Research
Philosophy
Each time we engage in a moderately complex task,
we likely enlist the help of an untold number of simpler visuo-motor
operations that exist largely outside of our conscious awareness.
Consider for instance the steps involved in preparing a cup of
coffee. For the sake of simplicity, assume that the coffee has
already been brewed and is waiting in the pot, and that all of the
essential accessories, an empty cup, a spoon, a carton of cream, and
a tin of sugar, are sitting on a countertop in front of you. What is
your first step toward accomplishing this goal? The very first thing
that you might do is to move your eyes to the handle of the coffee
pot, followed shortly thereafter by the much slower movement of your
preferred hand to the same target. Because the coffee pot is hot and
the handle is relatively small, this change in fixation is needed to
guide your hand to a safe and useful place in which to grasp the
object. After lifting the pot, your eye may then dart over to the
cup. This action is needed, not only to again guide the pot to a
very specific point in space directly over the cup, but also to
provide feedback to the pouring operation so as to avoid a spill.
After sitting the pot back on the counter (an act that may or may
not require another eye movement), your gaze will likely shift to
the spoon. Lagging shortly behind this behavior may be simultaneous
movements of your hands, with your dominant hand moving toward the
sugar tin and your non-preferred hand moving to the spoon. The spoon
is a relatively small and slender object that again requires
assistance from foveal vision for grasping; the tin is a rather
bulky and indelicate object that does not require precise Visual
information to inform the grasping operation. Once the spoon is in
hand and the lid to the tin is lifted, gaze can then be directed to
the tin in order to help scoop out the correct measure of sugar. To
ensure that the spoon is kept level, a tracking operation may be
used to keep your gaze on the loaded spoon as it moves slowly to the
cup. After receiving the sugar, and following a few quick turns of
the spoon, your coffee would finally be ready to drink (see Land et
al., 1998, for a similarly framed example).
Projects
Understanding the role of target memory in a search task
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Distracters are not the only items requiring memory
in a search task; observers must also keep the target pattern in
memory so that they know for what it is they are searching. The
effects of target memory on search remain largely unaddressed in the
literature, probably because most search studies have used very
simple stimuli (letters or oriented color bars) that can be quite
easily assigned a semantic label in long-term memory. If you are
searching for the letter "M", you need not hold in your working
memory all the Visual
information needed to relate each line segment
to all the others. Because this information already resides in your
long-term memory, all you really need to remember about the search
target is its semantic label. It is this potential for semantic
coding, and the minimal load that such coding would be expected to
place on working memory, that has led to the relative neglect of
target memory in the search literature. However, this rationale for
dismissing the influence of target memory on search becomes strained
when applied to more complex objects and less familiar patterns.
Real-world objects are Visual
ly very complex and cannot always be
adequately described by a single semantic label. For example,
imagine searching for a wrench. Given that wrenches can come in many
different sizes, shapes, and colors, the semantic label of "wrench"
will need to be supplemented with more specific Visual
details if it
is to yield an efficient search. These Visual
details, however,
might number in the dozens or hundreds of features; meaning that a
Visual
target definition might quickly fade from working memory and
therefore become unusable to the search process.
A study headed by Xin Chen has begun to tease apart the use of these
semantic and Visual
target definitions in a search task by
exploiting the differential effects that each should have on memory.
Our search stimuli are Chinese characters. To non-Chinese readers,
Chinese characters are fairly meaningless and unfamiliar Visual
patterns. Of course to Chinese readers these same characters are
meaningful symbols. Our search display consisted of a single Chinese
character presented to the center of the screen (the target),
surrounded by a ring of nine other Chinese characters. The task was
to indicate the presence or absence of the target among the
characters in this ring. To quantify memory in this task we
monitored the oculomotor behavior of observers and counted the
number of times that they chose to look back to the central target
during a search trial. Our expectation was that Chinese observers,
because they could define the target semantically, would not need to
refixate the central target because its semantic label would not
fade from working memory. However, because non-Chinese observers
would be limited to only a Visual
target definition, they might need
to periodically refixate the central target in order to refresh
their continuously fading memory for the Visual
pattern. Our data
confirmed these predictions (Chen & Zelinsky, 2003). Whereas Chinese
observers rarely shifted gaze back to the central target,
non-Chinese observers refixated the target after every three
fixations on distractors in the search ring. We can therefore
conclude that the search for Visual
ly complex and unfamiliar targets
is indeed affected by target memory, and that this effect can be
dramatic; with targets in this case fading from memory after
fixations on only three intervening distracters.