P335 Cognitive Psychology, Prof. John K. Kruschke

Visual Search

Experiment in the style of A. Treisman (1980s to present).

Search for a particular target item among an array of distractor items.

Stimuli and Procedure:
For this example, the items vary on two attributes: shape (square vs. circle) and color (black vs. white). The participant is told in advance what item she or he is searching for. In this example, the participant is told to look for a white circle. This is a case of conjunction search, because the target is defined by the conjunction of white with circular. Participants are told to respond as quickly as possible by pressing either the target-present key or the target-absent key.

Independent (Manipulated) Variables:
(1) The target could be present or absent. (2) The number of items in the array can be few or many. An example of four possible displays appears below:

Conjunction Search: Look for a White Circle.
Few ItemsMany Items

Dependent (Measured) Variable:
Response time (RT).

Typical results show a linear increase in response time as the number of items increases. Moreover, the slope of the line (i.e., the rate of increase) for target-absent trials in two times the slope of the target-present trials.

These results can be accounted for by a process called serial self-terminating search. A flow chart for this process is shown below:

Flow chart for serial exhaustive search.

The linear increase in RT is explained by the fact that for every item added to the display, the process must go through the loop that checks whether the item is the target. The double slope for target-absent items is explained by the fact that all the items must be checked when there is no target to be found, but if there is a target present, then on average only half the items must be checked until the target if found.

This theory emphasizes processing, and does not make strong commitments about representation. It assumes that items are represented in such a way that they can be separately attended to; i.e., the format is in fact as separate items. But beyond that, the theory doesn't specify whether the items are represented as templates or features or structures or whatever.

This experiment and theory provide a good example of the knowledge organization emphasized in the syllabus. See if you can fill in all the relevant information in the diagram at left.

Copyright © 2000 by John K. Kruschke