Spatial attention and audiovisual interactions in apparent motion

In this study, the authors combined the cross-modal dynamic capture task (involving the horizontal apparent movement of visual and auditory stimuli) with spatial cuing in the vertical dimension to investigate the role of spatial attention in cross-modal interactions during motion perception. Spatial attention was manipulated endogenously, either by means of a blocked design or by predictive peripheral cues, and exogenously by means of nonpredictive peripheral cues. The results of 3 experiments demonstrate a reduction in the magnitude of the cross-modal dynamic capture effect on cued trials compared with uncued trials. The introduction of neutral cues (Experiments 4 and 5) confirmed the existence of both attentional costs and benefits. This attention-related reduction in cross-modal dynamic capture was larger when a peripheral cue was used compared with when attention was oriented in a purely endogenous manner. In sum, the results suggest that spatial attention reduces illusory binding by facilitating the segregation of unimodal signals, thereby modulating audiovisual interactions in information processing. Thus, the effect of spatial attention occurs prior to or at the same time as cross-modal interactions involving motion information.     

Transient attention degrades perceived apparent motion

Transient spatial attention refers to the automatic selection of a location that is driven by the stimulus rather than a voluntary decision. Apparent motion is an illusory motion created by stationary stimuli that are presented successively at different locations. In this study we explored the effects of transient attention on apparent motion. The motion target presentation was preceded by either valid attentional cues that attract attention to the target location in advance (experiments 1-4), neutral cues that do not indicate a location (experiments 1, 3, and 4), or invalid cues that direct attention to a non-target location (experiment 2). Valid attentional cues usually improve performance in various tasks. Here, however, an attentional impairment was found. Observers' ability to discriminate the direction of motion diminished at the cued location. Analogous results were obtained regardless of cue type: singleton cue (experiment 1), central non-informative cue (experiment 2), or abrupt onset cue (experiment 3). Experiment 4 further demonstrated that reversed apparent motion is less likely with attention. This seemingly counterintuitive attentional degradation of perceived apparent motion is consistent with several recent findings, and together they suggest that transient attention facilitates spatial segregation and temporal integration but impairs spatial integration and temporal segregation.     

Object-based apparent motion

The interpretation of a dynamic visual scene requires integrating information within frames (grouping and completion) and across frames (correspondence matching). Fragmentary views of objects were used in five experiments. These views could not be matched with each other by any similarity transformation on the basis of their explicit visual features, but their completed versions were related by a rotational transformation. When the fragmentary images were successively presented to observers, it was found that they produced apparent motion in the picture plane and in depth. Thus, apparent motion is capable of establishing correspondence at the level of perceptually recovered objects in three-dimensional space.     

Visuotactile apparent motion

This article compares the properties of apparent motion between a light and a touch with apparent motion between either two lights or two touches. Visual and tactile stimulators were attached to the tips of the two index fingers that were held apart at different distances. Subjects rated the quality of apparent motion between each stimulus combination for a range of stimulus onset asynchronies (SOAs). Subjects reported perceiving apparent motion between all three stimulus combinations. For light-light visual apparent motion, the preferred SOA and the direction threshold SOAs increased as the distance between the stimuli increased (consistent with Korte's third law of apparent motion). Touch-touch apparent motion also obeyed Korte's third law, but over a smaller range of distances, showing that proprioceptive information concerning the position of the fingers is integrated into the tactile motion system. The threshold and preferred SOAs for visuotactile apparent motion did not vary with distance, suggesting a different mechanism for multimodal apparent motion.     

Visuotactile apparent motion

This article compares the properties of apparent motion between a light and a touch with apparent motion between either two lights or two touches. Visual and tactile stimulators were attached to the tips of the two index fingers that were held apart at different distances. Subjects rated the quality of apparent motion between each stimulus combination for a range of stimulus onset asynchronies (SOAs). Subjects reported perceiving apparent motion between all three stimulus combinations. For light—light visual apparent motion, the preferred SOA and the direction threshold SOAs increased as the distance between the stimuli increased (consistent with Korte’s third law of apparent motion). Touch—touch apparent motion also obeyed Korte’s third law, but over a smaller range of distances, showing that proprioceptive information concerning the position of the fingers is integrated into the tactile motion system. The threshold and preferred SOAs for visuotactile apparent motion did not vary with distance, suggesting a different mechanism for multimodal apparent motion.     

Color correspondence in apparent motion

To maintain figural identity during motion perception, the visual system must match images over space and time. Correct matching requires a metric for identifying "corresponding" images, those representing the same physical object. To test whether matching is based on achromatic (black/white) polarity and chromatic (red/green) color, observers viewed an ambiguous motion display and judged the path of apparent motion. Matching preserved black/white identity regardless of whether frames were viewed binocularly or dichoptically. Red/green identity was also preserved, but coherence of motion depended in part on the number of frames in the motion sequence and on the background luminance. These results suggest that correspondence is computed by a weighted metric containing terms for image features coded early in visual processing.     

Theories of apparent motion

Apparent motion is an illusion in which two sequentially presented and spatially separated stimuli give rise to the experience of one moving stimulus. This phenomenon has been deployed in various philosophical arguments for and against various theories of consciousness, time consciousness and the ontology of time. Nevertheless, philosophers have continued working within a framework that does not reflect the current understanding of apparent motion. The main objectives of this paper are to expose the shortcomings of the explanations provided for apparent motion and to offer an alternative explanation for the phenomenon.     

Perceptual organization in multistable apparent motion

Is motion perception based on a local piecemeal analysis of the image or do 'global' effects also play an important role? Use was made of bistable apparent-motion displays in trying to answer this question. Two spots were flashed simultaneously on diagonally opposite corners of a 1 deg wide square and then switched off and replaced by two spots appearing on the other two corners. One can either see vertical or horizontal oscillation and the display is bistable just as a Necker cube is. If several such bistable figures are randomly scattered on the screen and presented simultaneously, then one usually sees the same motion axis in all of them, suggesting the presence of field-like effects for resolving ambiguity in apparent motion. While viewing a single figure observers experience hysteresis: they tend to adhere to one motion axis or the other and can switch the axis only by looking away and looking back after 10-30 s have elapsed. The figure can be switched off and made to reappear at some other random location on the screen and it is then always found to retain its motion axis. Several such demonstrations are presented to show that spatial induction effects in metastable motion displays may provide a particularly valuable probe for studying 'laws' of perceptual organization.     

A motion aftereffect for long-range troboscopic apparent motion

The existence of a directional motion aftereffect (MAE) for long-range (LR) stroboscopic apparent motion (SAM) was examined with the use of a directionally ambiguous test stimulus. The spatial and temporal parameters were such that the LR, rather than the short-range, mechanism was likely to be implicated. MAEs were found for SAM, which were in the same direction, but somewhat weaker than those for a comparable stimulus in real motion. The MAEs for SAM were present only when good apparent motion was perceived, and could be shown also when only the unstimulated area between the two stroboscopic flashes was tested. The LR mechanism was further implicated, since the MAEs were also obtained under dichoptic adaptation conditions. It is concluded that the LR-motion mechanism does show a usual MAE under proper testing conditions.     

Color logic of apparent motion

Two shapes of either the same or different color will seem to be in smooth apparent motion with like-colored mates, at proper conditions of flash timing and spacing. An experiment is reported in which the condition was tested for unlike-colored pairs, for example red-green alternated with green-red. The question of interest was how the visual system would resolve the disparity of color. An 'intelligent' solution would rotate the shapes in three dimensions. Like-colored and unlike-colored parts were found to move and transform similarly, however, the resolution being dependent more upon timing than upon color. The motion of intelligence as it might be applied to vision is discussed in light of these results.     

Gender differences in apparent motion perception.

Distance disparity is a strong cue to element correspondence in apparent motion. Using a 2-AFC paradigm we have previously shown that shape similarity also plays a role. We now demonstrate a small gender difference in these effects: women are more sensitive to distance disparity, whereas men are more sensitive to differences in shape. Furthermore, in the competing presence of a shape cue, women's sensitivity to distance decreases while men's sensitivity is unaffected. These observations may be related to putative gender differences in the 'form' and 'motion-spatial relations' cortical pathways.     

Objects in the path of apparent motion

Experiments by several investigators suggest that forms between inducing stimuli for apparent motion may deflect motion from a straight path. In an experiment in which a binocularly viewed object was sometimes positioned between the inducing dots, we discovered that the degree of curvature of the path that is most likely to be reported depends upon the interstimulus interval whether or not the space between inducing dots contains an object. The interstimulus interval at which motion is most frequently reported is longer for curved paths of motion than for linear paths even though the spatial separation of inducing stimuli is held constant. The results suggest that apparent motion in depth is not based upon form.     

Primed and unprimed rebounding illusory apparent motion

Although sequences of uncorrelated random dots can yield a wide range of illusorily coherent motion percepts (including translation, rotation, contraction, expansion, shear, and rebounding motion), past priming studies have relied on two-alternative forced choice tasks that only measure unidirectional (positive or negative) priming effects. In Experiment 1 we showed that when participants are primed with unidirectional motion and given an additional option to report bidirectional (rebounding) motion, they do so frequently, suggesting that unidirectional motion can “default” to a rebounding percept. Furthermore, rebounding percepts are more prevalent during trials with long frame durations, suggesting a role for attention in forming and maintaining these illusory percepts. In Experiment 2 we compared rebounding percepts that followed unidirectional, drifting primes with rebounding percepts that followed bidirectional, rebounding primes, and found that these two types of illusory rebounding motion percepts differ systematically in their temporal structures. We argue that rebounding percepts following drifting primes can be understood as a breakdown of positive priming into an underlying oscillatory state, whereas rebounding percepts following rebounding primes may be understood either as (1) the initialization of the same oscillatory process, or (2) the entrainment of a two-step motion pattern by a higher-order mechanism.