Differential activation solution to the

The correspondence problem arises in motion perception when more than one motion path is possible for discontinuously presented visual elements. Ullman’s (1979) “minimal mapping” solution to the correspondence problem, for which costs are assigned to competing motion paths on the basis of element affinities (e.g., greater affinity for elements that are closer together), is distinguished from a solution based on the differential activation of directionally selective motion detectors. The differential activation account was supported by evidence that path length affects detector activation in a paradigm for which motion correspondence is not a factor. Effects on detector activation in this paradigm also were the basis for the successful prediction of path luminance effects on solutions to the motion correspondence problem. Finally, the differential activation account was distinguished from minimal mapping theory by an experiment showing that the perception of an element moving simultaneously in two directions does not depend on whether the two motions are matched in path-length determined affinity; it is sufficient that the activation of detectors responding to each of the two motion directions is above the threshold level required for the motions to be perceived. Implications of the differential activation solution are discussed for the stability of perceived motions once they are established, and the adaptation of perceived andunperceived motions.     

The influence of adaptation and stochastic fluctuations on spontaneous perceptual changes for bistable stimuli

Spontaneous perceptual change was studied by measuring the probabilities of the first two spontaneous pattern switches as a function of time following the onset of a bistable apparent quartet for which either horizontal or vertical motion is perceived. Contrary to the classical satiation hypothesis (Köhler & Wallach, 1944),differential time-dependent adaptation of the perceived compared with the unperceived motion directions was not necessary to account for the first spontaneous switch. In addition, adaptation of the perceived motion accompanied by recovery from adaptation of the unperceived motion was not necessary to account for the increased probability of the second spontaneous switch. It was concluded that regardless of possible adaptation effects, stochastic fluctuations are necessary for the actual reversal of activation levels that produces the spontaneous switch. When the difference in detector activation is reduced by differential adaptation of competing motion detectors (or by the occurrence of a prior spontaneous pattern change), smaller stochastic fluctuations are sufficient to reverse the relative activation of competing detectors. Thus, adaptation can increase the probability of spontaneous switches without directly causing them.     

Dynamic, state-dependent thresholds for the perception of single-element apparent motion: Bistability from local cooperativity

Previous studies have indicated that the formation of coherent patterns for multielement motion displays depends onglobal cooperative interactions among large ensembles of spatially distributed motion detectors. These interactions enhance certain motion directions and suppress others. It is reported here that perceiving one element moving between two nearby locations likewise is subject to cooperative influences (possibly facilitating and inhibiting interactions within alocal ensemble of overlapping detectors). Thresholds depending on luminance contrast were measured for a generalized singleelement apparent-motion stimulus, and evidence for spontaneous switching and hysteresis effects indicated that motion perception near the 50% threshold was bistable. That is, for conditions in which motion and nonmotion were perceived half the time, the two percepts were distinct; when one was perceived, it clearly was discriminable from the other. These results indicated that (1) single-element apparent-motion thresholds depended on the immediately preceding state of the ensemble of motion detectors responding to the stimulus, and (2) the stimulus activation of individual motion detectors always might be influenced by recurrent, cooperative interactions resulting from the detectors’ being embedded within interconnected ensembles.     

The dynamical foundations of motion pattern formation: stability,selective adaptation,and perceptual continuity

A dynamical model is used to show that global motion pattern formation for several different apparent motion stimuli can be embodied in the stable distribution of activation over a population of concurrently activated, directionally selective motion detectors. The model, which is based on motion detectors being interactive, noisy, and self-stabilizing, accounts for such phenomena as bistability, spontaneous switching, hysteresis, and selective adaptation. Simulations show that dynamical solutions to the motion correspondence problem for a bistable stimulus (two qualitatively different patterns are formed) apply as well to the solution for a monostable stimulus (only one pattern is formed) and highlight the role of interactions among sequentially stimulated detectors in establishing the state dependence and, thereby, the temporal persistence of percepts.     

On interference effects in concurrent perception and action

Recent studies have reported repulsion effects between the perception of visual motion and the concurrent production of hand movements. Two models, based on the notions of common coding and internal forward modeling, have been proposed to account for these phenomena. They predict that the size of the effects in perception and action should be monotonically related and vary with the amount of similarity between what is produced and perceived. These predictions were tested in four experiments in which participants were asked to make hand movements in certain directions while simultaneously encoding the direction of an independent stimulus motion. As expected, perceived directions were repelled by produced directions, and produced directions were repelled by perceived directions. However, contrary to the models, the size of the effects in perception and action did not covary, nor did they depend (as predicted) on the amount of perception–action similarity. We propose that such interactions are mediated by the activation of categorical representations.     

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

The relationship between local-level motion detection and higher level pattern-forming mechanisms was investigated with the motion quartet, a bistable stimulus for which either horizontal or vertical motion patterns are perceived. Local-level perturbations in luminance contrast affected the stability of the perceived patterns and, thereby, the size of the pattern-level hysteresis obtained by gradually changing the motion quartet's aspect ratio. Briefly eliminating luminance contrast (so nonmotion was perceived during the perturbation) eliminated pattern-level hysteresis, and briefly increasing luminance contrast (so motion was perceived during the perturbation) increased pattern-level hysteresis. Partially reducing luminance contrast resulted in bistability during the perturbation; pattern-level hysteresis was maintained when motion was perceived, and eliminated when nonmotion was perceived. The results were attributed to local motion/nonmotion perceptual decisions in area V1 affecting the magnitude of the activation feeding forward to motion detectors in area MT, where the stability of pattern-level perceptual decisions is determined by activation-dependent, future-shaping interactions that inhibit soon-to-be-stimulated detectors responsive to competing motion directions.     

Perceptual constancy during ocular pursuit: a quantitative estimation procedure

Perceptual constancy of visual motion is usually described as the degree of correspondence between physical and perceived characteristics of motion in the external world. To study it, one has to assess the relationship between physical motion, its retinal image, and its perception. We describe a quantitative estimation procedure for a measure K denoting the degree of perceptual constancy of background target motions noncollinear to the eye movements during ocular pursuit. The calculation of K is based on three vectors describing the target motion (1) as it is physically, (2) as it is mapped to the retina, and (3) as it is perceived, but only the direction of the perceptual motion vector has to be determined experimentally. K allows for quantitative comparison between experiments with a variety of parameters in visual motion displays.     

Adaptation to spiral motion in crowding condition

When a single, moving stimulus is presented in the peripheral visual field, its direction of motion can be easily distinguished, but when the same stimulus is flanked by other similar moving stimuli, observers are unable to report its direction of motion. In this condition, known as 'crowding', specific features of visual stimuli do not access conscious perception. The aim of this study was to investigate whether adaptation to spiral motion is preserved in crowding conditions. Logarithmic spirals were used as adapting stimuli. A rotating spiral stimulus (target spiral) was presented, flanked by spirals of the same type, and observers were adapted to its motion. The observers' task was to report the rotational direction of a directionally ambiguous motion (test stimulus) presented afterwards. The directionally ambiguous motion consisted of a pair of spirals flickering in counterphase, which were mirror images of the target spiral. Although observers were not aware of the rotational direction of the target and identified it at chance levels, the direction of rotation reported by the observers during the test phase (motion aftereffect) was contrarotational to the direction of the adapting spiral. Since all contours of the adapting and test stimuli were 90 degrees apart, local motion detectors tuned to the directions of the mirror-image spiral should fail to respond, and therefore not adapt to the adapting spiral. Thus, any motion aftereffect observed should be attributed to adaptation of global motion detectors (ie rotation detectors). Hence, activation of rotation-selective cells is not necessarily correlated with conscious perception.     

Perceptual constancy during ocutar pursuit: A quantitative estimation procedure

Perceptual constancy of uisual motion is usually described as the degree of correspondence between physical and perceived characteristics of motion in the external world. To study it, one has to assess the relationship between physical motion, its retinal image, and its perception. We describe a quantitative estimation procedure for a measure K denoting the degree of perceptual constancy of background target motions noncollinear to the eye movements during ocular pursuit. The calculation of K is based on three vectors describing the target motion (1) as it is physically, (2) as it is mapped to the retina, and (3) as it is perceived, but only the direction of the perceptual motion vector has to be determined experimentally. K allows for quantitative comparison between experiments with a variety of parameters in visual motton displays.     

Proximal velocity change as a determinant of space perception

How and to what degree does proximal velocity change determine perceived translatory motion in depth? This question was studied with a stimulus consisting of a single dot, moving in a straight horizontal path in a frontoparallel plane. Its motion corresponded to distai depth motion with constant speed. Ss reported verbally what they perceived. The results show that proximal velocity changes of this kind are, within certain limits, utilized by the visual system for the perception of translatory motion in depth. The limits were found to be determined by the absolute rate of change in proximal velocity. Further, it was found that the perceived motion track was usually bent, although all stimuli simulated depth motions along straight paths.     

The minimum principle and the perception of absolute,common, and relative motions

Wheel-generated motions have served as a touchstone for discussion of the perception of wholes and parts since the beginning of Gestalt psychology. The reason is that perceived common motions of the whole and the perceived relative motions of the parts are not obviously found in the absolute motion paths of points on a rolling wheel. In general, two types of theories have been proposed as to how common and relative motions are derived from absolute motions: one is that the common motions are extracted from the display first, leaving relative motions as the residual; the other is that relative motions are extracted first leaving common motions as the residual. A minimum principle can be used to defend both positions, but application of the principle seems contingent on the particular class of stimuli chosen. We propose a third view. It seems that there are at least two simultaneous processes—one for common motions and one for relative motions—involved in the perception of these and other stimuli and that a minimum principle is involved in both. However, for stimuli in many domains the minimization of relative motion dominates the perception. In general, we propose that any given stimulus can be organized to minimize the complexity of either its common motions or its relative motions; that which component is minimized depends on which of two processes reaches completion first (that for common or that for relative motions); and that the similarity of any two displays depends on whether common or relative motions are minimized.     

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.     

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.     

Correspondence matching in long-range apparent motion precedes featural analysis

It has been suggested that correspondence matching in long-range motion is mediated by a perceptually high-level, 'intelligent' system. This suggestion is based on findings that long-range motion can be perceived between stimuli that could not be detected by lower-level motion mechanisms acting on Fourier motion energy, and that correspondence matching is affected by featural similarities between motion tokens that would be invisible to low-level (Fourier) motion detectors. Here, the effects of spatial-frequency content, color, and binocular disparity on correspondence matching are investigated. It is shown that the effects of featural matches between motion tokens develop only over time and lag behind the effects of the relative proximity between motion tokens in the retinal projection. This suggests that correspondence matching in long-range apparent motion is mediated by a mechanism which acts initially on the retinal coordinates of the motion tokens only, but may be biased to favor matching tokens that are featurally similar through a slower top-down influence by higher-level processes.     

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.     

The how and why of what went where in apparent motion: modeling solutions to the motion correspondence problem

A model that is capable of maintaining the identities of individuated elements as they move is described. It solves a particular problem of underdetermination, the motion correspondence problem, by simultaneously applying 3 constraints: the nearest neighbor principle, the relative velocity principle, and the element integrity principle. The model generates the same correspondence solutions as does the human visual system for a variety of displays, and many of its properties are consistent with what is known about the physiological mechanisms underlying human motion perception. The model can also be viewed as a proposal of how the identities of attentional tags are maintained by visual cognition, and thus it can be differentiated from a system that serves merely to detect movement.     

Foundations of spatial vision: from retinal images to perceived shapes

Vision is based on spatial correspondences between physically different structures--in environment, retina, brain, and perception. An examination of the correspondence between environmental surfaces and their retinal images showed that this consists of 2-dimensional 2nd-order differential structure (effectively 4th-order) associated with local surface shape, suggesting that this might be a primitive form of spatial information. Next, experiments on hyperacuities for detecting relative motion and binocular disparity among separated image features showed that spatial positions are visually specified by the surrounding optical pattern rather than by retinal coordinates, minimally affected by random image perturbations produced by 3-D object motions. Retinal image space, therefore, involves 4th-order differential structure. This primitive spatial structure constitutes information about local surface shape.     

Development of the perception of invariants: substance and shape

Three experiments investigated the perception of substance and shape as invariant properties of objects by three-month-old infants. In experiment 1, infants were habituated to two differently shaped objects undergoing a rigid motion. After habituation of the infants, the objects were presented undergoing a different rigid motion, or undergoing a deforming motion, or undergoing the same rigid motion. Habituation was maintained to the new rigid motion, indicating that the two rigid motions were perceived as sharing an invariant property. Dishabituation, on the other hand, occurred when a deforming motion followed a rigid one. In experiment 2, infants were habituated to one shape undergoing two different rigid motions. After habituation, the shape was changed but the same two motions continued. Dishabituation occurred, compared to a group with no shape change, indicating that shape is distinguished as an invariant property over two rigid motions. In experiment 3, habituation to a shape undergoing two rigid motions was followed by a new shape presented motionless, or the same shape presented motionless. Cessation of motion did not prevent recognition of shape as invariant. Two properties of an object, substance and shape, thus appear to be detectable as invariant in an event sequence, an instance of "phenomenal doubling" at an early age.     

What is the speed to transparent and kinetic-boundary displays?

To compare transparent motion and kinetic boundaries with unidirectional motion, in many studies the relative motion is generated by superimposing or adjoining unidirectional motions oriented in opposite directions. The presumption, tacitly underlying this comparison, is that the two oppositely directed velocities are independent of one another as far as their speed is concerned, i.e. the speed of the relative motion is presumed to be equivalent to the speed of the unidirectional components. Here we report that the relative motion between dots moving in opposite directions augments perceived speed. A constant-stimuli procedure was used to pair transparent-motion or kinetic-boundary displays with unidirectional motion, and human observers were asked to match the speed of the relative and unidirectional motions. The results show that transparency and kinetic boundaries increase the perceived visual speed by about 50%, compared with the speed of the individual components.     

Dispersion in models of categorical perception

A signal detection theory model of auditory discrimination with a nonlinear mapping from stimulus continuum to perceptual continuum can account for the enhanced discrimination at the category boundary found in categorical perception. Properties of this transformation are specified by a unimodal “dispersion function”. Furthermore, it is shown that a system consisting of two acoustic feature detectors with an associated decision function is also a dispersive system, which models categorical perception of a stimulus continuum as well as boundary shifts under adaptation. The effect of detector adaptation on discrimination is discussed in view of three different types of decision variable and different types of detector noise.