Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySignal detectionPSY 310Greg FrancisLecture 12Is this a van, or a van and a car?Purdue UniversityExperimental psychology We spent the first part of the course looking at theneurophysiological properties of the visual system Now we turn to a discussion of the perceptualexperience itself What do we actually perceive? How do we describe it? How do we measure perceptual experiences? It’s not as easy as you might think! (But it’s not as bad as Fourier analysis either)Purdue UniversityMeasuring percepts There is a temptation to trust our intuition aboutwhat we see Introspection Philosophy However, what we report is often biased by more than just the “pure”perceptual experiencePurdue UniversityMeasuring percepts We tend to see objects in the world Our interpretation of those objects is based onmemory and learning It all seems effortless because it works correctlymost of the timePurdue UniversityMeasuring percepts But we know that the neurophysiology iscomplicated And with some stimuli the difficulties become clear What is the object here?Purdue UniversityThe perceptual processAttended stimulusEnvironmental stimulusActionRecognitionPerceptionProcessingTransductionStimulus onreceptorsProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityPerceptual process Notice that there is a difference between perception andrecognition You can see the black and white parts of this image But it is difficult to recognizePurdue UniversityPerceptual process With a bit of help it is obvious what the image is And once you know, you cannot see it any other wayPurdue UniversityReporting Percepts are subjective experiences I cannot know what you perceive unless I ask you to dosomething On the basis of your actions I can deduce whatyou perceive Description “I see something” You chose the correct label You push a certain button What kind of questions get the best deductions?Purdue UniversityMeasuring thresholds Suppose you wanted to identify the faintest spot of light thatsomeone can detect Method of constant stimuli Or method of adjustment, method of limits Present lots of spots of various intensities For each one the subject indicates whether he sees it or notPurdue UniversityMeasuring thresholds Do this for many trials, repeating stimuli For a faint stimulus, sometimes the subject willreport seeing it and sometimes notPurdue UniversityMeasuring thresholds Do this for many trials repeating stimuli For a faint stimulus, sometimes the subject willreport seeing it and sometimes not Make a graph0102030405060708090100Low HighPercent “I saw it” responsesSpot intensityProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMeasuring thresholds Identify a threshold by finding the intensity atwhich the subject reports seeing the spot 50% ofthe time0102030405060708090100Low HighPercent “I saw it” responsesSpot intensityPurdue UniversityMeasuring thresholds Identify a threshold by finding the intensity atwhich the subject reports seeing the spot 50% ofthe time0102030405060708090100Low HighPercent “I saw it” responsesSpot intensityPurdue UniversityWhy percentages? Why don’t you always see the same thing? For very faint stimuli there is noise from neural processesImageRetinaGanglion cellsOrientationcellsPurdue UniversityWhy percentages? Why don’t you always see the same thing? For very faint stimuli there is noise from neural processesImageRetinaGanglion cellsOrientationcellsPurdue UniversityWhy percentages? The noise intensity varies from trial to trial Sometimes the stimulus signal is much stronger than the noiseImageRetinaGanglion cellsOrientationcellsPurdue UniversityWhy percentages? The noise intensity varies from trial to trial Sometimes the noise is much stronger than the stimulus signalImageRetinaGanglion cellsOrientationcellsProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityWhy percentages? On some trials the noise will produce a pattern that looks like thestimulus Even though the stimulus was not shownImageRetinaGanglion cellsOrientationcellsPurdue UniversityMeasuring thresholds In experiments of this type, when no stimulus wasshown, subjects sometimes report that it wasshown!0102030405060708090100NoneLowHighPercent “I saw it” responsesSpot intensityPurdue UniversityEnvironmental noise It’s not just neural noise that can make a task difficult Suppose you are waiting for a phone call The phone is in another part of your apartment and you arelistening for its ring You may hear other sounds as well (noise) Washing machine Dishwasher Television Radio Other apartments You have to decide if a sound you heard was your phonewithin the noise or just the noise alone Detection is a discrimination taskPurdue UniversityDiscrimination In a discrimination task there are four kinds ofresponses that can be made: 1) Hit: the stimulus was presented, and youreported it was presented 2) Correct Rejection: the stimulus was notpresented, and you reported it was not presented 3) False alarm: the stimulus was not presented,but you reported it was presented 4) Miss: the stimulus was presented, but youreported it was not presentedPurdue UniversityDiscrimination The situation is complicated further because your reports ofwhether the stimulus was present depends on more thanjust the perceptual experience For example, if it is a very important phone call, you will“respond” to many sounds, even if you think they are unlikelyto be the phone Hits and false alarms will be frequent Misses and correct rejections will be rare If the call is not important and you are busy doing somethingelse, you might decide that what you heard was not thephone Hits and false alarms will be rare Misses and correct rejections will be frequentPurdue UniversityBias Likewise, some subjects may be biased to say “I saw thespot of light.” While other subjects may be biased to say “I didn’t see thespot of light.” These biases change the proportions of hits, correctrejections, false alarms and misses These biases may have almost nothing to do with perception Both subjects might describe what they see in exactly thesame way, but come to
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