Plato's Cave: Evidence for Completion in Depth

Evidence for Boundary and Surface Completion in Depth

If it is accepted that boundary completion and surface brightness filling-in occur by way of spatial diffusion, then the following evidence suggests that these mechanisms must occur not only in the plane of the visual field, but also in depth.

First is the evidence of the Kanizsa figure itself, which is reported by subjects to appear to hover or float some distance above the page.

More direct evidence is provided by the three-dimensional Kanizsa figure shown below, to be viewed binocularly, i.e. the left eye to view the left frame while the right eye views the right frame.

For those of you who cannot "free fuse" such stereo pairs, the sketch to the right illustrates how the percept looks. Equally interesting is the "porthole illusion" shown below, where the left and right eye images have been reversed, resulting in a percept as shown to the right, i.e. a white square viewed through four "portholes", where the inner rim of the portholes are completed perceptually in the same manner as the edges of the square were above.

Another neat example is this one, again a stereo pair, the percept is shown at the right. In this one we see boundary and surface completion around the ring going straight in to the paper.

Further evidence for three-dimensional completion is presented by Rock & Brosgole (1964). ...

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