Inferred retinal mechanisms mediating illusory distortions.

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.