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Abstract

We compared the mechanisms responsible for orientation discrimination
of stimuli defined by luminance and red/green isoluminant contrast in
three tasks. A 4-AFC paradigm was used to determine thresholds for
discriminating 1 cpd sinewave gratings differing in orientation,
contrast, or both. When measuring orientation thresholds as a function
of stimulus contrast, we found a decrease in thresholds with
increasing stimulus contrast. For three temporal frequencies (0 Hz, 1
Hz, and 8 Hz) the functions relating orientation thresholds to
stimulus contrast had similar shapes for luminance and isoluminant
gratings, indicating similar processing mechanisms. Thresholds for
stationary and slowly moving gratings were consistently lower for
isoluminant than for luminance gratings when contrast was expressed on
an absolute RMS-cone-contrast scale. When contrast was defined as
multiples of detection thresholds discrimination was slightly better
for luminance gratings. Thresholds for fast moving gratings were
similar irrespective of the definition of contrast. For both luminance
and isoluminant gratings, we found a marked "oblique-effect" when
measuring thresholds as a function of standard orientation. Finally,
we measured discrimination thresholds for gratings that varied in
contrast and orientation simultaneously. The shapes of the resulting
two-dimensional threshold contours were similar for luminance and
isoluminant gratings, indicating again that these stimuli undergo the
same neuronal processing. Performance of the observers could be
described by probability summation of the orientation and contrast
cues, resulting in an elliptical shape of the two-dimensional
threshold contours. In conclusion, our results show similar
performance for luminance and isoluminant gratings in three different
orientation discrimination tasks. The similarity in shape of the
different threshold functions presents strong evidence that a single
common mechanism might underlie orientation discrimination of
luminance and isoluminant stimuli.