Academic journal article Perception and Psychophysics

The Role of Memory and Restricted Context in Repeated Visual Search

Academic journal article Perception and Psychophysics

The Role of Memory and Restricted Context in Repeated Visual Search

Article excerpt

Previous studies have shown that the efficiency of visual search does not improve when participants search through the same unchanging display for hundreds of trials (repeated search), even though the participants have a clear memory of the search display. In this article, we ask two important questions. First, why do participants not use memory to help search the repeated display? Second, can context be introduced so that participants are able to guide their attention to the relevant repeated items? Experiments 1-4 show that participants choose not to use a memory strategy because, under these conditions, repeated memory search is actually less efficient than repeated visual search, even though the latter task is in itself relatively inefficient. However, when the visual search task is given context, so that only a subset of the items are ever pertinent, participants can learn to restrict their attention to the relevant stimuli (Experiments 5 and 6).

To interact with the world, we often have to perform visual search tasks on a regular basis. For example, in our daily lives, we may try to find a car in a car park or a face in a crowd. Moreover, we create artificial visual search tasks of great social importance (e.g., finding a tumor in a mammogram or a hidden weapon in an airport baggage scan). In order to improve upon these tasks, we need to understand the mechanisms that occur when we search for an item. For this reason, researchers have investigated the process of visual search in the laboratory. Typically, participants are asked to respond to a prespecified target item among a variable number of competing distractor items. The reaction time (RT) taken to respond to the target item is used as a measurement of search speed. If we plot RT against the number of items in a display (the set size), we can plot the slope of the RT × set size function, which gives us a measure of search efficiency. If attention can be deployed readily to the target item, independently of the number of distractor items, we expect the search slope to be shallow, approaching 0 msec/item. Efficient slopes are characteristic of feature searches, in which a target item can be separated from the distractors by means of a unique and salient feature (e.g., a red circle among green circles or a horizontal line among vertical; see Treisman & Gelade, 1980).

In other visual search tasks, there is a cost to adding more distractor items to the search task, and slopes are significantly greater than zero. Such displays include search for a target that is made up of a conjunction of features (e.g., search for a red circle among green circles and red squares) or search for a target letter among heterogeneous distractor letters. In conjunction search tasks, in which some feature information can guide search (Wolfe, 1994; Wolfe, Cave, & Franzel, 1989), slopes are intermediate in the efficiency they show, around 5-15 msec/item (e.g., Treisman & Sato, 1990; Wolfe, 1998). Tasks such as letter search, lacking guiding features, tend to produce search slopes showing an inefficiency of around 30-50 msec/item (e.g., Kunar & Humphreys, 2006; Theeuwes, Kramer, & Atchley, 1998). These slopes are typical of tasks involving stimuli that are large enough to be identified in peripheral vision. If each item must be fixated, search is much less efficient, because efficiency is limited by the relatively slow rate of saccadic eye movements.

In earlier work, Wolfe, Klempen, and Dahlen (2000) investigated how the efficiency of a heterogeneous letter search task changed over time in two search conditions: a repeated search task and an unrepeated search task (see Figure 1). In this variant of a standard visual search task, a target probe was presented at the beginning of each trial to identify the target letter for each trial. In both conditions, participants had to indicate whether the target probe was present or absent from the search set on each trial. …

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