Perceiving the centroid of configurations on a rolling wheel

Naive beliefs in baseball: systematic distortion in perceived time of apex for fly balls

When fielders catch fly balls they use geometric properties to optically maintain control over the ball. The strategy provides ongoing guidance without indicating precise positional information concerning where the ball is located in space. Here, the authors show that observers have striking misconceptions about what the motion of projectiles should look like from various perspectives and that they estimate when the physical apex of a fly ball occurs to be far later than actual, irrespective of baseball experience. Their estimations are consistent with the highest point they are looking at as the ball approaches, not with the physical apex. These findings introduce a new and robust effect in intuitive perception in which people confuse their perceptual perspective with the physical situation that they mentally represent.

Center of mass attracts attention

Using the spatial cueing technique, this study demonstrates that the center of mass (centroid) of a visual scene has a special ability to attract attention even when there is no object presented at this location. Four boxes formed an imaginary square and were presented to the left or right hemifield. After the cueing in one box, a target appeared in one of the four boxes and, in addition, at centroid. Fastest reaction times were observed at centroid, irrespective of whether this centroid was also occupied by a box. Reaction times at the uncued locations varied according to their relative positions to centroid and fixation. No inhibition of return effect was observed when the cue was at centroid.

Attention-based visual routines: sprites

A central role of visual attention is to generate object descriptions that are not available from early vision. Simple examples are counting elements in a display or deciding whether a dot is inside or outside a closed contour (Ullman, Cognition 18 (1984) 97). We are interested in the high-level descriptions of dynamic patterns - the motions that characterize familiar objects undergoing stereotypical action - such as a pencil bouncing on a table top, a butterfly in flight, or a closing door. We examine whether the perception of these action patterns is mediated by attention as a high-level animation or 'sprite'. We have studied the discrimination of displays made up of simple, rigidly linked sets of points in motion: either pairs of points in orbiting motion or 11 points in biological motion mimicking human walking. We find that discrimination of even the simplest dynamic patterns demands attention.

Perception of moving lines: interactions between local perpendicular signals and 2D motion signals

An oblique line translating vertically behind a horizontal rectangular aperture is perceived as moving in the horizontal direction, i.e., in the line-ending's direction. When a feature is added on the line, and thus provides a vertical unambiguous motion signal, the line's perceived direction is still horizontal. In parallel, the feature appears to slide obliquely along the line (Wallach, 1935). We first show that this finding which we call "the sliding effect" is robust and easy to replicate for different orientations of the rectangular aperture (up to about 20 deg from vertical). This effect also occurs with an invisible circular aperture. In this case, using an adjustment task, observers have great difficulty extracting the actual direction of the feature (a gap or a dot on the line). Instead, a systematic bias towards the direction of "sliding" is observed. This misperception of the feature's velocity is markedly reduced or even suppressed when the circular aperture is outlined or when a visible circle is drawn around this invisible aperture. The line's perceived direction is always roughly orthogonal to the line's orientation regardless of the presence of a visible circular outline. This latter result is important because it shows that the feature on the line does not disambiguate the perpendicular signals extracted along the contour, even in conditions where this feature's motion is almost correctly perceived. Altogether, these results suggest that line-endings are not used by the visual system in the same way as a feature on the line when it comes to determining the line's perceived direction.

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.

An invariant for wheel-generated motions and the logic of its determination

Observers appear to perceive the paths of abstract centers of point-light configurations in making judgments about movement. For configurations on rolling wheels a metric was derived that described the relative vertical motion of this point. It was hypothesized that the smaller the metric the more the stimulus should appear to move in a wheel-like manner with linear translation. In two experiments observers viewed pairs of stimuli and were asked to select either the event that appeared most wheel-like or the one that hopped the most. Viewers consistently selected the stimulus with the smaller metric as being more wheel-like, with a frequency that increased with the difference between metrics. The inverse of this pattern was found for those observers requested to select the stimulus that hopped most. In a second set of two experiments observers drew the translational paths of these stimuli. Their drawings corresponded to the motion paths of configural centroids. Together, these results strongly suggest that observers perceive the translational component of the motion of the configurations as the path described by their centroids, or geometric centers. We propose that this description of the stimulus event is determined by the logical ordering of information extraction provided by the perceptural system, and discuss this logic and cases where it seems evident.