Visual Search

Background

This applet lab explores aspects of attention in a visual search task. It is a classic experiment that makes strikingly clear the time needed to bring attention to bear on different regions of visual space. The basic idea is to ask a participant to search a visual image for a particular item and to respond as quickly as possible once they find the item, or to respond as quickly as possible when they are certain the item is not in the image.

Searches are divided into two types, those that require selective use of attention and those that do not. In the latter, the target item seems to "pop out" of the display and the participant can respond quickly. Notably, this pop out effect allows the participant to respond quickly even when the number of other (distractor) items is increased. In the other type of displays it seems that the participant is forced to study each item individually until the target item is found. In these cases the target item does not pop out, and search time increases with the number of distractor items.

Controlling whether attention is needed or not is accomplished by the type of target and distractor items. In the experiment below, the target is always a red letter O. For the feature condition the distractors are always letter O's of a different color. As you will see, the red color seems to pop out of the image to quickly identify the location of the target. To require attention, the distractor items are made more complex. Some of the distractors are red letter N's while others are letter O's of a non-red color. Because some of the distractors are red, the red O no longer pops out and the participant must search through all the items to find the one that is both red and a letter O. This type of search is a called conjunctive search because the target is a conjunction of features in the distractors.

After running the experiment below a plot will appear that shows the time required for you to find the target ("Yes" condition) or realize that the target was not there ("No" condition). These times will be plotted against "SET SIZE", which is the number of items on the screen. There will be a total of four curves (yes--feature, no--feature, yes-conjunctive, no--conjunctive) plotted. You should find that for both feature conditions changes in SET SIZE have little effect on search time. For the yes--conjunctive condition, you should find that search time increases with SET SIZE. For the no-conjunctive condition, you should find that search time increases with SET SIZE, and at a faster rate than the yes-conjunctive condition.

Instructions

Start by clicking on the Practice button. A window will appear. After positioning the mouse in the window press the <space> bar. A fixation cross will appear in the middle of the window, stare at it. A short time later (less than a second) O's and N's of various colors will appear on the screen. Your task is to determine if there is a red letter O among the letters. When you see a red letter O, press the z key on your keyboard as quickly as possible. When you are certain there is not a red letter O in the window, press the / key on your keyboard, again as quickly as possible.

After pressing the z or / key, the letters will be replaced with some text describing if you were correct and how long (in milliseconds) it took you to respond.You can then press the <space> bar to start the next trial.

After you finish the set of practice trials, you should close the window (either with the Close menu option or with the method used by your computer system). You can then either run another set of practice trials or click on the FEATURE button to run a set of experimental trials. After running the feature trials, click on the CONJUNCTIVE button to run additional experimental trials. When you finish both the feature and conjunctive trials, a window will appear that plots your data.

In every trial your goal is simply to determine if a red letter O is present. If it is press z, if it is not press /. If you are frequently incorrect, try to delay your response until you are more certain that you are correct.