Chapter 1. Signal Detection Experiment

Introduction

Cognitive Tool Kit
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Signal Detection Experiment

In this experiment you are going to be asked if you saw a dot. Unlike most of our experiments, in this one you will be getting points for how well you do. Your task is to get as many points as possible. If you are correct, you will get points, if you make mistakes you might lose points. You will see a table like the one below:

If you are attentive, you should be able to accumulate
approximately 1500 points during this block of trials.
The target is present: The target is absent:
You say the target is present: 50 points -25 points
You say the target is absent: -25 points 50 points

The target is the dot. So if you say the dot is on the screen and it is present, you get 50 points. If it is on the screen and you say it is not, you lose 25 points. Each block of trials could have different scoring rules, sometimes you might not lose as much or not win as much, so pay attention to the table of scores presented before each trial. Use these point values to try to play the game to get as many points as possible.

Experiment Setup

Instructions

Instructions

For a short period of time, you will be shown a screen full of squares. Some screens may include a single circle. You are to determine if the circle was present. Before each block of trials, you will be told how your successes and failures will be scored. Your task is to accumulate as many points as possible. For each situation, a goal number of points you should be able to achieve for each situation will be indicated.

Keyboard Responses

Key What Response Means
z Yes, the circle was shown.
m No, the circle was not shown.

Experiment

Begin Experiment

Results

Results

Debriefing

Debriefing

Signal detection theory describes how we go about determining if something happened in a situation where it is not clear whether something happened. For example, you are downstairs without a phone, watching a movie, and at an intense moment, you thought you heard the phone ring in another room. You have to decide if the phone rang. Signal detection theory separates this decision into two elements: how easy is it for you to detect the stimulus (your sensitivity) and how much you get by detecting the stimulus (your criterion). To illustrate criterion, think of waiting to hear about a possible date: You want to hear the phone. Or think of hoping to avoid a call from your parents: You don’t want to hear the phone. In this experiment, we tried to manipulate both sensitivity and criterion by changing how many points you got for hits and correct rejections and how many points you lost for false alarms and misses.

To change your sensitivity, we controlled the color of the circle target. In one condition, the circle was the same color as the squares. This target is hard to detect and you should have a low sensitivity to this target. In other conditions the circle is also a different color, which should make it easier to detect, that is, increase your sensitivity to the target.

The data was plotted on ROC curves, which show the results for all of one sensitivity. So all of the conditions where the circles were the same color as the squares are on one line, and the conditions where the circles were a different color are on a different line. The different points on the line represent the different ways that point values were assigned and represent different criterion values. The conditions where the circles were a different color from the square should be farther from the diagonal on the ROC figure. If you need to review, go back to the results of the Signal Detection Theory activity, where you can play around with an ROC curve.

References:

Green, D. M., & Swets, J. A. (1966). Signal detection theory and psychophysics. Oxford, England: John Wiley.

Quiz

Quiz

Question 1.1

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1
Correct.
Incorrect.
The independent variable is the value that is changed by the experimenter. In this case, the color of the target was manipulated.

Question 1.2

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1
Correct.
Incorrect.
In this trial, the stimulus has been presented, so this trial must be either a hit or a miss because when the target stimulus is present, these are the only possibilities. You say that the stimulus is present, so the correct answer is hit.

Question 1.3

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1
Correct.
Incorrect.
In this trial, the stimulus has not been presented, so this trial must be either a correct rejection or a false alarm because these are the only possibilities if the target is absent. You say that the stimulus is present, so the correct answer is false alarm.

Question 1.4

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Correct.
Incorrect.
The proportion of hits is determined from the participant’s report that there is a stimulus on trials where there is a stimulus. So this variable is measured from the subject’s performance, which makes it a dependent variable.

Question 1.5

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Correct.
Incorrect.
Your criterion is the correct answer. If there are penalties for false alarms and misses, you should be stricter; if you do not have penalties for false alarms and misses, you should be more lax.