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The Contribution of Provisual Actions to Judgments of form in Depth

An action is ‘provisual’ if it is meant to improve the information that can be obtained through vision. Eye and head movements are the most studied examples, but it is more a general phenomenon, including for example manoeuvring for position. The effect of allowing provisual manipulation was compared with more traditional ways of facilitating perception of form in depth. Stimuli were parallel projections of a square and a rectangle (whose proportions ranged from 1 : 1.1 to 1 : 1.4), both slanted in depth, presented side by side on a computer screen. Nine subjects judged which was which. Information varied along three dimensions: (1) surface type—outline, shaded, and textured; (2) dimensionality—structures were either 2-D, or had a ‘leg’ at each corner (normal to the surface); (3) movement—static, passive (rotary) motion, and provisual (where subjects manoeuvred the objects using a mouse). Surface effects were weak (p=0.047). Dimensionality had a stronger effect (p=0.0003). The effect of passive movement was comparable, but provisual manipulation gave much better discrimination, irrespective of surface or dimensionality, reaching a ceiling with rectangle proportions around 1 : 1.3. The subjects' usual strategy was to manoeuvre structures into a roughly frontoparallel position. The judgment is almost trivial in that position, but finding it is an interesting theoretical problem—particularly given the demonstration that it can be achieved from parallel projections of a few coplanar points. A model is being developed based on neural networks capable of learning the probabilities associated with viewpoints and trajectories.