Vector analysis and process combination in motion perception

Experiments are reported supporting an altered explanation of the vector analysis that occurs in certain motion displays discovered by Johansson (1950). What seemed the result of a perceptual vector analysis is ascribed to the outcome of two different, independent stimulus conditions to which such displays can give rise because of external vector analysis. The different stimulus conditions are configurational change on the one hand and one of the subject-relative stimulus conditions on the other. In two of Johansson's displays, conditions for configurational change were altered by adding stationary reference points in the surround of the displays. Veridical perception of the displays resulted in a majority of instances. We also found that the different motions that result from configurational change and from subject-relative stimulation may combine to form unitary perceived motions and that this happens quite frequently under some conditions.     

Resolving ambiguities in orientation, motion, and depth domains.

Three different perceptual systems--orientation, motion, and depth--can recover a global perceptual organization from spatially correlated random multielement patterns. In all three cases the global structure composed of random elements is evaluated by mechanisms performing measurements in the energy domain within appropriately defined local space-time areas. The selective increase in energy of one fraction of the elements may dramatically change the whole perceptual organization of the stimulus. In specially devised patterns one and the same element can belong to two or more separate perceptual organizations, the perceptual salience of one of which can be reinforced by a luminance increment of the elements comprising it. If a stimulus provides two different perceptual organizations to which each element could potentially belong, one of four possible solutions of the existing ambiguity will occur: suppression, rivalry, mixture, or parity. Two superimposed global orientation patterns either suppress or dominate over each other but cannot be seen simultaneously or in a mixed form. Characteristic of the depth system is that it allows multiple binocular matchings and parity of possible perceptual solutions. Finally, if a stimulus provides two or more paths along which each element may appear to move, the perceived global motion direction is determined by a mixture of directions of these competing motion paths. Dissimilarities in these ways of resolving ambiguities may be based on different principles defining regularity and coherence of an object in the orientation, motion, and depth domains.     

Effects of similarity on apparent motion and perceptual grouping

Effects of similarity in colour, luminance, size, and shape on apparent motion and perceptual grouping were examined in part 1 in two parallel experiments on the same seven subjects. In both experiments, the effect of similarity was compared with that of proximity in competitive, bistable stimulus situations. A combination of a larger horizontal separation between the homogeneous stimulus elements and a smaller constant vertical separation between heterogeneous stimulus elements produced two kinds of apparent motion (or perceptual grouping) with equal probabilities. Such matched separations between homogeneous stimulus elements were obtained by the double staircase method in various stimulus conditions. In both experiments on apparent motion and perceptual grouping matched separation was found to increase as the difference between the heterogeneous stimulus elements increased. High correlations (0.71 to 0.94) of matched separations were found between apparent motion and perceptual grouping in four stimulus series: colour, luminance, size, and shape. Six of the seven subjects were also tested in part 2. Here, the effects of differences were found to work additively across different perceptual attributes in both phenomena, when multiple differences were combined in heterogeneous elements. The experimental results are discussed from the point of view that apparent motion is an example of perceptual constancy.     

Multiplicative effects of intention on the perception of bistable apparent motion

When viewing ambiguous displays, observers can, via intentional efforts, affect which perceptual interpretation they perceive. Specifically, observers can increase the probability of seeing the desired percept. Little is known, however, about how intentional efforts interact with sensory inputs in exerting their effects on perception. In two experiments, the current study explored the possibility that intentional efforts might operate by multiplicatively enhancing the stimulus-based activation of the desired perceptual representation. Such a possibility is suggested by recent neurophysiological research on attention. In support of this idea, when we presented bistable apparent motion displays under stimulus conditions differentially favoring one motion percept over the other, observers' intentional efforts to see a particular motion were generally more effective under conditions in which stimulus factors favored the intended motion percept.     

Relational invariance and visual space perception: On perceptual vector analysis of the optic flow

The outlines of the theory of perceptual vector analysis in visual space and motion perception in its present state of development are presented. This theory is exclusively founded on the outcome of systematic experimental research extended over more than three decades. Some informative experiments and their results are briefly described, starting with some decisive findings reported in Johansson (1950). A fundamental difference between this approach and the traditional pictorial way of describing the proximal stimulus is that it accepts abstract mathematical relations in spatial change over time in the optic flow hitting the receptor organs as specification of the proximal stimulus rather than the traditional interpretation of cues in static images. In its present form it is able to establish functionally valid mathematical correspondence between the light-reflecting environment, geometrical transformations in the optic flow at the retina and the percept.     

Perceptual acceleration of objects in stream: evidence from flash-lag displays

An object in continuous motion is perceived ahead of the briefly flashed object, although the two images are physically aligned (Nijhawan, 1994), the phenomenon called flash-lag effect. Flash-lag effects have been found also with other continuously changing features such as color, pattern entropy, and brightness (Sheth, Nijhawan, & Shimojo, 2000) as well as with streamed pre- and post-target input without any change of the feature values of streaming items in feature space (Bachmann & P?der, 2001a. 2001b). We interpret all instances of the flash-lag as a consequence of a more fundamental property of conscious perception in general: acceleration of the speed with which samples of perceptual information become represented in explicit format immediately after the stimulation onset. Decreased visual latency of the samples of stimulus information from the streamed input leads to the relative perceptual lag for the separately flashed stimulus because it is not preceded by adjacent sensory input that would have accelerated its perception. Experimental support for the notion of perceptual acceleration is reviewed.     

Inhibition of return is not detected using illusory line motion

Inhibition of return (IOR) is the name that has been assigned to a response time (RT) delay to a stimulus presented at a recently stimulated spatial location. A commonly held explanation for the origins of IOR is that perceptual processing is inhibited and that this inhibition translates into slower RT. Three experiments with 10 subjects were used to directly test this perceptual explanation. The first two experiments assessed the level of perceptual facilitation present in the IOR paradigm using the frequency and latency of illusory line motion judgments. Contrary to the predictions of the perceptual view, the line motion and RT measures revealed only speeded processing at previously stimulated spatial locations. Experiment 3 required a simple detection response and used the same stimulus and timing parameters as those in Experiments 1 and 2. IOR was present, replicating the recent finding that judgments based on perceptual qualities of the stimulus do not demonstrate a RT delay, whereas simple detection tasks do show RT inhibition at previously stimulated locations. These findings are discussed in relation to a number of hypotheses about the origin of the RT delay.     

Linking dynamical perceptual decisions at different levels of description in motion pattern formation: computational simulations

A two-level dynamical model of motion pattern formation is developed in which local motion/ nonmotion perceptual decisions are based on inhibitory competition between area V1 detectors responsive to motion-specifying versus motion-independent stimulus information, and pattern-level perceptual decisions are based on inhibitory competition between area MT motion detectors with orthogonal directional selectivity. The model accounts for the effects of luminance perturbations on the relative size of the pattern-level hysteresis effects reported by Hock and Ploeger (2006) and also accounts for related experimental results reported by Hock, Kelso, and Sch?ner (1993). Single-trial simulations demonstrated the crucial role of local motion/nonmotion bistability and activation-dependent future-shaping interactions in stabilizing perceived global motion patterns. Such interactions maintain currently perceived motion patterns by inhibiting the soon-to-be-stimulated motion detectors that otherwise would be the basis for the perception of an alternative pattern.     

When is motion 'motion'?

Examples of visual motion have become more and more abstract over the years, leading up to 'third-order' stimuli where direction is actually determined by the observer through top - down attention. But how far can this be pushed--are there motion stimuli that are yet more arbitrary and abstract? Actually, there is a broad class of 'conceptual motion' stimuli--things like a moving grating of faces, or a shifting pattern of words--that are perfect analogs to traditional 'perceptual motion' stimuli, solvable by the same motion computation and for which observers can readily make direction-of-motion judgments. Interestingly though, these do not produce a sensation of motion (among other automatic consequences of motion detection). Here we compare a luminance-based perceptual motion stimulus to a semantic-based conceptual motion stimulus to contrast the psychophysical hallmarks of these motion categories.     

Bottom-up and top-down factors of motion direction learning transfer

Perceptual learning of motion discrimination has long been believed to be motion direction specific. However, recent studies using a double-training paradigm, in which the to-be-transferred condition was experienced through practicing an irrelevant task, found that perceptual learning in various visual tasks, including motion direction discrimination, can transfer completely to new conditions. This transfer occurred when the transfer stimulus was subconsciously presented, or when top-down attention was allocated to the transfer stimulus (which was absent). In the current study, observers were exposed subconsciously, or directed top-down attention, to the transfer motion direction, either simultaneously or successively with training. Data showed that motion direction learning transferred to the transfer direction, and suggest that motion direction learning specificity may result from under-activations of untrained visual neurons due to insufficient bottom-up stimulation and/or lack of top-down attention during training. These results shed new light on the neural mechanisms underlying motion perceptual learning and provide a constraint for models of motion perceptual learning.     

Coherence and transparency of moving plaids composed of Fourier and non-Fourier gratings

We examined the perceptual coherence of two-component moving plaids. The gratings that constituted the plaids were either standard Fourier gratings (F), in which luminance was determined by a drifting sinusoid, or non-Fourier gratings (NF), in which the contrast of a random background was modulated by a drifting sinusoid. These NF gratings are examples of stimuli that generate a compelling percept of motion, even though they fail to elicit a motion signal from motion analyzers based on standard cross-correlation (Chubb & Sperling, 1988). Naive observers viewed three types of stimuli consisting of superpositions of these two components: (1) two standard drifting gratings (F/F), (2) two non-Fourier drifting gratings (NF/NF), and (3) one standard and one non-Fourier drifting grating (F/NF). As expected, the F/F stimulus yielded a compelling percept of coherent motion. The dominant percept of all the observers for the NF/NF stimulus was one of coherent motion, provided that both gratings were visible and of approximately equal contrast. None of the observers reported a dominant percept of coherent motion for the F/NF condition, over a wide range of contrasts for the two grating components and across two varieties of NF gratings. In view of the results of Albright (1992) and Albright and Chaudhuri (1989), that show that single cells in macaque V1 and MT respond to both F and NF motion, one cannot interpret our findings as evidence that F and NF motion are processed independently. Alternative, “higher level” interpretations based on the intrinsically ambiguous nature of the stimuli and physical laws governing the appearance of transparent objects are discussed.     

Coherence and transparency of moving plaids composed of Fourier and non-Fourier gratings.

We examined the perceptual coherence of two-component moving plaids. The gratings that constituted the plaids were either standard Fourier gratings (F), in which luminance was determined by a drifting sinusoid, or non-Fourier gratings (NF), in which the contrast of a random background was modulated by a drifting sinusoid. These NF gratings are examples of stimuli that generate a compelling percept of motion, even though they fail to elicit a motion signal from motion analyzers based on standard cross-correlation (Chubb & Sperling, 1988). Naive observers viewed three types of stimuli consisting of superpositions of these two components: (1) two standard drifting gratings (F/F), (2) two non-Fourier drifting gratings (NF/NF), and (3) one standard and one non-Fourier drifting grating (F/NF). As expected, the F/F stimulus yielded a compelling percept of coherent motion. The dominant percept of all the observers for the NF/NF stimulus was one of coherent motion, provided that both gratings were visible and of approximately equal contrast. None of the observers reported a dominant percept of coherent motion for the F/NF condition, over a wide range of contrasts for the two grating components and across two varieties of NF gratings. In view of the results of Albright (1992) and Albright and Chaudhuri (1989), that show that single cells in macaque V1 and MT respond to both F and NF motion, one cannot interpret our findings as evidence that F and NF motion are processed independently. Alternative, "higher level" interpretations based on the intrinsically ambiguous nature of the stimuli and physical laws governing the appearance of transparent objects are discussed.     

Implicit learning modulates selective attention at sensory levels of perceptual processing

Electrophysiological evidence suggests that attention can be modulated as early as 100 msec after stimulus presentation. However, it is not clear whether these changes are based primarily on stimulus properties such as perceptual load (i.e., the level of perceptual difficulty), or other properties, such as general attentional set or learned expectations concerning perceptual load. Using event-related potentials, this study examined how implicit learning of perceptual load conditions modulates selective attention at sensory levels of perceptual analysis. The results show significant differences in P1 amplitude recorded over occipital areas of the brain as a function of learned expectations of perceptual load, only when perceptual load could be reliably predicted by the preceding stimuli. Moreover, differences in processing were found when both low and high perceptual load conditions could be predicted. These findings suggest that implicit learning modulates the allocation of attention at early stages of perceptual processing.     

Visual texture perception and Fourier analysis

The relationship between the Fourier spectra of visual textures (represented by four hypothetical visual channels sensitive to spatial frequencies) and the perceptual appearance of the textures was investigated. Thirty textures were synthesized by combining various spatial frequencies of different amplitudes. Twenty subjects grouped the textures into 2, 3, 4, and 5 groups based on the similarity of their appearance. The groupings were analyzed by means of linear discriminant analysis using the activity of the four channels as predictor variables. The groupings were also examined by multidimensional scaling, and the resulting stimulus configuration was canonically correlated with the channel activity. The results of both analyses indicate a strong relationship between the perceptual appearance of the textures and their Fourier spectra. These findings suport a multiple-channel spatial-frequency model of perception.     

Perceptual unit formation in simple motion patterns

The present study investigated the proximal constraints that determine perceptual unit formation under minimal stimulus conditions. Projections of three moving dots, which could form two possible two-dot configurations, were presented to naive observers. In a forced-choice situation, their task was to report which two-dot configuration was perceived as a distinct perceptual unit. The results showed that common motions (arbitrary translations and rigid rotations in the frontoparallel plane) have stronger grouping power as compared to different relative motions (expansions/contractions, or simultaneous expansions/contractions and deformations in the frontoparallel plane). It was found that proximal changes of distances between elements in two-dot structures reduce grouping power. Changes of proximal directions, however, did not affect unit formation in two-dot structures at all. The effect of vector algebraic combinations on grouping power in three-dot strutures was also investigated. Evidently, visual vector analysis splits up motion combinations into their constituents, and in come cases this contributes to additive effects.     

A theory of phenomenal geometry and its applications

The geometry of perceived space (phenomenal geometry) is specified in terms of three basic factors: the perception of direction, the perception of distance or depth, and the perception of the observer's own position or motion. The apparent spatial locations of stimulus points resulting from these three factors thereupon determine the derived perceptions of size, orientation, shape, and motion. Phenomenal geometry is expected to apply to both veridical and illusory perceptions. It is applied here to explain a number of representative illusions, including the illusory rotation of an inverted mask (Gregory, 1970), a trapezoidal window (Ames, 1952), and any single or multiple point stimuli in which errors in one or more of the three basic factors are present. It is concluded from phenomenal geometry that the size-distance and motion-distance invariance hypotheses are special cases of the head motion paradigm, and that proposed explanations in terms of compensation, expectation, or logical processes often are unnecessary for predicting responses to single or multiple stimuli involving head or stimulus motion. Two hypotheses are identified in applying phenomenal geometry. It is assumed that the perceptual localization of stimulus points determines the same derived perceptions, regardless of the source of perceptual information supporting the localizations. This assumption of cue equivalence or cue substitution provides considerable parsimony to the geometry. Also, it is assumed that the perceptions specified by the geometry are internally consistent. Departures from this internal consistency, such as those which occur in the size-distance paradox, are considered to often reflect the intrusion of nonperceptual (cognitive) processes into the responses. Some theoretical implications of this analysis of phenomenal geometry are discussed.     

The importance of pattern information for the resolution of depth-ambiguous apparent motion

A single apparent motion display can result in the perception of a rigid three-dimensional motion or a plastic, two-dimensional motion. Previous studies have found that the principal determinant of the perceptual outcome is the temporal properties of the apparent motion stimulus. Here it is shown that the form properties of the stimulus are another determinant and that, in some situations, they may become a more powerful determinant than the temporal properties.     

Common and relative components of reflected light as information about the illumination, colour, and three-dimensional form of objects

Abstract.— A model for perceptual analysis of the proximal stimulus into common and residual components, earlier described in a series of papers by Johansson and his co-workers for motion and space perception, is applied to colour perception. Though still premature, this application seems to make colour constancy a natural result of the analysis in the same way as size and form constancy is solved by Johansson's vector analysis in the frame of reference of projective geometry. Applications of the model to Land's Mondrian experiments, to simultaneous contrast, and to the Cornsweet-OBrien effect are outlined. The information from variations and invariances in reflected light (besides the geometrical projection) is discussed with reference to Gibson's ecological optics.