Illusions as neuro-signs

Abstract

One of the reasons visual scientists find illusions fascinating is that their pictorial simplicity belies the complexity of visual processing. Illusions can direct us towards neurophysiological interpretations at many levels, which I have called ‘neuro-signs’. The most useful visual illusions are those that point to the simplest neural level possible, that is, those that provide signs of neural processing at early stages of the visual system, such as in the retina itself. The region of the retina that can change the activity of nerve cells throughout the visual system when light falls on it is called the ‘receptive field’ of those nerve cells. Primate retinal ganglion cells have receptive fields that are concentric and antagonistic — if the centre of a receptive field is excited by light the surround is inhibited (called ‘surround inhibition’), or vice versa. Are there any visual signs of such neural antagonism? There could be. In the black and white grid pattern shown below, a variety of illusory dots can be seen at the intersections of the black and white grid. In the upper half, dark grey dots appear at the white intersections between the black rectangles, although a light meter would not register such effects. This is called a Hermann grid; its reverse in the lower half, where illusory light grey dots are seen in the black intersections, is called a Hering grid. In both cases, the dots at an intersection disappear as soon as it is fixated, but they remain at those nearby. The dots are more difficult to see at the smallest intersections in both grids. Because of this effect, it is thought the illusory dots might be related to the distance from the centre of the retina (or ‘retinal eccentricity’); receptive fields have been found to be much smaller around the centre of the retina than more peripherally. Indeed, the optimal separation for producing the illusory dots increases with retinal eccentricity in a similar manner to the increase in receptive field size. But why do the illusory dots appear at all? One suggestion is that concentric receptive fields of the same dimensions will be stimulated differently when they fall at an intersection between four rectangles than when flanked by two rectangles — there would be more surround inhibition at an intersection than elsewhere. This would make a light intersection darker than its neighbours and a dark intersection lighter. Such an interpretation does not account for the dots that are visible between rectangles that are simply outlines, although these dots are not as compelling. When coloured rectangles are placed on a grey background (see above), the illusory dots at the intersections take on the colour of the rectangles: they appear to be light green in the grey intersections of the upper half, and reddish in the grey intersections of the lower half. This is because some receptive fields at the retinal ganglion cell level display colour opponency — the centre can be excited by, say, red and the surround inhibited by green; others operate on blue–yellow opponency. Hermann–Hering grids provide signs of a link between visual perception and neurophysiology. The link is made at an early stage of visual processing — the differences in the activity of the retinal ganglion cells — with the assumption that the neural signals for these differences are preserved in their transmission to the visual cortex of the brain. N Wade, Department of Psychology, University of Dundee, Dundee DD1 4HN, UK. This is the second in a short series of articles by Nicholas Wade, describing some common visual illusions.