Chapter 1. Signal Detection Experiment

Introduction

Cognitive Tool Kit
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Signal Detection Experiment

Signal detection theory describes how we go about determining if something happened in a situation where it is not clear that 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. You have to decide if the phone rang. The two factors involved are how easy it is to hear the phone (your sensitivity), and how much you want to hear the phone (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 will try to manipulate both sensitivity and criterion to illustrate how this theory can be used in the laboratory to examine some cognitive issues involved in our basic perception.

References:

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

Experiment Setup

Figure 1.1

Instructions

Instructions

You will be shown a screen full of squares for a short period. 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

Figure 1.2

Results

Results

Figure 1.3

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.