A Craik-O'Brien-Cornsweet illusion for visual depth.

A stereo analogue of the Comsweet tuminance illusion was discovered. and measured by a null method. Two flat vertical textured surfaces in the frontoparallel plane met at a vertical boundary, at which the left-hand surface curved slightly forward and the right-hand surface curved back by an equal amount. The protruding left edge was jointed to the receding right edge by a step. Result: although the two flat surfaces were equidistant, the left surface appeared to be about half a centimetre nearer to the observer than the right surface. In the Craik-O’Brien-Comsweet illusion, two fields of equal luminance meet at a border whose luminance profile is shaped like a double spur (Fig. la). As a result, the whole of the right field appears brighter than the whole of the left field (Craik, 1966; O’Brien, 1958; Cornsweet, 1970). This illusion is probably caused by lateral inhibition between visual channels sensitive to retinal luminance. The spur-shaped profile can be thought of as a luminance ramp downwards (light to dark), superimposed on a spatial luminance step upwards (dark to light)(Fig. 2). The illusion can be produced by projecting the spur-shaped pattern shown in Fig. 3(a), with the projection lens covered by a strong cylindrical lens which astigmatizes or smears the picture vertically. The astigmatizing lens converts the vertical height of any aperture in the slide into a iong vertical zone of appropriate luminance, because the wider the sfit in a vertical direction, the greater the luminous flux. Thus, the geometrical pattern of the white bar in Fig. 3(a) is converted into a pattern of light density: a contour profile is converted into a luminance profile. In practice. it is convenient to use a sheet of horizontally ribbed or beaded Plexiglas (lenticular screen from Lamac International, 12 West 18th St, New York, NY 10011, price about f3). This material is a Fresnel cylindrical lens, Iike an optician’s “Maddox rod”. The technique is further described by Anstis and Comerford (i975) and Anstis (1976). Figure 3(b) is identical to 3(a), except that the right-hand half of the bar has been tb)