Exploring the link between time to collision and representational momentum.

Previous research has shown that when a moving object is occluded prior to collision with another stationary object, observers tend to consistently underestimate the physical/actual time to collision (TTC). We examined whether mislocalization of the initial point of disappearance plays any role in this underestimation. To assess the contribution of mislocalization, we coupled a standard TTC paradigm with a representational-momentum paradigm. Representational momentum refers to the tendency to mislocalize the remembered stopping point of a moving object as being too far forward along an actual or implied path of motion. Using identical displays we found large representational-momentum shifts and consistent underestimation of TTC. When the displays were modified to disambiguate the point of disappearance, representational momentum was absent or significantly reduced and underestimates of TTC were effectively eliminated. These results strongly suggest that the represented point of disappearance is an important factor in TTC estimation and that systematic forward displacement of this point may partially explain frequently observed underestimation.

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.

An implicit measure of undetected change.

Several paradigms (e.g. change blindness, inattentional blindness, transsaccadic integration) indicate that observers are often very poor at reporting changes to their visual environment. Such evidence has been used to suggest that the spatio-temporal coherence needed to represent change can only occur in the presence of focused attention. However, those studies almost always rely on explicit reports. It remains a possibility that the visual system can implicitly detect change, but that in the absence of focused attention, the change does not reach awareness and consequently is not reported. To test this possibility, we used a simple change detection paradigm coupled with a speeded orientation discrimination task. Even when observers reported being unaware of a change in an item's orientation, its final orientation effectively biased their response in the orientation discrimination task. Both in aware and unaware trials, errors were most frequent when the changed item and the probe had incongruent orientations. These results demonstrate that the nature of the change can be represented in the absence of awareness.

The visual perception of human locomotion.

To function adeptly within our environment, we must perceive and interpret the movements of others. What mechanisms underlie our exquisite visual sensitivity to human m ovement? To address this question, a set of psychophysical studies was conducted to ascertain the temporal characteristics of the visual perception of human locomotion. Subjects viewed a computer-generated point-light walker presented within a mask under conditions of apparent motion. The temporal delay between the display frames as well as the motion characteristics of the mask were varied. With sufficiently long trial durations, performance in a direction discrimination task remained fairly constant across inter-stimulus interval (ISI) when the walker was presented within a random motion mask but increased with ISI when the mask motion duplicated the motion of the walker. This pattern of results suggests that both low-level and high-level visual analyses are involved in the visual perception of human locomotion. These findings are discussed in relation to recent neurophysiological data suggesting that the visual perception of human movement may involve a functional linkage between the visual and motor systems.