Introduction
Click on the white portion of the screen at right. (The display shows a central fixation cross surrounded by four squares, equally spaced diagonally to appear to the top left, top right, bottom right, and bottom left of the fixation point.) Between 400 and 1800 milliseconds after you click, a small red circle (a “test probe”) will appear in one of the four squares surrounding the fixation cross. Your job is to click again anywhere in the white portion of the screen as soon as possible after the probe appears. The computer measures the time you take to respond to the probe (your reaction time, or RT). Once you respond, you can start another trial by clicking again. Try a few trials before moving on. Note that your results are not recorded in the introduction.
This task may seem so simple as to be meaningless. But when a wrinkle or two are added, these sorts of probe detection experiments can provide us with important information about how attention is allocated to different points in space.
The user can click the links to explore several cueing experiments that use the probe detection paradigm. We will describe the activity below as if you are the user completing it. The following sections will describe the important takeaway information from this activity.
Activity Description
In all the tasks in this activity, you begin by clicking on the white portion of the screen and fixating your gaze on the fixation cross in the center of the screen. Between 400–1800 milliseconds (that’s between 0.4 and 1.8 seconds) later, a test probe—a small red circle—will appear somewhere on the screen. A cue may also appear somewhere on the screen after you start the trial but before the probe appears. Do not move your eyes to look at the cue! Keep your eyes fixed on the center of the white portion of the screen until the probe appears.
Your task is simple: as soon as you see the test probe, click the white portion of the screen again (you don’t have to click in one of the boxes; just click anywhere in the white area of the screen, as quickly as possible). You will then see a green check mark if you responded appropriately, or the words “TOO FAST!!!” if you responded before the test probe appeared. Do as many trials as you like and then read about what the results of the tasks tell us about attentional mechanisms.
Probe Detection with Simple Cueing
In this version of the task, a small blue dot will appear in one of the four small squares after you start the trial and before you see the test probe. The blue dot is a cue, and the cue will correctly indicate the location of the test probe on 70% of the trials. On the other 30% of the trials, the cue will indicate a location where the probe does not appear. So on average, your performance will benefit by trusting the cue.
You will find your attention drawn to the cued location, but you should not move your eyes during the trial. Keep your eyes fixated on the cross in the center of the white portion of the screen throughout each trial. As in all the other tasks in this activity, your job is to click again anywhere in the white portion of the screen as soon as you see the test probe (the red circle).
After you’ve responded, your average RTs for validly and invalidly cued trials will be shown; that is, when the cue correctly and incorrectly indicates the probe location. Do you think you’ll be faster to respond when the cue is valid?
About Simple Cueing Results
If you do enough trials of this task (at least 30), you should begin to see that you are indeed faster to respond to validly cued trials than to invalidly cued trials. If we also included a condition in which there was no cue (as in the introduction to this activity), we would find that validly cued trials are responded to faster than trials with no cue, while invalidly cued trials are responded to slower than trials with no cue.
These results, which have been replicated many times in the perception literature, indicate that visual attention is automatically drawn to the location of the cue. If the cue is valid, it allows you to respond to the probe more quickly, because you will be “prepared” to see the probe in this location. If the cue is invalid, however, you will have to shift your attention from the cued location to the probe location before responding, which increases your response time.
Probe Detection with Symbolic Cueing
In this version of the probe detection task, the cue will consist not of a blue dot, but of an arrow in the center of the screen pointing to one of the four possible probe locations. As in the other versions of the task, this cue will be valid on 70% of the trials and invalid on the other 30%. Remember to keep fixating on the center of the screen throughout the trial, then click the white portion of the screen again as soon as you see the test probe (but not before).
Do you think this cue will be as effective as the blue dot in drawing your attention to one location or another?
Probe Detection with Object Cueing
This task will feature the same blue dot cue as in the simple locational cueing task. The difference, as you can see, is that in this task, the cue and probe will appear not in a small square, but in the top or bottom portion of a tall, thin, gray rectangle.
How do you think this will alter the effect of cues?
About Object Cueing Results
In analyzing the results of this task, we are interested in not just whether the cue is valid or invalid, but also in where an invalid cue appears. In half of the invalid trials, the cue appears directly above or below the probe location. This means that even though it is invalid, the cue still falls on the same object (i.e., within the same gray rectangle) as the probe. On the other half of the invalid trials, the cue appears directly to the left or right of the probe location—on a different object.
You may find, as numerous researchers have, that you are faster to respond when the invalid cue is on the same object as the probe than when the invalid cue fell on a different object, even though the cue is equally distant from the probe in both cases.* This phenomenon is known as object-based attention. These kinds of results indicate that attention is not simply a “spotlight” that illuminates whatever falls in its beam. Instead, attention selects particular objects (or perhaps, portions of objects) for further scrutiny by other parts of the visual system.
*Your web browser is not an ideal system for conducting experiments, so you may not be able to replicate the classic results of this type of experiment here. However, the more trials you do, the more likely it is that your results will approach those found by perception researchers.