We measured thresholds and reaction times for detecting a target, defined b
y orientation contrast, as a function of the number of elements displayed s
imultaneously and of the linear orientation gradient present in the display
. This test served to evaluate how well the human visual system is able to
ignore smooth gradients in orientation - similar to what it does with gradi
ents of luminance or wavelength. Smooth orientation gradients are common in
natural environments as opposed to the usual laboratory (search) experimen
ts. It turns out that targets defined by a discontinuity in the transition
between line orientations can be processed in parallel, i.e. that 'search'
times increase by between 0.5 and 6 ms, on average, Fer additional element
displayed, irrespective of the number of elements. But thresholds of orient
ation difference for the detection of the target increase linearly with the
orientation gradient present in the display, and tend to increase more str
ongly for small gradients, indicating a special bonus for (near) collineari
ty. The averaged data follow a Weber-law type while this is not true for th
e individual observers' data. These results show that the visual system is
indeed able to detect targets based on orientation contrast, rather than on
absolute orientation [cf. Nothdurft (1985). Vision Research, 25, 551-560],
but that the orientation gradient cannot be ignored. (C) 2000 Elsevier Sci
ence Ltd. All rights reserved.