Purpose.
Previous work indicated that search for a simple
conjunctive (color/size)
target was more efficient when the distractors were spread
across three depth
planes (at or in front of the plane of fixation) rather than
when all
distractors were in the same depth plane (Scharff, 1998). The current series of
experiments
investigated whether this conclusion would be supported if a
more complex
conjunctive target was used, one of the depth planes was behind
the plane of
fixation, and if the distractors were given random disparities
(within two
ranges) versus disparities fixed within in one of three planes
of
disparity. Method. Reaction times were measured for
five participants
for an L-conjunction search task (the targets and distractors
were all the same
shape but varied in orientation).
The
number of distractors in the same depth plane (or range) as the
target varied between
5 and 30, with the remainder of the distractors randomly placed
in the other
two depth planes (or ranges).
Depth
conditions were again repeated so that the target could be in
the front, middle
(plane of fixation) or back depth position.
Results. The single depth plane
conditions resulted in
a serial search pattern. When
distractors
were spread across different depths, searches were faster but
there
was a stronger effect of number of distractors when the target
and distractors
were placed in a depth range rather than when placed in one of
the disparity
planes. However,
regardless of depth
configuration, searches including depth were faster than single
depth searches
including the same number of distractors.
Overall, target location (front, middle, back) did not
affect search times
for any of the experiments, and prior knowledge of the target
depth plane did
not systematically affect search times. Conclusions. Serial searches are
more efficient when the
distractors are placed across multiple depth planes. This suggests that
multiple depth planes (or
disparity channels) may be processed simultaneously during
visual search. Because
prior knowledge of target depth had
an inconsistent effect, it is suggested that we are not
efficient at limiting
searches to one specific depth plane.