FOR all the fun we have with them, illusions do serious work in illuminating how our brains work, and in particular how perception works. They may also help us understand how consciousness developed, and tell us about our “neuro-archaeology” and the behaviour patterns laid down in the nervous system over evolutionary time.
But let’s concentrate on perception: it is tricky enough. I’ve tried to classify illusions in a way that shows the principles underlying them, starting with physical causes, moving on to physiological disturbances of neural signals, and finally to cognitive processes – where the brain tries to make sense of sensory signals, not always successfully.
The distinction between physiological and cognitive is not straightforward. It’s rather like the distinction between how a machine works and what it does. For example, a can opener needs two descriptions: the mechanism of levers and cutters, and what this does to open a can.
That distinction between physiological and cognitive has “real-world” consequences. Think of the placebo effect, which suggests close connections between the physiological and the cognitive-psychological. So different types of illusions could be significant in ways we do not yet know. That’s why I have constructed my Peeriodic Table of Illusions (bad pun intended) thus: blindness, the ambiguities, instability, distortion, fiction, and paradox, plus their causes.
We know what we see is very different from the images on our retinas because perceptions are scaled, like maps. So what sets the scaling for seeing the sizes and shapes of surrounding objects? Using ambiguity illusions I found that the scaling in Ponzo and Muller-Lyer illusions can be set from visual cues, such as the convergence of lines by perspective, or from the current perception of distance. The fact that the same retinal image can give more than one perception, as when perceptions “flip”, is useful because it lets us separate “bottom-up” (from the eye) from “top-down” (from the brain) processes. This way we know that a perceptual change without a change in the eye must be top-down, from the brain, and not bottom-up, as there is no change in the image….