Illusions as neuro-signs

Abstract

The term ‘visual illusion’ is mainly associated with a set of figures produced in the late nineteenth century, when the likes of Zöllner and Poggendorff described their eponymous phenomena (see Curr Biol 1998, 8:R509). Müller-Lyer described his illusion in 1889 and a variant of it is shown below. The vertical line is bisected by the lower corner of the diamond but the lower half looks considerably longer than the upper half. This simple and compelling illusion of length has been studied experimentally perhaps more than any other but its basis remains elusive. An illusion like the blue and grey one below was illustrated by Wundt in 1898 (although it is called the Titchener illusion). The two grey discs are the same size but the size contrast with the surrounding ones results in their perceptual inequality. The last figure is a variant of the illusion published by Ponzo in 1912. Converging lines provide powerful allusions to depth or distance in pictures and they can have distorting effects on our judgments of size. The outline circles throughout the figure are the same size but the outermost ones appear largest. The circles also appear distorted in shape because of the angles at which they intersect the inclined lines. Visual illusions are generally classified as distortions of orientation or size and, for a long time, the former have been taken to provide the better indications of neural function (in this series, referred to as neuro-signs). But the situation is now changing. In the past decade, functional distinctions between two pathways of neural processing in the visual cortex have been made. These are called the dorsal and ventral streams (see Curr Biol 1998, 8:R489–R491). Perceiving the location of an object was attributed to the dorsal stream, whereas the ventral stream determined its identity — the ‘where’ and ‘what’ dimensions of vision. A recent interpretation of these streams has been added in terms of perception and action: the dorsal stream is said to be concerned with motor control and the ventral with perceptual representation. The difference between the way illusions of size look to us and the way we respond to them is taken to support the latter distinction. Whereas our perception is distorted by these patterns, our motor control is not. If discs were placed on the grey circles in the blue and grey figure, in order to pick them up, observers would adjust their fingers to the appropriate physical width rather than to their perceived inequality. Thus, we might be able to fool the eye, but not the fingers. N Wade, Department of Psychology, University of Dundee, Dundee DD1 4HN, UK.