Independent processing of visual form and motion

Masking within and across visual dimensions: psychophysical evidence for perceptual segregation of color and motion

Visual masking can result from the interference of perceptual signals. According to the principle of functional specialization, interference should be greatest when signal and mask belong to the same visual attribute (e.g., color or motion) and least when they belong to different ones. We provide evidence to support this view and show that the time course of masking is visual attribute specific. First, we show that a color target is masked most effectively by color (homogeneous target-mask pair) and least effectively by motion (heterogeneous pair) and vice versa for a motion target. Second, we show that the time at which the mask is most effective depends strongly on the target-mask pairing. Heterogeneous masking is strongest when the mask is presented before the target (forward masking) but this is not true of homogeneous masking. This finding supports a delayed cross-feature interaction due to segregated processing sites. Third, lengthening the stimulus onset asynchrony between target and mask leads to a faster improvement in color than in motion detectability, lending support for a faster color processing system and consistent with reports of perceptual asynchrony in vision. In summary, we present three lines of psychophysical evidence, all of which support a segregated neural coding scheme for color and motion in the human brain.

A solution to the tag-assignment problem for neural networks

Purely parallel neural networks can model object recognition in brief displays – the same conditions under which illusory conjunctions (the incorrect combination of features into perceived objects in a stimulus array) have been demonstrated empirically (Treisman 1986; Treisman & Gelade 1980). Correcting errors of illusory conjunction is the “tag-assignment” problem for a purely parallel processor: the problem of assigning a spatial tag to nonspatial features, feature combinations, and objects. This problem must be solved to model human object recognition over a longer time scale. Our model simulates both the parallel processes that may underlie illusory conjunctions and the serial processes that may solve the tag-assignment problem in normal perception. One component of the model extracts pooled features and another provides attentional tags that correct illusory conjunctions. Our approach addresses two questions: (i) How can objects be identified from simultaneously attended features in a parallel, distributed representation? (ii) How can the spatial selectional requirements of such an attentional process be met by a separation of pathways for spatial and nonspatial processing? Our analysis of these questions yields a neurally plausible simulation of tag assignment based on synchronizing feature processing activity in a spatial focus of attention.

The duration of 3-d form analysis in transformational apparent motion

Transformational apparent motion (TAM) occurs when a figure changes discretely from one configuration to another overlapping configuration. Rather than an abrupt shape change, the initial shape is perceived to transform smoothly into the final shape as if animated by a series of intermediate shapes. We find that TAM follows an analysis of form that takes 80-140 msec. Form analysis can function both at and away from equiluminance and can occur over contours defined by uniform regions as well as outlines. Moreover, the forms analyzed can be 3-D, resulting in motion paths that appear to smoothly project out from or into the stimulus plane. The perceived transformation is generally the one that involves the least change in the shape or location of the initial figure in a 3-D sense. We conclude that perception of TAM follows an analysis of 3-D form that takes approximately 100 msec. This stage of form analysis may be common to both TAM and second-order motion.

The perception of biological motion across apertures

To understand the visual analysis of biological motion, subjects viewed dynamic, stick figure renditions of a walker, car, or scissors through apertures. As a result of the aperture problem, the motion of each visible edge was ambiguous. Subjects readily identified the human figure but were unable to identify the car or scissors through invisible apertures. Recognition was orientation specific and robust across a range of stimulus durations, and it benefited from limb orientation cues. The results support the theory that the visual system performs spatially global analyses to interpret biological logical motion displays.

Directional bias in the perception of translating patterns

Recent findings suggest that the visual system is biased by its past stimulation to detect one direction of motion over others. Three experiments were designed to investigate whether this bias is mediated by the direction or by the velocity of the past stimulation, and whether this bias is offset by contradictory pattern or depth information. Observers were presented with two solid or random-dot patterns that moved across a display screen in antiphase. As the two patterns reached the center of the screen, they became superimposed in such a way that their subsequent directions were ambiguous. Results from experiment 1 showed that the probability of perceiving these patterns as continuing to move in the same directions was significantly greater when they moved at a constant velocity than when they moved at a variable velocity. Results from experiments 2 and 3 revealed that this directional bias was reversed only gradually as an increasing amount of contradictory pattern information was introduced, but that this reversal was quite abrupt when a relatively small amount of contradictory depth information was introduced. Collectively, these results suggest that a directional bias in the perception of moving patterns is mediated not only by the direction of the previous stimulation, but also by the velocity of that stimulation. Moreover, the analyses of pattern and motion information appear relatively independent during the early stages of visual processing, but the analyses of depth and motion information appear considerably more interdependent.

Moving contexts do affect the perceived direction of apparent motion in motion competition displays

Three experiments were performed to test Ullman's [The Interpretation of Visual Motion. MIT Press, Cambridge, Mass. (1979)] independence hypothesis, used in the minimal mapping theory of motion correspondence. Subjects were required to detect the direction of motion (left vs right) of an element in a motion competition display. In control conditions, threshold interelement distances were obtained for this task in the absence of any moving context. In experimental conditions, context elements that moved either to the left or to the right were added to the display. This resulted in changes in thresholds (relative to the control condition) that indicated that a context moving in one direction increased the probability of seeing the competition element move in the same direction. The magnitude of the context effect was shown to be related to the proximity of the context to the competition display, as well as to the number of elements in the context. These results are in conflict with Ullman's independence hypothesis. An alternative model of the motion correspondence process, which uses information about interdependencies between element movements, is briefly discussed.