In this report we describe the results of an experiment in which we demonstrated that a powerful and compelling stereoscopic experience is elicited with very brief (< 1 msec) stimulus durations. The observers were highly successful in recognizing briefly flashed, stereoscopic, random-dot surfaces in the absence of monocular contours. The results are shown to be closely related to the range of depths for any stimulus form; however, the recognition thresholds were nonmonotonic as a function of disparity. Previous investigators have disagreed about the existence of a temporal threshold for stereopsis. We believe that prior findings suggesting that stereopsis cannot occur at short exposure durations are probably due to inadequate control of fixation disparity. Therefore, there is poor dichoptic image registration when a stereoscopic stimulus is presented. The present results also raise difficulties for any theory of stereopsis that requires eye movements.How long must a stimulus last for stereoscopic perception to occur when there are no monocular cues? There is some dispute over this question in the literature. Lehmkuhle and Fox (1980), who studied the effect of stereoscopic depth on metacontrast masking (produced by random-dot stereograms), demonstrated perceived depths at stimulus durations as low as 50 msec. On the other hand, Richards (1977) reported that depth in briefly presented random-dot stereograms cannot be perceived (unless monocular cues are present) when stimulus durations are less than 200 msec. Richards asserted that eye movements are necessary for many subjects to see random-dot stereograms.In this paper, we demonstrate that brevity of exposure is not an obstacle to high-quality stereoscopic depth perception even when there are no monocular cues, provided that the dichoptic display is properly registered on corresponding retinal points at the moment the dichoptic stimulus is presented. That is, the retinal disparity at the fixation point should be close to zero, and the line of sight should not have to be adjusted to achieve zero fixation disparity. We proved this point by presenting a dichoptic, random-dot stereogram for a total duration of less than 1 msec in an experimental situation in which subjects' eyes were appropriately preconverged. In pilot studies, it was shown that stimuli ofthis kind did not lead to any measurable stereoscopic shape recognition if the eyes were not converged at the necessary small fixation

This study explored the ability of observers to recognize very sparsely sampled, stereoscopic, dotted surfaces. The observers' performance exceeded that of a simple surface-fitting algorithm that served as a first approximation model to the solution ofthe perceptual problem. We explored various attributes ofthe stimulus surfaces, but were unable to find any single attribute that could account for the measured performance. Therefore, we propose that the observers used several global surface attributes jointly to distinguish one form from another. Collectively, these attributes define what is meant by form in this study. We suggest that our results argue for a globally precedent, multidimensional model ofform reconstruction as opposedto a local-feature, unidimensional interpretation.How do observers reconstruct percepts from abstractions or caricatures that only sparsely sample a stimulus object? For many decades it has been quite clear that the human visual system has a powerful ability to fill in, close, and perceptually complete partial stimuli. Examples of completion phenomena are ubiquitous in geometric illusions, in the work of the Gestalt psychologists (as discussed, for example, in the work of Wertheimer, 1923, and Kohler, 1929/1947 and in many reports by contemporary perceptual researchers. Modern artists also take advantage of this ability to perceptually complete by suggesting complex images with simple abstractions of reality rather than by photographic reproduction of the full details of a scene.The ability to reconstruct images is so powerful that it is often difficult to convince an observer that an object that was perceived was not, in fact, present. In one wellknown example, a carefully designed demonstration is required to convince an observer that each retina possesses a 5 0-wide lacuna, Pursuing another proof of our reconstructive abilities, Townsend and Ashby (1982) showed that observers viewing stimuli that were close approxi-