The Medawar Lecture 2001 knowledge for vision: vision for knowledge

Abstract

An evolutionary development of perception is suggested-from passive reception to active perception to explicit conception-earlier stages being largely retained and incorporated in later species. A key is innate and then individually learned knowledge, giving meaning to sensory signals. Inappropriate or misapplied knowledge produces rich cognitive phenomena of illusions, revealing normally hidden processes of vision, tentatively classified here in a 'periodic table'. Phenomena of physiology are distinguished from phenomena of general rules and specific object knowledge. It is concluded that vision uses implicit knowledge, and provides knowledge for intelligent behaviour and for explicit conceptual understanding including science.

Figure 1

Titchener illusion. The two inner circles are the same size though appear different, by contrast with the larger and the smaller circles. There are related contrast effects, sometimes showing up as illusions, for all modalities and all the senses.

Figure 2

Hollow face illusion. A hollow face mask is seen as a convex face. This is striking evidence of the power of visual knowledge; the more realistic the face, the stronger the effect. There is a general tendency to convexity (as most objects are convex), but this is most dramatic for faces. It is clearly a cognitive phenomenon, the significance of which has only recently been realized.

Figure 3

Impossible triangle model. Although it exists, as a three-dimensional object, it looks impossible. It looks impossible from view points where the sides seem to touch at the corners. This is because of the visual rule that things touching are (probably) the same distance. Here they are not, so here this touching rule misleads. So the perceptual hypothesis is generated from a false assumption, giving a cognitive paradox.

Figure 4

Ins-and-outs of vision. The general notion is that perceptions are hypotheses of what might be out there. We suppose that bottom-up signals, top-down knowledge and sideways rules combine to generate object perceptions as predictive hypotheses. Failures of behaviour can correct knowledge. Prevailing perceptual hypotheses can work downwards, to modify even basic experience, or ‘qualia’ such as brightness or colour. (This is found from illusions of ‘flipping’ ambiguity, where perception changes though the input stimuli are unchanged.) Perceptions are much like hypotheses of science: filling gaps of data; being predictive to unsensed features, and into to the future; selected by probabilities and conferring probabilities (Gregory 1970, ; Hoffman 1998; this is essentially Bayesian).

Figure 5

Neglect. The left half of objects is missing. Half is missing even though the eyes continue to move freely. This must surely be significant for considering how objects are seen.

Figure 6

Ouchi illusion. The two regions are not locked together, but move independently. The inner region moves around independently when the figure is moved, or with movements of the eyes. Why does this not generally occur? For the visual channels have different delays, depending on level of illumination and dark adaptation. Is there a mechanism of ‘border locking’, which fails for this figure, with its regions of orthogonal lines and different spatial frequencies?

Figure 7

Café wall illusion. Found in the tiles of a nineteenth century café wall in Bristol. The parallel ‘mortar’ lines appear to converge alternatively, forming long wedges. For this effect, the mortar lines must be thin (less than 10 s of arc at the eye). There must be luminance—not only colour—contrast of the tiles. The distortion of the ‘mortar lines’ reverses when alternative rows of tiles are shifted sideways by half a cycle.

Figure 8

Gestalt laws of grouping. Dots form patterns from ‘closure’, ‘proximity’, ‘common fate’ (when moving, as leaves of a tree) and so on. These may be innate rules.

Figure 9

Dalmatian dog. The dog is hard to distinguish from the pebbles. All object perception is problem-solving—here, one sees the difficulty.

Figure 10

(a) Necker cube. This flat figure appears to be a three-dimensional cube, although it has no perspective. As there is no evidence for which face is near or far, the brain entertains the alternative possibilities—flipping spontaneously from one to the other. These are alternative perceptual hypotheses, entertained in turn by the brain as it cannot make up its mind. (b) Duck–Rabbit. The best-known ‘ambiguous object’ is seen sometimes as a duck, and at others, as a rabbit. Presumably this ‘flipping’ depends on evidence for a duck and for a rabbit of equal probabilities. As it cannot be two things at once, it flips between the duck and the rabbit possibilities.

Figure 11

Ames Room. An odd-shaped room designed so that from a critical position its image (in a single eye) is the same as a normal rectangular room with each feature being expanded linearly with increased distance. As the eye's image is the same, it must look like a corresponding normal room, until objects are placed in it. Then the brain must decide whether the room is an odd shape or the objects are odd sizes. The room usually wins.

Figure 12

Ink blots. The changing patterns show dynamics of perception. Whether or not these are sufficiently characteristic of the individual to be clinically useful is a controversial topic.

Figure 13

Ponzo illusion. The simplest and clearest perspective distortion illusion. As for all these illusions, features depicted though not necessarily seen as more distant, are expanded.

Figure 14

Müller-Lyer illusion. The double ‘arrow’ with diverging ‘fins’ looks longer than the converging-fins arrow, though they are equal. The theory accepted here is that this is a perspective illusion, the ‘fins’ being perspectives of corners, inside and outside, respectively. Like the Ponzo (figure 13), this represented distance gives expansion: expansion for the inside corner and contraction for the outside corner, according to the perspective depths signalled by the ‘arrows’, accepted as perspective corners.

Figure 15

Judd's illusion. Here, the ‘arrows’ are not opposed, as in the Müller-Lyer (figure 14), but point in the same direction. This gives tilt in depth, and the central dot is displaced by the usual expansion with depicted depth (normally giving size constancy).

Figure 16

Zöllner tilt illusion. The repeated short tilted lines signal depth, as perspective corners, like the risers and treads of a staircase. It is suggested that the represented tilted depth introduces asymmetry, much as for the Judd illusion (figure 15). Again the asymmetry is in the representation, not in the pattern, which is symmetrical, as it repeats. Does this ‘save’ Curie's principle?

Figure 17

Poggendorff displacement illusion. The thin oblique line (which seems to be a single perspective line) is displaced across the horizontal rectangle.

Figure 18

Glass effect. A random dot pattern, superimposed on itself and slightly rotated, appears to have circles. Simple displacement gives lines by visual grouping. This seems to be a low-level grouping phenomenon.

Figure 19

Kanizsa's ghostly triangle. The missing ‘portions of cake’ of the black discs, as they line up, are accepted as evidence for a (non-existent) nearer occluding surface–which is created by the visual brain. Probability favours this, but here the brain has made the wrong bet. This seems to be a quite low-level cognitive creation.

Figure 20

Saunder's parallelogram. The inner oblique lines XA and AY are the same length, though they look different (XA looks much longer). This visual illusion could upset conceptual ‘seeing’ of a geometrical proof.