Objects approaching at the same speed, on the same trajectory, but at different distances from an observer, have different angular speeds at the eye. To recognize that the objects' approach speed is the same despite the differences in retinal motion, the observer must "factor out" the distance of each object. We examine whether observers can do so in three relative speed judgement experiments. In the first experiment we use a traditional psychophysical impoverished point-light display. In the second we use an un-typically rich cue-laden display. In the former case, observers are unable to accurately estimate speed, in the latter their performance is much improved. These two experiments, taken together, establish the range of possible performance. We then test performance in a display designed to provide the cues available in a typical natural ball-catching task. We find that observers are unable to make accurate judgements in this case. These results raise the question of how observers catch balls without accurate estimates of approach speed; we conclude with a discussion of potential solutions.

Depth aftereffects produced by prolonged inspection of an object in depth can be mediated by monocular and binocular depth cues. The adapting mechanisms responsible for such effects have not yet been fully determined. Theories of binocular depth aftereffects typically posit a role of an adaptive horizontal disparity sensitive mechanism, implying multiple cue-specific mechanisms for depth aftereffects. Here we examined whether binocular depth aftereffects can be attributed to such a cue-specific mechanism. In Experiment 1 we did so using a technique allowing us to maintain horizontal disparities and vergence constant for our adaptation stimuli, whilst manipulating simulated depth by virtue of a vertical disparity induced-depth effect. We found that depth aftereffects were almost identical to those produced by adaptation to stimuli of equivalent depth produced by conventional horizontal disparity modulations. In Experiment 2, we examined depth aftereffects following adaptation to apparently frontal surfaces produced by different combinations of horizontal and vertical disparity modulations. Aftereffects were close to zero. These results suggest that binocular depth aftereffects are not due to adaptation of a horizontal disparity sensitive mechanism, and we argue that adaptation occurs at the level of a 3D shape sensitive mechanism derived from multiple cues. Experiment 3 was a control to examine whether the two types of adaptation stimuli in Experiment 1 were indeed perceptually the same, since in theory they may differ if vertical disparities influenced metric depth scaling. We found no evidence of this, and concluded that the two classes of stimuli used in Experiment 1, though consisting of very different patterns of disparity, were perceptually equivalent.

Event-Related Potential (ERP) studies using delayed match-to-sample tasks have demonstrated the presence of two components, N270 and N400, possibly reflecting the sequential processing of multiple sources of endogenous mismatch. To date, studies have only investigated secondary sources of mismatch between a single cue and target. In this study, we used distractor stimuli to investigate the effect of a secondary source of mismatch distinct from the task relevant stimulus. Subjects performed such a task in two paradigms in which the cue and target could match or mismatch -producing a source of task-relevant endogenous mismatch. In the second paradigm, task-irrelevant distractors were added -producing an additional, distinct source of irrelevant perceptual mismatch. A mismatch-triggered negativity was elicited in both paradigms, but was delayed and enhanced in magnitude in the distractors present paradigm. It is suggested that the distractors may differentially affect mismatch responses through the generation of an automatic but task-irrelevant mismatch response.

A pair of projectiles travelling on parallel trajectories produce differing patterns of retinal motion when they originate at different distances. For an observer to recognise that the two trajectories are parallel she must "factor out" the effect of distance on retinal motion. The observer faces a similar problem when physically parallel trajectories originate at different lateral positions; here direction must be "factored out". We report the results of a series of experiments designed to determine if observers can do this. The observers' task was to judge whether the direction of travel of an approaching sphere (test trajectory) was to the left or right of parallel to a previously shown trajectory (reference trajectory). In the first set of experiments the reference and test trajectories started from different lateral positions. In the final experiment they started from different distances. From the pattern of judgements we determined a set of perceptually parallel trajectories. Perceptually parallel trajectories deviated significantly from physically parallel. We conclude that under circumstances comparable to those encountered when catching a ball in flight, observers do not have access to accurate estimates of trajectory direction.