Lecture 19 - Signal Detection Theory 11-19

Lecture 19 - Signal Detection Theory 11-19 - Signal...

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Signal Detection Theory "Detection represents a source of uncertainty near the threshold of Perception" Disadvantages Of Classical ψϕ isics Not good enough to evaluate observer's performance and/or behavior – Forced choice : when signal was supposed below threshold it was thought that the Ss responded at random – Failed to take into account false detection
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Questions – Sensitivity of observer (discriminability)? – Criteria of decision? SDT was developed to address these questions
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Basic idea –In a binary identifcation task there are two states oF the world (signal or noise) and two types oF responses ( yes or no ). A Subject can make two types of errors – say Yes when a noise alone is presented – say No when a signal is presented The frequencies of these two types of error will be determined by two factors : – Sensitivity of observer (discriminability) Criteria of decision
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STATE OF THE WORLD SIGNAL NOISE YES NO RESPONSES P(H) + P(M) = 1 HIT FALSE ALARM CORRECT REJECTION MISS ERROR ERROR P(FA) + P(CR) = 1
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Example Two inspectors inspect parts to detect defective pieces 100 parts are inspected 90 are good 10 are bad MISS FA Total errors Inspector 1 5 1 6 Inspector 2 1 8 9 Worst error? Who is the best inspector? What is their discriminability and decision criteria?
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Outline Of The Theory Two phases in Perception : Identifcation - Decision. Decision requires a criterion to attribute received inFormation to either one oF the normal theoretical distributions resulting From random variation oF sensory inFormation corresponding respectively to a noise alone or to a signal superimposed to noise. Decision Limit "Neural evidence"
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Very often signal barely emerges from noise Dim Bright Noise Signal NEURAL ACTIVITY | | | | | |||||||||||||||| | Critical Value (amount of neural evidence Xc that represents the decision limit) no signal signal present NO < X c < YES varies continuously whether or not signal is present because of NOISE External or Internal
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PROBABILITY OF OBSERVING X Xc Noise Signal + Noise NO YES Correct Rejection Hit FA Miss
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Lecture 19 - Signal Detection Theory 11-19 - Signal...

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