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PSYC212_lecture4_2018.pptx

PSYC212_lecture4_2018.pptx - LECTURE 3 PSYCHOPHYSICS AND...

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LECTURE 3 – PSYCHOPHYSICS AND ELEMENTS OF NEUROPHYSIOLOGY (CONTINUED)
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1. Quick recap – Fechner’s law 2. Thresholding techniques 3. Magnitude rating 4. Signal detection theory 5. Elements of neurophysiology LECTURE 3 - PSYCHOPHYSICS AND ELEMENTS OF NEUROPHYSIOLOGY
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SIGNAL DETECTION THEORY # of instances Looks like there is a square Noise Square + noise Principle # 1: - Sometimes, the noise will increase/decrease the subjective perception of the signal (e.g. square). - Sometimes, even if there is just noise, it will look like there is square. Principle # 2: - When reporting on your subjective experiences, you have to set yourself a criterion for saying « yes » vs « no ». SDT: - What is your « sensitivity », regardless of your responce criterion?
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SIGNAL DETECTION THEORY What is the sensitivity of a « biomarker » made out of 11 Antigens (composite score)? What cut-off score should we use to detect ovarian cancer? If it is to low, we will have too many dalse positives…if it is too high we will have too many misses… In the graph, each step represent testing hits and false alarms for a given criterion. This can help selecting the threshold as a function of the level of hits vs false alarms that you want to have (e.g. for a diagnostic tests FA are better than misses). You can also derive a general sensitivity index (d’) that helps compare the marker with other available markers. False Alarms Hits
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SIGNAL DETECTION THEORY Noise Square+noise Looks like there is a square Looks like there is a square Looks like there is a square Looks like there is a square
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SIGNAL DETECTION THEORY Noise Square+noise Looks like there is a square Looks like there is a square Looks like there is a square Sensitivity (d’): how well can you distinguish the stimulus from the noise? Your sensitivity to a stimulus is illustrated by the separation between the distributions of your response to noise alone and to signal plus noise Sensitivity: D’~= Hits – False Alarms
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SIGNAL DETECTION THEORY Noise Square+noise Looks like there is a square Looks like there is a square Looks like there is a square The criterion for saying yes vs. no can change as a function of several motivational factors related to the consequences of making a false alarm or a miss. Bias refects the general tendency to say more/less « yes » than « no ». However, this doesn’t change the sensitivity. 100$ for the 1st person who finds the square! Electric shock if error! Shock + money Sensitivity: D’~= Hits – False Alarms
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SIGNAL DETECTION THEORY HITS FALSEALARMS Example: d’ = 0.75 Receiver operating characteristic (ROC) curves: In studies of signal detection, the graphical plot of the hit rate as a function of the false alarm rate. Chance performance will fall along the diagonal.
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