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.

Flicker-Defined Forms in the Ternus Display

Odic and Pratt (2008, Perception, 37, 1790-1804) proposed that the type of movement seen in the bistable Ternus display depends on the elements' temporal summation of contrast relative to the background. To test this theory, participants viewed a flicker-defined Ternus display where the elements had no temporal summation of contrast. Participants also viewed a luminance-defined control condition. Five interstimulus intervals (ISIs) (0, 20, 40, 60, and 80 ms) and two stimulus durations (SDs) (200 and 400 ms) were used in each condition. If temporal summation of contrast does not influence perceived group and end-to-end movement in flicker-defined forms, it was expected that the frequency of their reports would be equal to those in the luminance- defined control condition at the same ISIs and SDs. As predicted, the main effect of condition was not significant and participants reported both percepts at expected rates in both conditions, contrary to the predictions of Odic and Pratt (2008).

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.

Detecting sudden changes in dynamic rotation displays

Four experiments and controls were run in order to determine the ability of the visual system to detect slight changes in three-dimensional (3D) rotating stimuli in comparison to two-dimensional (2D) controls. A small number of observers (between 5 and 8) viewed computerized displays of pixel-defined transparent rotating spheres or circular patches of pixels drifting linearly in opposite directions. Halfway through the circuit of rotation a letter was briefly displayed and the rotation continued with some change introduced. Our results showed that for horizontal shifts of the stimulus on the X-axis, changes in the axis of rotation, and additions/deletions of pixels, observers were better at detecting the changes associated with 3D motion than 2D motion. There was no good 2D control for approaching and receding stimuli, but on the basis of other results it was concluded that 3D movement had no advantage. It is suggested that rotation in 3D is more readily monitored by the visual system than simultaneous 2D motions in opposite directions.

Orientation anisotropy in the Ternus phenomenon

The Ternus display is an ambiguous, multi-element, apparent-movement animation which can be perceived in two alternative organizations. One of these involves the simultaneous movement of several elements and is called group movement. The other involves the movement of one element only and is called element movement. The present study examined the influence of retinal eccentricity and organizational orientation on the percentage of group-movement reports obtained across a range of interstimulus intervals. Six participants observed 48 variations of the Ternus display over 100 replications and judged the type of movement seen on each trial. Analysis showed that retinal eccentricity had no effect on the appearance of the display. However, compared with horizontally organized elements, obliquely organized and vertically organized elements yielded progressively more group movement across all but the shortest interstimulus intervals. These results represent an orientational anisotropy in the processing of Ternus apparent movement.

Spatial correspondence and relation correspondence: grouping factors that influence perception of the Ternus display

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 use of these principles to explain apparent movement are discussed.

Factors influencing the ability to detect motion reversals in rotation simulations

Five experiments probed the conditions under which observers fail to report instantaneous reversals in the direction of motion of pixels that define the rotation of a transparent sphere or plane. Our results showed that the extent to which rotation reversals were not reported depended upon whether observers used strict or lax criteria to make their judgments, the degree of perspective present in the rotation simulations, and the percentage of pixels that actually reversed direction. Furthermore, we found failures to report rotation reversals both with stimuli whose pixels were confined to smooth surfaces and scattered within volumes. Reversal detection with planar stimuli, unlike sphere stimuli, depended upon the orientation of the stimulus at the moment of reversal. Treue et al. (1995) postulated a surface-interpolation process as the explanation for the apparent insensitivity of observers to such reversals. However, we suggest that other stages of processing (e.g. a structure-from-motion process) are required to account for these results.

Effects on face recognition of spatial-frequency information contained in inspection and test stimuli

Researchers often assume a critical band of spatial frequencies is required for face recognition. Also, many studies have not measured the contrast required for recognition. On Day 1, observers viewed high-pass-filtered (HP), low-pass-filtered (LP), or unfiltered (UF) faces. On Day 2, they viewed a variety of faces, some of which were LP filtered, HP filtered, and UF. Observers adjusted contrast until they achieved both detection and recognition. Observers were most accurate and sensitive when filtered faces agreed in spatial-frequency content across days. Faces differing in spatial-frequency content were least well recognized. Unfiltered faces always fell between the 2 extremes. Observers generally used less contrast to recognize unfiltered than filtered faces. Correspondence of information between inspection and testing seemed more important than any particular range of frequencies.

Do variables that affect similar bistable apparent-movement displays result in similar changes in perception?

Two bistable apparent-movement displays (i.e. ones that generate two qualitatively different kinds of movement percepts under different conditions) were compared. They were designed to be as similar as possible spatially, and were studied with identical stimulus manipulations to see whether changes in balance between their bistable percepts would be similar. Results show that the two displays had different response characteristics to the same stimulus manipulations. Two models of motion perception that have previously predicted at least one kind of bistable apparent motion were considered in terms of how well they address the current data. As yet, neither model has been shown to predict the motion states and bistable behavior of the two displays studied here. It is concluded that results of the type described here (specifically, differences in the psychophysical functions yielded by two structurally similar but qualitatively different bistable displays) present a challenge for theories of motion perception.

Buildup and decay of a three-dimensional rotational aftereffect obtained with a three-dimensional figure

Gaps in past literature have raised questions regarding the kinds of stimuli that can lead to three-dimensional (3-D) rotation aftereffects. Further, the characteristics of the buildup and decay of such aftereffects are not clear. In the present experiments, rotation aftereffects were generated by projections of cube-like stimuli whose dynamic perspective motions gave rise to the perception of rotation in unambiguous directions; test stimuli consisted of similar cubes whose rotation directions were ambiguous. In experiment 1, the duration of the adaptation stimulus was varied and it was found that the 3-D rotation aftereffect develops with a time constant of approximately 26 s. In experiment 2, the duration between adaptation and testing was varied. It was found that the 3-D rotation aftereffect has a decay constant of about 9 s, similar to that observed with 2-D motion aftereffects. Experiment 3 showed that the rotation aftereffects were not simple depth aftereffects. To account for these aftereffects and related data, a modification of an existing neural-network model is suggested.

Discrimination of rotation from linear motion: influence of interference gratings and dichoptic viewing

Observers viewed a screen on which a number of patches containing horizontally moving pixels were presented. Target patches consisted of pixels whose motion simulated the rotation of a cylinder around its Y-axis; distractors, of pixels whose motion was linear. The task was to assess whether a display contained a target patch. Exp. 1 showed that the superposition of the displays with drifting sine-wave gratings interfered with rotation detection in a spatial- and temporal-frequency specific manner. In Exp. 2, dichoptic presentation greatly reduced the interference of the drifting gratings. It is argued on various grounds that the process(es) responsible for the detection and discrimination of rotating structure in depth exists at a low level of the visual system.

Motion capture of stationary lines by apparently moving terminators

Several studies and observations of a new form of motion capture are reported: frames containing identical rows of evenly spaced vertical lines are alternated in a standard apparent-motion paradigm. However, one vertical line in the first frame has short horizontal 'terminators' attached; the terminators are shifted to a different line in the second frame. Alternation that includes an unpatterned, nonzero interstimulus interval results in perceived motion of a vertical line along with the terminators. This motion can 'cross over' other stationary vertical lines and persists when light-filled interstimulus intervals and gaps between lines and terminators are introduced. It can also be obtained with different line sizes and spacings. The present motion capture does not appear to rely on a global-frame effect. Alternative explanations are considered.

Background spatial frequency influences perception of luminance-based apparent movement

Apparent movement occurs when stimulus figures are presented at different spatial locations at different times. In the present studies, the periodicity of the background upon which stimuli were presented was manipulated. Subjects viewed a bistable movement display presented on a background consisting of a variable-frequency grating. The type of movement perceived varied systematically with the spatial frequency of the background, suggesting that more than the target stimuli are processed in the generation of apparent movement.

The detection of stimuli rotating in depth amid linear motion and rotation distractors

In three experiments, observers watched displays consisting of two or more areas that contained unidirectionally moving pixels. In half of the displays, one area of pixels contained movement that corresponded to the projection of the front surface of a rotating cylinder. The total duration of the displays and the number of stimulus areas per display were varied. The subjects' task was to indicate whether or not a given display contained rotation. When the display time required to reach 75% accuracy was determined, it was found that the number of stimuli per display had no effect; nor did it interact with other variables. One control experiment eliminated "pixel crowding" at the edges of the rotating cylinders, with little effect on the results. Another control experiment found that the ability to discriminate rotating from linear motion declines with distance away from fixation. A fourth experiment showed that under conditions similar to the first three, subjects can make accurate shape discrimination, thereby suggesting that three-dimensional information contributed to the decisions made in the original experiments. On the basis of these results and previous data, it is suggested that in the present experiments structure was recovered from motion by the short-range process, ad that this recovery engages attention to a relatively constant extent, regardless of the number of stimuli contained in a display. Shape discrimination based on structure from motion may required a more effortful form of attention.

The role of background in mediating apparent movement

The spatial structure of the background upon which figures engage in apparent movement has been a neglected variable in research. In this pilot study, the presence of a sine-wave-grating as background had a strong influence on the quality of apparent movement that was perceived by 3 observers.

A comparison of varieties of "second-order" motion

Bistable apparent-movement displays were created using four different kinds of "second-order" stimuli in which figures were defined by binocular disparity, spatial phase shifts of periodic luminance distributions, relative motion, and texture-element orientation differences. For each display, characteristics of the local structure of the figures, backgrounds, or both were varied. For each experimental condition, the type of apparent movement seen as a function of interstimulus interval was measured, and it was found that the relationship between perceived apparent movement and interstimulus interval differed across the types of displays viewed. The results suggest that the transformations between first-order stimulus properties and second-order motion may be too complex to imply a single uniform class of second-order motion detectors. Alternative physiological accounts of the results are discussed.

Visual-haptic relations in a two-dimensional size-matching task

Visual and haptic perceptions of the two-dimensional size of square stimuli were compared using cross-modal, intramodal, and bimodal matching tasks. In a repeated-measures factorial design, 12 women participated in five matching tasks involving various combinations of vision and haptic touch; five sizes of standard squares were matched with a comparison range of 10 squares during each task. Analysis showed that, for stimuli with side lengths of .75 in. and smaller, matching accuracy was superior when vision was used during the matching task regardless of the modality used during inspection. When haptic touch was used in the matching task, accuracy was better when vision had been used during inspection than when it had not. These results were consistent with those of previous studies comparing size perception by vision and other forms of touch. The over-all relationship between matched size and inspected size was best accounted for by a third-order polynomial function.

Perception of three-dimensional angular rotation

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 (20 degrees-70 degrees). DLs were lowest (7%) when points were on the surface and when at least one sphere rotated around the y-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 the x-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.

Comments on Cavanagh and Mather (1989): coming up short (and long)

Cavanagh and Mather (1989) reviewed literature concerning the possible distinction between short- and long-range processes in motion perception and concluded that the distinction cannot be supported. Instead, they proposed that motion perception be considered on the basis of detectors for first-order (luminance, color) and second-order (first-order motion, texture, stereo) stimulus attributes. They supported their position with studies of motion based on second-order stimuli. The present paper contends that when experiments permitting the investigation of both processes in the same display are included and when criteria are examined in their totality rather than one-by-one, the original short-range/long-range distinction can be retained. Furthermore, it is argued that the first-order/second-order distinction does not represent a theoretical advancement and that studies of second-order motion can be interpreted in terms of the older distinction. It is concluded that the short-range/long-range distinction is useful and should not be abandoned.

Effects of adaptation to apparent movement on recovery of structure from motion

Past work on the recovery of three-dimensional structure from dynamic two-dimensional images has led to inconsistent conclusions regarding the contributions of the short-range and long-range motion processes. In the present experiments, subjects adapted to displays (either four lines or 50 randomly positioned pixels) whose spatiotemporal parameters were chosen to favor either the short-range or long-range process. Adaptation periods were followed by test displays that simulated the rotation of a four-pixel random object about the vertical gamma-axis. The dependent measure was the angle of rotation between successive frames of the rotation display at which percepts of three-dimensional structure broke down. Both the original data and derived measures based on best-fitting polynomials showed small but consistent effects: Compared to control conditions, adaptation to short-range motion reduced the angle at which percepts of structure broke down; adaptation to long-range motion increased them. It is suggested that both low-level (i.e. short-range) and high-level (long-range) processes contribute to the recovery of structure from motion.

The effects of position cues on the appearance of stimulus elements in a bistable apparent movement display

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.

Global cooperativity of the short-range process in apparent movement: evidence obtained with contour-containing stimuli

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 can be 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.

Auditory sensitivity and tone-sequence reproduction in oral contraceptive users and nonusers

In two stages spanning a single academic year, 10 women using oral contraceptives and 11 not using oral contraceptives were tested on their auditory threshold sensitivity to six frequencies (250, 750, 1500, 3000, 6000, and 8000 Hz) as a function of phases of the menstrual cycle. 11 women (5 users and 6 nonusers) were also tested on their ability to reproduce successively longer tone sequences. Results showed that there was little variation due to phase of the menstrual cycle, with the exception that oral contraceptive users showed lower relative thresholds at low frequencies during the postmenstrual phase. Over-all, oral contraceptive users had higher absolute and relative thresholds than nonusers, except at 3000 Hz. Although not statistically significant, results on the tone-reproduction test showed an enhanced ability to reproduce tones during the premenstrual phase for women using oral contraceptives. Results are discussed in relation to previous findings.

Dependence of apparent movement of a subjective figure on the perceptual fate of inducing elements

In a series of demonstrations, two stimulus frames that contained subjective figures were alternated. It is shown that the perception of apparent movement of a subjective figure depends upon the configuration of the inducing stimuli and whether or not conditions of presentation favor the short-range or long-range process in apparent movement. Those conditions that favor the long-range process result in global apparent movement of the subjective figure. However, those conditions that favor the short-range process may prevent apparent movement of the subjective figure, or may result in a kind of apparent movement that is qualitatively different from that seen when similar physical contours are alternated. These results are interpreted in terms of the assumed differences between the short-range and long-range processes.

Contrast requirements for picture recognition: converging evidence for a spatial frequency hypothesis

Based upon perceptual studies, the present hypothesis was that different ranges of spatial-frequency information constitute different sources of information for recognition memory. In Experiment 1, 40 subjects were tested with sets of focused and unfocused pictures as inspection and test stimuli. In addition to reporting whether each test picture was believed to be a member of the inspection set or a novel picture, each subject was allowed to adjust the contrast of the stimulus until such a judgment could be made. In Experiment 2, subjects made similar judgments when inspection or test stimuli were flickered (perceptually enhancing low spatial frequencies) or unflickered. Results from both studies were consistent with the experimental hypothesis. Other studies were reviewed, which, together with the present data, lend converging evidence to the spatial-frequency hypothesis.

Preliminary findings suggesting cyclic changes in visual contrast sensitivity during the menstrual cycle

Visual contrast thresholds to both stationary and moving gratings of three spatial frequencies (2, 4, and 16 cyc/deg) were measured over a 32-day period in two women displaying normal menstrual cycles and in two noncycling control subjects. The time-series data of each subject in each condition were Fourier analyzed and the resulting amplitude spectra showed differences between the two sets of subjects. The spectra of the control subjects were relatively flat, whereas those of the experimental subjects showed a number of peaks at several harmonics (periods). Conservative significance tests suggested that the peaks in the spectra of the cycling women were larger than might be expected by chance. The data also suggested that changes in sensitivity were greatest for 4-cyc/deg gratings, those nearest the peak of the normal contrast sensitivity function.

Evidence for color-contingent organizational aftereffects

An effort was made to produce negative organizational aftereffects in response to a perceptually ambiguous stimulus that can be seen as a cube in either of two orientations. 6 subjects were adapted to alternative disambiguated versions of the ambiguous cube, shown either in green or magenta light. When tested with the ambiguous figure shown in either green or magenta light, 5 subjects showed negative aftereffects while one subject showed a positive aftereffect. The aftereffects showed an average change in preferred organization of 25%. The results are interpreted in terms of both cognitive and hard-wired processes.

A three-dimensional motion aftereffect produced by prolonged adaptation to a rotation simulation

After adaptation to a perspective simulation of a square plane rotating in depth, an ambiguous rotation simulation (ie one containing no perspective information) appears to rotate in the direction opposite that of adaptation. The strength of this three-dimensional motion aftereffect (MAE) is proportional to the amount of perspective available in the adaptation display and, in the dark, decays to about 75% of its initial strength within about 546 s. The nature of the testing situation and a control experiment suggest that the three-dimensional MAE is not caused by retinal adaptation of two-dimensional directionally selective mechanisms.

Failure to find an absolute retinal limit of a putative short-range process in apparent motion

Previous studies of apparent movement have concluded that the short-range process does not operate when stimulus displacements exceed 15-20' arc. In the present experiments, we studied a bistable apparent-movement display, one of whose perceptual organizations is mediated by short-range process. In both experiments it was found that the perceptual organization mediated by the short-range process could be made dominant at stimulus displacements well in excess of the proposed spatial limit, provided the stimulus elements were made larger. It is concluded that the spatial limit of the short-range process is a relative, not absolute, one. Current knowledge regarding the short-range process is reviewed, and an hypothesis regarding the functional utility of the short-range process in compensating for the effects of small eye tremors is advanced.

Correspondence strength in apparent movement as a function of short-range and long-range processes: comments on Ullman (1980)

It is argued that correspondence strength is a joint function of the relative activity of short-range and long-range processes and not solely a function of a single correspondence process, and this puts Ullman's interpretation of his results in doubt.

Perception of eye scans with the Müller-Lyer stimuli: evidence for filter theory

Ginsburg's filter theory successfully accounts for the perceptual distortions perceived in a wide range of illusions and bistable phenomena. Essentially, the theory proposes that illusory distortions are the natural consequence of low-pass spatial filtering (based upon the human modulation transfer function) of the physical stimulus. With regard to the Müller-Lyer illusion, predictions based upon filter theory and human scan-path data are in accord. However, data linking filter theory's predictions regarding perceptual experiences associated with the illusion to the eye-scan results have been missing. In the present experiment subjects provided subjective estimations of their own eye scans while viewing each of the following stimuli: the fins-out member of the Müller-Lyer illusion, the fins-in member of the Müller-Lyer illusion, and a finless horizontal line (variations of each stimulus consisted of one, two, and three line segments). The analysis of these data supported three predictions that were derived from filter theory. Potential problems facing filter theory are also addressed.

A new bistable motion illusion based upon 'kinetic optical occlusion'

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 approximately less than 50 ms) or he saw right-to-left stroboscopic movement of successive bars (with IBIs approximately greater than 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 under appropriate conditions. In a series of experiments it was found that the transition IBI for the original display did not depend upon whether the observer was tracking the sequence of events or fixating a stationary point in the display. The transition IBI was an increasing function of spatial frequency above about 3 cycles deg-1, and depended upon whether the display was focused or optically blurred. Empirically determined transition IBIs correspond well to estimates of the integration times of visual mechanisms studied in other paradigms.

High fundamental spatial frequencies and edges have different perceptual consequences in the 'group/end-to-end' movement phenomenon

A subject viewing two alternating frames, each containing, say, three vertical stripes in a horizontal row, displaced laterally by one cycle in one frame with respect to the others, perceives either the three stripes moving left-right-left in unison (group movement) or one stripe moving from one end of the display to the other and the two overlapping stripes stationary (end-to-end movement). At suitable temporal parameters of presentation (frame duration, interstimulus interval) the perception of the display is bistable. Experiments have shown that the relative strengths of these alternative movement sensations depend upon the fundamental spatial frequency of the display and upon stimulus waveform. Square-wave stimuli, which have energy at high spatial frequencies, had effects opposite to those produced by increases in fundamental spatial frequency. Amblyopes differed from normal viewers only in the perception of the square-wave stimuli.

The effects of spatial and temporal factors on the perception of stroboscopic rotation simulations

In the present two experiments subjects viewed discontinuous, i.e. stroboscopic, simulations of a transparent sphere partially filled with randomly positioned luminous dots and rotating about the y axis in depth. Over a range of stimulus conditions, such simulations elicited coherent sensations of continuous rotation and internal volume of the sphere. By manipulating both spatial and temporal variables in the simulations, it was attempted to define the boundary conditions for which corresponding elements of a simulation are perceptually paired from frame to frame to yield coherent sensations of rotation and depth. The results indicated that the process that matches or pairs corresponding elements in a three-dimensional simulation cannot be identified with the process that matches corresponding elements in similar two-dimensional displays. Furthermore, temporal factors influenced the perception of these simulations more than did spatial factors, and rotation-judgment accuracy and perceived depth were different functions of temporal frequency. Over a range of temporal frequencies, such three-dimensional simulations are apparently processes in a manner similar to that in which objects undergoing real motion are processed.

Optical defocusing reverses perceptual organization

At a certain point during the defocusing of a reversible figure, the organization of the figure reversed unexpectedly, and the reversal seemed more abrupt than a spontaneous reversal. The critical point is approximately constant over time for a given subject, and a similar point occurs during refocusing. This phenomenon is discussed in terms of inhibition between competing neural structures.

Possible role of transient and sustained visual mechanisms in the determination of similarity judgments

Two studies examined the role played by transient and sustained visual mechanisms in the determination of similarity judgments produced in response to pairs of geometrical stimuli. In two experiments, subjects were trained to attend to two dimensions of a set of stimuli and to assign similarity ratings with respect to those two dimensions only. An INDSCAL multidimensional scaling analysis of the subsequent similarity ratings showed that subjects emphasized global blob, or low spatial frequency-dependent, dimensions of the stimuli when they were presented for brief durations (20 msec.), irrespective of the dimensions to which the subjects and had been trained to attend. This finding suggested that low spatial frequency selective transient mechanisms dominated the perceptual processes which underlie the similarity judgments. When the stimulus duration was raised to 50 msec. so that sustained mechanisms could also make a signficicant contribution to the perceptual processes underlying the similarity judgments, subjects emphasized only those dimensions on which they had been trained. The implications of the present findings for the concepts of selective attention and automatic activation were discussed.

Apparent movement of successively generated subjective figures

In the present studies a pair of random-dot frames was constructed so that two areas in the first frame (f1) were correlated with two areas in the second frame (f2). The alternation of the pair of frames (an f1--f2 sequence) gave rise to two subjective figures. When two pairs of randomdot frames (an f1--f2 sequence and an f3--f4 sequence), each of which produced two subjective figures in different locations, were thmeselves alternated, the subjective figures from the f1--f2 sequence interacted with the subjective figures from the f3--f4 sequence to produce apparent movement. With any one of the four general kinds of displays which we constructed, subjects usually perceived only one of two types of subjective-figure movement. The type of movement that was perceived with a given display depended primarily upon the degree of change (across the interval between an f1--f2 and an f3--f4 sequence) of the internal structure of the successively generated subjective figures. Relative intensity differences between the subjective figures and their backgrounds influenced the type of apparent movement seen, whereas variations in the density of elements in a display did not. We tentatively propose a two-stage model to explain the apparent movement of the subjective figures: the first stage is assumed to generate the subjective figures by means of a cross-correlation of the intensity distributions of the two frames within an f1--f2 sequence and within an f3--f4 sequence; on the basis of inputs from the first stage, the second stage generates apparent movement signals for the subjective figures.