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.