A functional role for illusory colour spreading in the control of focused visual attention.

In cases of modal completion, illusory colour spreading fills in the surface of a subjectively completed shape. In amodal completion, shapes are likewise completed, but now behind a partial occluder, so that filling in of illusory colour to the completed region no longer arises. We consider the possible functional effects that illusory colour spreading may exert on later stages of vision, and argue that comparisons of modal with amodal completion may be particularly revealing in this regard. It is hypothesised that cueing the inducers of a modally completed object should attract attention to the entire object, including the completed region, owing to the colour spreading there. By contrast, changes to the inducers of a comparable amodally completed object should only attract attention to the inducing regions themselves. This prediction is supported by findings in two experiments with stereoscopic displays, with control conditions ruling out nonattentional accounts, or explanations in terms of stereo disparity alone rather than the presence versus absence of illusory colour. We argue that illusory colours get filled in at quite early stages during modal completion, precisely so that later stages of vision, such as focused attention, can then be driven by the completed regions in the same way as for uniform regions that are physically present in the image.