Inferred retinal mechanisms mediating illusory distortions MARCO J.H. PUTS, JOELPOKORNY, and VIVIANNE C. SMITH Visual Science Laboratories, University of Chicago, Chicago (Received September 7, 2003; Accepted February 20, 2004) Abstract The Zoellner illusion is a geometric distortion occurring when nonorthogonal inducing lines appear to tilt veridically parallel bars. The retinal pathways contributing to such illusions are unknown. The goal of this experiment was to investigate the retinal origin of the illusion. This was accomplished by determining the contrast gain for illusion thresholds. The magnocellular (MC-) and parvocellular (PC-) pathways exhibit different contrast gains, and this difference can be used psychophysically to identify the pathway. The stimulus pattern was four vertical bars with a series of inducing lines. The bars were always 5% higher in contrast than the inducing bars. The pattern was presented on a larger pedestal. Two paradigms were used. In the pulsed-pedestal paradigm, the observer adapted to the background and the pedestal and pattern were presented together as a brief pulse. In the steady-pedestal paradigm, the observer adapted to the continuously presented pedestal and the pattern appeared as a brief pulse. The contrast between the pedestal and the pattern was varied to obtain thresholds for two criteria: perceiving the directions of the inner inducing lines, and perceiving the distortion of the bars. The results for both criteria were similar in shape, but displaced in sensitivity. Detection of the directions of the inner inducing lines was 0.16–0.29 log unit more sensitive than perception of the illusion. The data for the pulsed-pedestal paradigm depended on the contrast between the pedestal and the pattern and produced a shallow V-shape. These results were associated with mediation in the PC-pathway. The data for the steady-pedestal paradigm depended on the pedestal luminance in a linear relation and showed similar sensitivity to the data for the pulsed-pedestal paradigm. Perception of the illusion required 10–15% Weber contrast. Keywords: Illusions, Magnocellular, Parvocellular, Contrast gain, Equiluminance, Isoluminance Introduction Geometrical illusions usually occur in figures when nonorthogonal line crossings bring about distorted percepts. Examples include the Zoellner, Hering, and Wundt illusions (Fisher, 1968; Wade, 1982; Gregory, 1997). The retinal origins of these distortions are un- known but there is a longstanding debate about whether these illusions disappear at equiluminance. Lehman (1904) first reported that visual illusions involving geometrical distortions disappear under equiluminance. Liebmann (1927) observed that, besides the loss of the illusion, everything in the scene became diffuse, without any distinction between figure and ground. Also objects tended to behave in a “ghostly” manner. Parts of the stimulus seemed to disappear, or change in shape. Liebmann argued that the disap- pearance of the illusion under equiluminance was due to changes in scene appearance. Gregory (1977) observed no loss of any of the distortion illusions (including the Zoellner illusion) under equiluminance. However, Gregory noticed what he called the “jazzing” of single lines, which may be similar to the effects described by Liebmann. Li and Guo (1995) also found that the Zoellner illusion did not disappear under equiluminance. Additionally, they used a match- ing task to measure the luminance contrast necessary to perceive distortion in an achromatic pattern and reported a threshold of 15%. Westheimer et al. (1999) found that the tilt illusion and the Poggendorff illusion changed little in magnitude until the inducing contours were barely detectible. Livingstone and Hubel (1987) used observations under equilu- minance to infer whether certain processes were mediated via the parvocellular (PC-) or the magnocellular (MC-) pathway. They found that geometrical distortions and stereo depth disappeared at equiluminance, and decided that both were mediated by the MC- pathway. Geometrical distortions can be seen as the result of monocular depth cues (Livingstone & Hubel, 1987; Gregory, 1997); hence they could be mediated by the same pathway as stereopsis. Reasoning about PC- or MC-pathway mediation of processes on the basis of equiluminance studies is precarious (Mollon, 1982). First, there is abundant physiological evidence that under many conditions the MC-pathway responds to equiluminant ex- change of middle- and long-wavelength lights (Schiller & Colby, 1983; Lee et al., 1989; Kaiser et al., 1990; Dobkins & Albright, 1995). Second, failure to find an effect at equiluminance may reflect intrusion of luminance signals due to chromatic aberration. Address correspondence and reprint requests to: Marco J.H. Puts, Visual Science Laboratories, The University of Chicago, 940 East 57th Street, Chicago, IL 60637, E-mail: mputs@operamail.com Visual Neuroscience (2004), 21, 321–325. Printed in the USA. Copyright © 2004 Cambridge University Press 0952-5238004 $16.00 DOI: 10.10170S0952523804213219 321