Visual Search for Features and Conjunctions

Overview

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University

How do people find objects in cluttered visual scenes? Think, for example, of looking for keys on a messy desk, finding the ripest-looking fruit at the grocery store, locating your car when you can’t quite remember where you parked it, or finding an old friend at an airport exit gate. Clearly, an understanding of visual perception is going to play a role in any answers, and more specifically, an understanding of visual attention will be crucial.

Visual attention refers to the ability to focus in on just part of an image, mustering one’s processing resources selectively to determine whether the thing being looked for—the target, in the standard experimental jargon—is present. To study search and attention, experimental psychologists have developed a widely used paradigm known (unsurprisingly) as visual search.

Psychologists have also motivated a great deal of research by the intuition that any good theory of search is going to have, to explain why some things are easy to find and others are hard to find. So in the context of the visual search paradigm, perceptual psychologists have often focused on contrasting easy searches with more difficult ones. The most influential contrast is between what researchers call a feature search and a conjunction search.

Procedure

1. Stimulus design

The experiment includes two types of trials. In half of the trials—the feature search trials—participants will be asked to find a red bar among blue ones. So render a set of 40 displays, placing the red bar randomly in each, and randomly placing either 3, 6, 9, or 12 blue bars as well. The number of blue bars is the distractor load. There will be equal numbers of trials (10 in this case) for each load.
The experiment also requires target absent trials, trials that do n

Results

Note that response times in feature search trials are relatively unaffected by distractor load (Figure 3). In contrast, conjunction search response times increase linearly. In fact, the slope of that function describes the amount of extra search time it takes, on average, for each additional distractor in the scene. In this case, it looks like about 50 ms per item. Similarly, both searches take about 200 ms with only three distractors present. This suggests that a uniform amount of time is necessary to g

Application and Summary

In the real world, understanding how visual search works has many important applications. For example, major research programs are currently applying an understanding of visual search in the laboratory to understand and improve how doctors search for certain telltale signatures when they look at an x-ray or MRI scan. Similar research programs look at how TSA personnel search through scans of passenger baggage at the airport, and even how athletes locate their teammates on a field.