TRICKS OF THE LIGHT

Another powerful example of the effects of knowledge in perception . Rotating the page through 180 degrees reverses the effect. The blobs that appeared concave now appear convex, and vice versa (Ramachandran, 1995). Notice that the pattern of shading of the blobs is ambiguous. In the upper part of the figure, it could be produced if protruding blobs were illuminated from above, or if receding blobs were illuminated from below (and vice versa, for the lower half of the figure). Yet we tend to perceive them as protruding blobs illuminated from above. This is because our visual system tends to ‘assume’ (on the basis of previous probabilities) that objects in our world are lit from above (as they are in natural surroundings by our single sun), and this assumption governs the perception of ambiguous shading. Presumably, someone who lived on a plane where the only illumination came from luminous sand on the planet’s surface would see the blobs on the upper part of figure 8.16 as receding and the blobs on the lower part as protruding. If the gradient of shading is switched from vertical to horizontal, then all the blobs, whether on the top or bottom, tend to be seen as protruding. This suggests that, once the direction of illumination is clearly not vertical, it tends to be ignored. Instead, another assumption dominates perception, namely that ambiguous blobs protrude (the same assumption about vertices that governs perception of the Necker cube). Although the assumption that objects are lit from above by a single light source is important, it does not always govern our perceptions, even when it is clearly applicable. Gregory (1997) has pointed out that it may be defeated by other knowledge about very familiar objects – in particular, human faces. Gregory drew attention to the fact that the hollow mask of a face does not usually appear hollow. Instead, the receding nose appears to protrude. It is only when the mask is viewed from a short distance that stereoscopic depth information (i.e. information from both eyes) is able to overcome the ‘assumption’ that noses always protrude. What would happen if this assumption about noses were to conflict with the assumption that objects are lit from above? When the rear of a hollow mask is lit from below, the nose appears to protrude and to look as though it is lit from above, in line with both assumptions. But when the lighting is from above, the nose still appears to protrude, even though it also appears to be lit from below. Clearly the assumption that noses protrude is stronger than the assumption that objects are usually lit from above. This is probably because we have no day-to-day experience of non-protruding noses, but we occasionally experience objects lit from below by reflected or artificial light.

[Richard Gregory (1923– ) is a well-known supporter of cognitive constructionist approaches to understanding perception. Originally trained in philosophy as well as psychology, he has summarized and reviewed much experimental evidence (some of which he has provided himself ) for the ‘intelligence’ of the visual system in interpreting its input, and related this ‘top-down’ view of perception to its philosophical context. His books, especially Eye and Brain, have fired generations of students with an enthusiasm for the study of perception]