

Illusion reversal rate as a function of subjective depth

Undergraduate students (N = 48) served as subjects in a test of Gregory's theory of illusions. Twenty-four students made judgments about the subjective depth of three reversible illusions--Necker cube, Book, and Pyramid--under the conditions of complete versus incomplete illusions and illusions without depth cues versus with depth cues. An additional 24 subjects recorded the three illusion reversal rates under slightly altered conditions. Differences were found among the various illusions with respect to complete versus incomplete illusions, reversal rate, subjective judgments of depth, and certain correlation values. Support for Gregory's theory of illusion as displaced or misleading depth perception is offered by these results.     

Explicit and implicit memory for rotating objects

Although both the object and the observer often move in natural environments, the effect of motion on visual object recognition ha not been well documented. The authors examined the effect of a reversal in the direction of rotation on both explicit and implicit memory for novel, 3-dimensional objects. Participants viewed a series of continuously rotating objects and later made either an old-new recognition judgment or a symmetric-asymmetric decision. For both tasks, memory for rotating objects was impaired when the direction of rotation was reversed at test. These results demonstrate that dynamic information can play a role in visual object recognition and suggest that object representations can encode spatiotemporal information.     

Differences in top-down influences on the reversal rate of different categories of reversible figures

Understanding the mechanisms underlying the multistability of reversible figures may provide valuable insights into the normal functioning of our visual system. The proposed factors that control the perceptual alternations of reversible figures can be classified into bottom-up and top-down processes. In the present study, we report differences in top-down effects on the reversal rate depending on whether a structural perspective (Necker cube, Schr?der staircase) or a meaningful content (duck/rabbit figure, chef/dog figure) is subject to the reversal phenomenon. In order to activate top-down mechanisms explicitly the subjects had the instruction to bring the reversal rate under voluntary control. The results indicated that both slowing down and speeding up the rate of alternations was more effective for the content-reversal figures (duck/rabbit, chef/dog) than for the rather abstract perspective-reversal figures (Necker cube, Schr?der staircase). In order to investigate the effect of meaningfulness in figure/ground reversals, the effect of the same instructional variable was also determined for Rubin's vase/faces and the Maltese cross. The results showed a similar tendency as in the case of the comparison between perspective reversals and content reversals. Possible cognitive processes that may play a role in top-down influences on figure reversal and theoretical implications of these findings for the interaction of bottom-up and top-down processes are discussed.     

Attention and depth perception

The Necker cube is a line drawing with two possible solutions in depth perception. The process of interpreting a two-dimensional line drawing as a three-dimensional object was investigated using the Necker cube. Attention was directed to a local feature of a briefly presented cube, ie an angle at a vertex. The attended angle was perceived as a front part of the cube and other parts were interpreted so as to match this interpretation. Results show that the local feature to which attention was directed was interpreted first and then global features and other local features were interpreted so as to agree with the local feature interpreted initially. This suggests that the three-dimensional interpretation of the line drawing was made sequentially from the local feature to global structures.     

Wobble cones and wobble holes: the stereokinetic effect revisited

It is well-known that patterns of eccentric circles when slowly rotated give rise to compelling three-dimensional impressions of cones or conical holes which can 'wobble' as the pattern rotates. The wobble can be considered as part of the overall phenomenon of depth elicited from a rotating display, the 'stereokinetic' effect (SKE). This paper considers the three-dimensional appearance as being the result of the sliding of contours and thus it imitates the motion parallax found in real three-dimensional objects in motion. New variants of SK figures are used to examine these points. An analogy with computer programs is proposed which questions earlier views on the location of perceptual invariance.     

The effect of touch on a visually ambiguous three-dimensional figure

The effect of haptic (i.e. tactile-kinaesthetic) information on the perception of an ambiguous visual figure was investigated. The figure used was a self-luminous, threedimensional wire cube viewed in darkness. It was found that visual reversals still took place when the cube was explored with the hands, but the reversal rate was reduced and the time increased during which the cube looked as it “really” was.     

Attentional sampling of multiple wagon wheels

Attending to a periodic motion stimulus can induce illusory reversals of the direction of motion. This continuous wagon wheel illusion (c-WWI) has been taken to reflect discrete sampling of motion information by visual attention. An alternative view is that it is caused by adaptation. Here, we attempt to discriminate between these two interpretations by asking participants to attend to multiple periodic motion stimuli: The discrete attentional sampling account, but not the adaptation account, predicts a decrease of c-WWI temporal-frequency tuning with set size (with a single periodic motion stimulus the c-WWI is tuned to a temporal frequency of 10 Hz). We presented one to four rotating gratings that occasionally reversed direction while participants counted reversals. We considered reversal overestimations as manifestations of the c-WWI and determined the temporal-frequency tuning of the illusion for each set size. Optimal temporal frequency decreased with increasing set size. This outcome favors the discrete attentional sampling interpretation of the c-WWI, with a sampling rate for each individual stimulus dependent on the number of stimuli attended.     

Perception of three-dimensional shape specified by optic flow by 8-week-old infants

Sensitivity of 8-week-old infants to optical flow specifying the shape of a three-dimensional object was assessed. Infants viewed kinetic random-dot displays that specified three-dimensional cubes. The cubes were identical except for the presence or absence of an interior corner. Half of the infants viewed the full display. The other half viewed the central region of the displays, where the flow specifying the presence or absence of the corner differed. Infants in the full-view condition looked significantly longer to a novel cube than to the familiar cube following habituation. In contrast, infants in the partial-view condition looked equally to the novel and familiar cubes, ruling out the possibility that infants who viewed the full displays merely discriminated differences in motion in the central region of the two displays. These findings suggest that infants as young as 8 weeks perceive three-dimensional object shape from optic flow.     

Object-centred orienting of attention

Abstract

In the present study two experiments are reported in which the subjects were presented on a computer screen with a two-dimensional line drawing that is perceived as a three-dimensionalobject (i.e. a cube). The cube could be seen as stationary, as rotating about the y-axis (Experiment 1A), or as rotating about the x-axis (Experiment 1B). The subject's attention was directed by a visual precue to a vertex of the cube. As the cube rotated, the precued location moved in viewer-centred co-ordinatesm, but the local feature of the cube that had been precued (i.e. a given vertex) did not move in object-centred co-ordinates. The imperative stimulus was presented at the precued location (valid trials) or at an uncued location (invalid trials). Precued and uncued locations were determined in object-centred coordinates. The subjects were required to signal detection of the imperative stimulus by pressing the space bar on the computer keyboard. There were also control conditions in which the procedure was identical, but the cube was not visible. When the cube was visible, valid trials were faster than invalid trials, regardless of whether it was seen as stationary or rotating. It was concluded that the subjects could allocate spatial attention in object-centred co-ordinates. There wer also indications that responses for invalid trials were faster when the imperative stimulus was presented on the same face as the precue than when it was presented on the opposite face.     

Demonstrations of spatiotemporal integration and what they tell us about the visual system

Five sets of displays are presented on the journal website to be viewed in conjunction with the text. We concentrate on the factors that give rise to the integration and disruption of the direction of apparent motion in two-dimensional and three-dimensional space. In the first set of displays we examine what factors contribute to the integration and disruption of apparent motion in the Ramachandran/Anstis clustered bistable quartets. In the second set we examine what factors give rise to the perception of the direction of motion in rotating two-dimensional wheels and dots. In the third and fourth sets we examine how the depth cues of shading and disparity contribute to the perception of apparent motion of opaque displays, and to the perception of rotating unoccluded displays, respectively. In the fifth set we examine how the depth cue of motion parallax influences the perception of apparent motion. Throughout, we make inferences about the roles which various parallel pathways and cortical areas play in the perceptions produced by the displays shown.     

Piecemeal organization and cognitive components in object perception: perceptually coupled responses to moving objects

In three experiments, observers who were instructed to perceive one of two alternative depth arrangements of a three-dimensional wire cube fixated near one of two intersections that differed in the degree to which they specified the cube's veridical depth organization. In order to separate perceptual effects from experimenter effects, we measured indirect reports about variables perceptually coupled to perceived depth rather than direct reports about perceived depth. In all three experiments, reversal durations at the two intersections differed, even though the two were parts of a single object. In addition, reversals varied with viewers' intentions. Thus, the unit of perceptual organization may be smaller than the entire object, and viewers' intentions can influence the perception of real moving objects. In additional analyses, reversal durations were separated into two components: nonelective instability and malleability; the question of whether these two components of ambiguity are functionally distinct could not be decided.     

How to keep a reversible figure from reversing: teasing out top-down and bottom-up processes

The nature of processes underlying our perception of reversible figures was examined through two experiments investigating the effects of prior exposure conditions on an observer's report of figural reversal. In experiment 1, observers were adapted over several minutes to an unambiguous version of a rotating Necker cube prior to the presentation of the standard ambiguous figure. Results indicated that adaptation produced an immediate bias to perceive the ambiguous figure in the opposite configuration (ie reverse bias) and to reduce reports of reversal over the test period. The introduction of a brief delay between the adaptation and test periods revealed that this bias is a highly transient effect and is only clearly evident when the adaptation and test figures are matched in size. In experiment 2, observers were primed with an unambiguous figure for a few seconds prior to the presentation of the standard ambiguous figure. In this case, the obtained bias strongly favored the observer's reporting the ambiguous figure to be in the same configuration as the adapting figure (ie positive bias); and neither introducing a delay period nor changing figure size had any effect. We conclude that these experiments reveal the distinct roles of transient, retinally localized neural processes as well as more stable, global processes under specifiable conditions.     

Movement perception in computer-driven visual displays

Computer-driven visual displays (CDVDs), like television and movies, produce stroboscopic rather than continuous physical movement. The success with which the perception of motion is produced depends or. factors such as the fineness of the raster and the temporal and spatiai reiationships of the stimulus points. For a given velocity, the more points there are on the movement trajectory, and the closer their spacing, the better is the perceived movement. Moderately slow retinal velocities (on the order of .4 to .8 deg/sec) produce the highest quality of perceived movement. One can discriminate among possible subclasses of movement detectors by presenting a complex sequence of intensities at two or more points and varying their cross correlation. Motion between two areas can be perceived even when there is zero correlation between the spatial patterns in each location. Perceived motion can be of rotation, as well as of translation. The two-dimensional shadow of a rotating three-dimensional wire figure is perceived as a rotating, rigid, three-dimensional wire figure (the kinetic depth effect). A three-dimensional “shadow” of a hypothetical four-dimensional wire figure also has been produced; it was not seen as rigid.     

Effects of sleep deprivation on voluntarily controlled reversal rate of ambiguous figures

Abstract.— The effects of sleep deprivation (SD) on the reversal rate of the Necker cube and Rubin's Vase-Faces were studied both with instructions to maximize and to minimize the rate. Three viewing conditions were used: Free eye movements, eyes on a fixation point and reversals of an after-image of the figure. SD was found to increase the rate under minimizing instructions and to decrease the rate under maximizing instructions. The results were interpreted in terms of an accelerated satiation process and an impairment of the attentional control.