Visual Search
Searching for a visual target can be easy or difficult depending on a number of factors. Searching for a needle in a haystack is not difficult because the needle is small. If the needle were red or shaped like a ball, it would be easier to find. Searching for the needle is difficult because in order to find it the needle must be distinguished from the hay around it using two features, color and shape, which must be processed at the same time (i.e., in parallel).
Targets and Distractors
In this lab activity, you will be asked to look for a specific target among distractor stimuli. The target may differ from distractors in one or more of its features, and there may be a few or a large number of distractors, making the visual search less or more difficult. By examining differences in response times for target detection across conditions, psychologists have been able to understand better many aspects and limitations of visual information processing.
The Visual Search Paradigm
You will be exploring the visual search paradigm by Treisman and Gelade (1980). On every trial, you will be looking for a green circle. A trial will begin with fixation, and when ready to begin, press the spacebar. Then you will be presented with a series of red and green shapes. If you see a green circle in the display, press the âDâ key. If you do not see a green circle, press the âKâ key. Try to answer as quickly and accurately as possible. There will be 10 practice trials and 80 experimental trials after which you can view your responses.
Feature Search
One type of search that you did on some trials in this activity is called a feature search. This is where all the distractors were red circles, and a green circle was present or absent. In this type of search, the number of items presented does not affect response time. This is because color is processed automatically or preattentively and it is easy to group items based on color without having to direct attention to each item individually. This effect is known as the pop-out effect and is a key characteristic of feature search. In addition to color, luminance, orientation, motion direction, and velocity, all produce a pop-out effect in visual search experiments, suggesting that these visual features are all processed automatically and without attention.
Conjunction Search and Feature Integration Theory
On other trials, you did a very different type of search, called a conjunction search. Response times on conjunction search trials for both target present and target absent conditions increase as the number of items in the display increases. The Feature Integration Theory (FIT) proposed by Treisman and Gelade (1980) explains the main finding by hypothesizing that while individual features such as color and shape are processed in parallel, when those features are combined into a single object such as a target that contains features present in the other distractor items in the array, attention is required. In the conjunction search, where it is necessary to bind multiple features of green and circle to determine if an item is the target, you need to direct attention to one item at a time one by one until the target is found. When more items are in the display, you must pay more attention to more items before discovering the target. Therefore, in conjunctive search, response times increase with the number of items in the array. Response times are also longer on target absent trials because participants must look at all the items in the array to be certain that there is no target present. On target present trials, participants will only search through the array until they find the target, which, on average, will be after looking at about half the items in the array.