Department of Psychology, University of Bristol, 12a Priory Road, Clifton, Bristol. Summary Following Hermann magritte : Tentative de l’impossible PDF Helmholtz, who described visual perceptions as unconscious inferences from sensory data and knowledge derived from the past, perceptions are regarded as similar to predictive hypotheses of science, but are psychologically projected into external space and accepted as our most immediate reality.
The large contribution of knowledge from the past for vision raises the issue: how do we recognize the present, without confusion from the past. This danger is generally avoided as the present is signalled by real-time sensory inputs – perhaps flagged by qualia of consciousness. Intelligence and Knowledge Philosophy and science have traditionally separated intelligence from perception, vision being seen as a passive window on the world and intelligence as active problem-solving. It is a quite recent idea that perception, especially vision, requires intelligent problem-solving based on knowledge. There is something of a paradox confounding intelligence and knowledge, for one thinks of knowledgeable people as being specially intelligent and yet more knowledge can reduce the intelligence needed for solving problems.
So illusions are important for investigating cognitive processes of vision. Acceptance that knowledge makes a major contribution to human vision is recent, remaining controversial. This applies even more to the machine vision of artificial intelligence. For von Helmholtz, human perception is but indirectly related to objects, being inferred from fragmentary and often hardly relevant data signalled by the eyes, so requiring inferences from knowledge of the world to make sense of the sensory signals. It is a key point that vision is not only indirectly related to objects, but also to stimuli. It is perhaps better named the law of specific qualities: any afferent nerve signals the same quality or sensation whatever stimulates it. Thus we see colours not only from light but also when the eyes are mechanically pressed, or stimulated electrically.
An essential problem for vision is perceiving scenes and objects in a three-dimensional external world, which is very different from the flat ghostly images in eves. A striking example is illustrated in the following section. This is even more striking with the actual rotating mask. This does not, however, show that knowledge has no part to play in vision. Rather, it shows that conceptual and perceptual knowledge are largely separate. Young was a pioneer who stressed the importance of handling knowledge for understanding brain function, and that there may be a ‘brain language’ preceding spoken or written language. If the essential feature of the brain is that it contains information then the task is to learn to translate the language that it uses.
But of course this is not the method that is generally used in the attempt to understand the brain. Classifying must he important for learning and perception, for it is impossible to make inductive generalizations without at least implicit classes. It is extraordinarily hard to give a satisfactory definition of an ‘illusion’. As science’s accounts of reality get ever more different from appearances, to say that this separation is ‘illusion’ would have the absurd consequence of implying that almost all perceptions are illusory. There are two clearly very different kinds of illusions: those with a physical cause and cognitive illusions due to misapplication of knowledge. Illusions due to the disturbance of light, between objects and the eyes, are different from illusions due to the disturbance of sensory signals of eye, though both might be classified as ‘physical’.
Extremely different from both of these are cognitive illusions, due to misapplied knowledge employed by the brain to interpret or read sensory signals. For cognitive illusions, it is useful to distinguish specific knowledge of objects, from general knowledge embodied as rules. Ins-And-Outs’ To the usual terms ‘bottom-up’ signals and ‘top-down’ knowledge, we add what might be called ‘sideways’ rules. As usual, signals from the eyes and the other senses are ‘bottom-up’. Conceptual and perceptual object knowledge are shown in separate ‘top-down’ boxes. Knowledge as embodied in the general rules.
Perceptual learning seems to work largely by feedback from behaviour. Classifying Illusions Appearances of illusions fall into classes which may be named quite naturally from errors of language: ambiguities, distortions, paradoxes, fictions. It may be suggestive that these apply both to vision and to language, because language possibly grew from prehuman perceptual classifications. To classify causes we need to explain the phenomena. There is no established explanation for many illusions, but even a tentative classification may suggest where to look for answers amid may suggest new experiments.