Perception

Because of the aperture problem, local motion measurements must be combined across space. However, not all motions should be combined. Some arise from distinct objects and should be segregated, and some are due to occlusion and should be discounted because they are spurious. Humans have little difficulty ignoring spurious motions at occlusions and correctly integrating object motion, and are evidently making use of form information to do so. There is a large body of theoretical and empirical evidence supporting the importance of form processes involving junctions in the way motion is integrated. To assess the role of more complex form analysis, we manipulated nonlocal form cues that could be varied independently of local junctions. Using variants on diamond and plaid stimuli used in previous studies, we found that manipulations distant from the junctions themselves could cause large changes in motion interpretation. Nonlocal information often overrides the integration decisions that would be expected from local cues. The mechanisms implicated appear to involve surface segmentation, amodal completion, and depth ordering.

We tested the ability of children 3–5 years of age to recognise biological motion displays. Children and adults were presented with moving point-light configurations depicting a walking person, four-legged animals (dogs), and a bird. Participants were able to reliably recognise displays with biological motion, but failed in the identification of a static (four consecutive frames taken from each sequence) version. The results indicate that, irrespective of the highly reduced and unusual structural information available in point-light displays, biological motion is sufficient for reliable recognition of human and non-human forms at an age as early as 3 years. Moreover, 5-year-olds exhibit the ceiling level of recognition. The findings are discussed in the context of the neuropsychological and brain mechanisms involved in biological motion perception.

Three experiments were conducted to explore how translational and radial background motion affected visual localisation. In experiment 1, subjects were asked to indicate the apparent position of a small spot of light flashing against a background of vertical stripes, at a varying point in time before and after rapid translational motion of the background to the left or right. When the spot was flashed before the background motion, subjects mislocalised it toward the central fixation point. An interesting finding was that this mislocalisation occurred in most cases when the background moved in the direction opposite to the visual half-field in which the spot was flashed. That is to say, a spot flashed on the right side of the fixation point was mislocalised when its background moved to the left, and not when it moved to the right; and the converse was also true. In experiment 2, concentric circles were used as the background, and moved in a contracting or expanding direction. The results indicated that mislocalisation toward the central fixation point occurred when a spot was flashed before contracting motion of the background. The same mislocalisation was observed for the spot flashed in the lower visual field, but not when it was flashed in the upper visual field (experiment 3). It is concluded that the mislocalisation is a visual illusion induced by a transient background motion toward the central fixation point.

When viewing a wide-angle visual display, which rotates in the frontoparallel plane around the line of sight, observers experience an illusory shift of the direction of gravity; this shift leads to an apparent tilt of the body and displaces allocentric space coordinates. In this study, subjects adjusted an indicator to the apparent horizontal while viewing a rotating display. To determine whether top–down processes could affect the illusion, the subjects were asked to visualize a rotating configuration of dots onto a blank central portion of the moving visual field. Visualizing dots and actually viewing the dots deflected the spatial judgment in very similar ways. These results demonstrate that top–down processing can affect allocentric space coordinates.

We try to explain perceptual continuation and depth in the visual-phantom illusion in terms of perceptual transparency. Perceptual continuation of inducing gratings across the occluder in stationary phantoms could be explained with unique transparency, a notion proposed by Anderson (1997 Perception 26 419–453). This view is consistent with a number of previous reports including that of McCourt (1994 Vision Research 34 1609–1617) who criticized the stationary phantom illusion from the viewpoint of his counterphase lightness induction or grating induction, which might involve invalid transparency. Here we confirm that the photopic phantom illusion (Kitaoka et al, 1999 Perception 28 825–834) really gives in-phase lightness induction and involves bistable transparency. It is thus suggested that perceptual continuation and depth in the visual-phantom illusion depend on perceptual transparency.

In four experiments, participants made speeded same – different responses to pairs of face photographs showing the same woman or different women with the same expression or different expressions. Compared with responses to positive pairs, negative pairs were matched more slowly on identity than on expression. A secondary finding showed that face expressions (same, different) influenced identity responses, and identities influenced expression responses, equally for positive and negative pairs. The independence of this irrelevant-dimension effect from the contrast effect supports the conclusion required by the main finding—that negation slows perceptual encoding of surface-based information used for identification more than it does encoding of edge-based information used for expression recognition.

Cueing paradigms have become popular in assessing the processes of attention. In two experiments we manipulated (i) the contrast of the target, and (ii) the similarity between the targets discriminated. We used a cue that would isolate the exogenous component of attention. Both a reduction in target contrast and an increase in target similarity raised overall reaction times by a similar amount; however, the target contrast manipulation produced a much greater cueing effect compared with the target similarity manipulation. The results suggest that manipulation of target contrast changes the attention cueing effect at a stage of attracting attention to a location of the target (the ‘move’ stage), rather than at a later processing stage.

What happens to the pictorial content of fixations when we move our eyes? Previous studies demonstrate that observers are very poor at detecting changes in natural scenes that occur across saccades, blinks, and artificial interruptions (‘change blindness’). They suggest that the visual ‘snapshots’ of what is on the retina during a fixation are not retained and fused over successive fixations. I find similar results when volunteers are performing the complex real-life task of making a cup of tea. Volunteers can access the snapshot of the current fixation but not those of previous fixations. I suggest that volunteers are reporting the content of a low-level visual store that holds a veridical snapshot of the current fixation, rather than the retina itself. The snapshots are not ‘wiped’ by the saccade and remain in the buffer until they are overwritten by a new snapshot. The overwrite occurs in an all-or-none manner and can be at any time within the first 400 ms of each new fixation, with 50% of overwrites being within the first 100 ms.

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.