Illusions as neuro-signs.

Abstract

Visual neuroscience is generally concerned with neural and/or behavioural responses to contours. For the physical stimulus, contours are defined as differences in light intensity, or luminance, like those between black letters on white paper. One set of stimuli, however, involves seeing discontinuities of brightness where no luminance differences exist. These illusory contours can be produced by gaps in figures (like the missing sectors in the blue image above). The white triangular shapes that radiate from the centre appear brighter than the white background, even though all the white areas have the same luminance. The shapes are produced by the white sectors of the blue discs and by the terminations of the concentric circles. The illusory contours even appear to curve between the discs rather than follow the shortest straight line. They can also be induced by line terminations, as in the case of the central vase shape of the image below, where the colour of the lines appears to spread into the area so defined.Figure 3View Large Image Figure ViewerDownload Hi-res image Download (PPT) Illusory contours behave much like physical ones — they interact with one another and can produce tilt after-effects (see Curr Biol 1998, 8:R509-510). When their orientations change systematically it can be sufficient to determine a shape, as in the red image above. The illusory radiating spokes (implied by arcs of circles) are straight on one side but change direction on the other. Because the changes occur at regular points on the illusory line they can define a head in profile (the left-facing head can be seen more easily if the image is viewed out of focus); the red colour appears to extend over the whole head area. It might seem unlikely that such phenomena would direct us towards neural levels of visual processing (what I have called ‘neuro-signs’ in this series) but this could be the case. Some single cells in areas V1 and V2 of the brains of macaque monkeys respond to aligned gaps or aligned discontinuities of line endings. There are probably neuro-signs for illusory contours because many of the objects we encounter in the environment do not have boundaries which are sharply distinguishable from the background. N Wade, Department of Psychology, University of Dundee, Dundee DD1 4HN, UK.