Limitations on the parallel guidance of visual search: color x color and
orientation x orientation conjunctions.
Wolfe JM; Yu KP; Stewart MI; Shorter AD; Friedman-Hill SR; Cave KR.
Journal of Experimental Psychology: Human Perception and
Performance,1990 Nov, 16(4):879-92.
In visual search for a conjunction it is much more difficult to search
for the conjunction of 2 colors or 2 orientations than for Color x
Orientation or Color x Shape conjunctions. The result is not limited to
particular colors or shapes. Two colors cannot occupy the same spatial
location in Color x Color searches. However, Experiments 6 and 7 show that
Color x Shape searches remain efficient even if the color and shape are
spatially separated. Our guided search model suggests that in searches for
Color x Shape, a parallel color module can guide attention toward the
correct color, whereas the shape module guides attention toward the correct
shape. Together these 2 sources of guidance lead attention to the target.
However, if a target is red and green among red-blue and green-blue
distractors, it is not possible to guide search independently toward red
items and green items or away from all blue items.