There are many celebrated examples of ambiguous perceptual configurations such as the Necker cube that abruptly and repeatedly ''switch'' among possible perceptual states. When such ambiguous configurations are presented intermittently, observers tend to see the same perceptual state on successive trials. The outcome of each trial apparently serves to ''prime'' the outcome of the following. We sought to determine how long the influence of a past trial persists by using ambiguous motion quartets as stimuli. We found large, significant effects of all four most recent trials, but the results were not consistent with any priming model. The results could be explained instead as perceptual completion of two kinds of temporal patterns, repeating and alternating. We conclude that the visual system does not passively remember perceptual state: it analyzes recent perceptual history and attempts to predict what will come next. These predictions can alter what is seen.ambiguous figures ͉ apparent motion ͉ hysteresis ͉ priming V isual perception under ordinary circumstances is an ongoing process. Current visual information is integrated with past information as part of a perceptual cycle (1), and it is not surprising that, for example, the outcome of a given trial in a psychophysical experiment is affected by what has occurred in recent trials. The observer's response time on a particular trial, for example, is significantly affected by recent task history (2-8). This trial-to-trial effect of the past on the present is particularly pronounced in the perception of motion quartets, a commonly used apparent motion stimulus. A motion quartet consists of a brief display of two tokens presented at opposite ends of a diameter of an invisible circle followed a short time later by presentation of two other tokens on a possibly different diameter. With proper choice of timing, the observer sees apparent motion carrying one token of each pair to a token of the other (Fig. 1). The direction of perceived motion implies a correspondence between each token in the first pair and one of the tokens in the second. This pairing of tokens represents the visual system's solution to the motion correspondence problem (9 -11).The perceived motion is compelling, but it can also be ambiguous. When the tokens are all identical and the angle between the diameters is Ϸ90°, many observers are as likely to see movement in the clockwise direction as they are in the counterclockwise direction. By varying , the experimenter can vary the probability that the observer will perceive movement in one direction or the other. When is near 180°, motion is almost always seen as counterclockwise, and when it is near 0°, motion is almost always seen as clockwise.The perceived direction of motion is affected by proximity, the similarity between potentially corresponding tokens (9-14), and the direction of motion perceived during recent trials. Ramachandran and Anstis (15) found that the tendency for the observer to perceive the same direction of motion persisted even wit...

For humans to effectively interact with their environment, it is important for the visual system to determine the absolute size and distance of objects. Previous experiments performed in full-cue, real-world environments have demonstrated that blind walking to targets serves as an accurate indication of distance perception, up to about 25 m. In contrast, the same task performed in virtual environments (VEs) using head-mounted displays shows significant underestimation in walking. To date, blind walking is the only visually directed action task that has been used to evaluate distance perception in VEs beyond reaching distances. The possible influence of the response measure itself on absolute distance perception in virtual environments is currently an open question. Blind walking involves locomotion and the egocentric updating of the environment with one's own movement. We compared this measure to blind throwing, a task that involves the initiation of a movement directed by vision, but no further interaction within the environment. Both throwing and walking were compressed in the VE but accurate in the real world. We suggest that distance compression found in VEs may be a result of a general perceptual origin rather than specific to the response measure.

In 4 experiments, the authors varied the extent and nature of participant movement in a virtual environment to examine the influence of action on estimates of geographical slant. Previous studies showed that people consciously overestimate hill slant but can still accurately guide an action toward the hill (D. R. Proffitt, M. Bhalla, R. Gossweiler, & J. Midgett, 1995). Related studies suggest that one's potential to act may influence perception of slant and that distinct representations may independently inform perceptual and motoric responses. The authors found that in all conditions, perceptual judgments were overestimated and motoric adjustments were more accurate. The virtual environment allowed manipulation of the effort required to walk up simulated hills. Walking with the effort appropriate to the visual slant led to increased perceptual overestimation of slant compared with active walking with the effort appropriate to level ground, while visually guided actions remained accurate.