Perceiving persistence: Can multiple objects be tracked predictively?
Objects go in and out of view all the time, either because of eye blinks, saccades, or temporary occlusion. How are such objects continuously tracked and followed? Do our visual systems come equipped with predictive tracking devices? The question was addressed with a multiple object tracking paradigm in which subjects attempted to track four briefly disappearing targets that moved among four distractors. Tracking performance was best when objects reappeared where they disappeared, and worse when reappearing further from the points of disappearance. Test it for yourself!
Here is a movie where all objects reappear where they disappear. Track the blinked objects:
Here is a movie where all objects reappear as if they had moved during the disappearance interval:
For further evidence for the non-predictive nature of parallel tracking, see Keane & Pylyshyn, 2006.
Multiple object tracking and automatic grouping
Multiple object tracking has almost always been used to explore attention but we show that a version of the paradigm–which we call Multiple Vertex Tracking–can also be used to explore contour completion. In this paradigm, a target and distractor object orbit in each quadrant of the screen. In one condition, each target forms illusory contours with two neighboring distractors and tracking becomes harder:
When the above elements are rotated by 48 degrees (relative to the display above), illusory contours are removed, and tracking becomes easier:
Illusory contours are not the only way that targets and distractors automatically group during tracking (Erlikhman et al., 2013). Here is a movie demonstrating the distracting effect of color grouping:
In so-called depth inversion illusions (DIIs), the visual system perceives a hollow object as convex, presumably from having internalized the statistical regularity that most objects in the world protrude outward. Symptomatic people with schizophrenia are less likely to rely on this convexity assumption and therefore experience depth inversion illusions to a lesser extent (Keane et al., 2013). We found that resistance to DIIs was especially pronounced among those with severe psychotic symptoms. An example of the illusory stimulus is provided below, courtesy of my collaborator Thomas Papathomas. One side of the object is an ordinary convex mask and the other concave side produces an illusory appearance of convexity.