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Assignment

Sensation and Perception: Signal Detection

Psychologists have always been interested in the relationshipbetween physical stimuli and the cognitive interpretations of thesensations and perceptions these stimuli produce. This field ofstudy is called psychophysics. One of the major contributions ofpsychophysics is signal detection theory (SDT). The concepts behindSDT had a very practical beginning. They were first developed as away to help the military pick the best sonar and radar operatorsduring World War II. For example, detection of an enemy plane on aradar screen requires picking a target “signal” (the enemy plane)among lots of other extraneous signals (called “noise”) that mayshow up on the screen. Humans vary in their ability to do this, andSDT provides a way to analyze this variability.

From a psychological perspective, SDT is really a way tofactor a human’s decision-making behavior (called “criteria” or“bias”) into a perception activity that involves sensitivity to astimulus. Consider a lab experiment in which a subject wearsheadphones and is asked to indicate whether or not a very weaksound has been presented on a given experimental trial. Typically,there is ambient background noise (called “white noise”) present onevery trial, so the subject must pick out the sound within thecontext of the background noise. The subjects respond “Yes” if thesound is heard or “No” if it isn’t. The sound could either havebeen present on the trial or not. If the sound is present and thesubject decides “Yes”, this is a correct response, called a Hit. Ifthe sound is present and the subject decides “No”, this is anincorrect response, called a Miss. If the sound is absent on thetrial and the subject decides “Yes”, this is what is called a FalseAlarm. If the sound is absent on the trial and the subject decides“No”, this is a Correct Rejection.

We could arrange the possibilities from our example in asimple chart, which is called an SDT table:

Stimulus (Sound) Actually Present?

Subject’s Response

Yes (Signal + Noise)

No (Noise Only)

“Yes, I hear it”

Hit

False Alarm

“No, I don’t hear it”

Miss

Correct Rejection

Now, let’s put some real numbers into our example. Supposethat there are 100 trials in our experiment, with 50 trials,randomly selected, in which the sound is present (called a Signal +Noise trial, because the stimulus sound is presented “on top of”the background noise) and 50 trials on which the sound is notpresent (called a Noise only trial, because the only thing presentis general background white noise). If, on the 50 Signal + Noisetrials, the subject said “Yes” on 40 trials and “No” on 10 trials,then that subject got 80% (40/50) Hits and 20% (10/50) Misses. Notethat the Hits and Misses are complementary, so if we know the Hitpercentage, we can find the Miss percentage by subtracting the Hitpercentage from 100%. Similarly, if on the 50 Noise only trials,the subject said “Yes” to 20 trials and “No” to 30 trials, thenthat subject made 40% (20/50) False Alarms and 60% (30/50) CorrectRejections. Once again, the percentages are complementary.

Now let’s see how we can use these concepts to differentiatethe detection abilities of humans. Assume we run both Subject A andSubject B through our above example of 100 trials, 50 with thesound present and 50 with it absent. Let’s say Subject A correctlydetects the target sound 25 times, and Subject B correct detects it17 times. The question is: “Who is doing better?” You might want tosay Subject A since he got more Hits, but the frequency of FalseAlarms clearly needs to be factored in. If Subject A has 20 FalseAlarms and Subject B has 5 False Alarms, then B is better atdistinguishing the trials in which the sound is present from thetrials in which the sound is absent. Specifically, these resultswould seem to indicate that A is pretty much guessing that thesound is present but is wrong (i.e., exhibits a False Alarm) asoften as right. Subject B is more selective about saying the soundis detected but rarely says the target sound is there when it isnot. Thus, it could be argued that B is in actually doing better atthe task.

This example suggests that we need a measure of performancethat includes both Hit rates and False Alarm rates in order tosuccessfully differentiate among the signal detection abilities ofdifferent people. To this end, researchers have developed a measureof signal detection sensitivity called d’ (pronounced d-prime) thatcan be computed for an individual who has participated in an SDTtask. While the derivation of this measurement is well beyond thescope of PSYC, it is important to understand that the larger thevalue of d’ the better the subject is at distinguishing the targetsignal from the noise.

The computation of d’ is fairly complex, but I have providedan Excel spreadsheet for the calculation given Hit and False Alarmrates. To continue with our example above, Subject A has acalculated d’ = 0.253. Subject B has a calculated d’ = 0.869. (Youcan verify these numbers using the spreadsheet.) This shows thatSubject B, while having a lower Hit rate, is actually the betteroverall performer in our SDT task.

The experiment you are to conduct for Assignment #1 (with youas the subject) measures face recognition abilities of people usingthese same SDT principles. Humans have an uncanny ability torecognize faces of people they have previously seen, and SDT is agood tool to investigate individual differences in thisability.

Procedure

Questions/Tasks

1. Explain in a paragraph or two the concept of signaldetection.

2. What is the independent variable(s) in thisexperiment?

3. What is the dependent variable in this experiment?

4. Define “Hit”, “Miss”, “False Alarm” and “Correct Rejection”in the context of this experiment.

5.Using your scored results, construct an SDT table thatsummarizes your performance. Your table should have the same formatas the table on the top of page 2 in this handout with the onlydifference being that you are to put your performance percentagesin each cell of the table.

6. Using the “d’ Calculator” Excel spreadsheet, find your d’value and record it here. This is a measure of your “sensitivity”(i.e., skill) in the face recognition task. While a higher value isassociated with greater sensitivity and skill, it is more usefulfor comparative purposes, so you might want to ask your fellowstudents in PSYC what their d’ values were – and, of course, bragif yours is higher than theirs!

7. Consider a radiologist who is in charge of reading lungx-rays to detect possible cancerous tumors. Using the Presence ofStimulus as the Presence of Cancer in this case, build an SDT table(like the one above) that shows the four possible outcomes inresponse to reading a patient’s x-ray. Consider the two possibleerrors (False Alarm and Miss) that the radiologist could make. Inthe context of the task, explain these two errors and comment onthe ramifications of each one. Many medical people would argue thatone of the mistakes has more serious consequences than the other.Which one? From an SDT perspective, how might you “coach” or“train” a radiologist in order to minimize the more consequentialmistake?