Understanding the role of target memory in a search task
Distractors 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 Visually 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 Visually 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 distractors.
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).
eye movements and visual cognition