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Paul Faure

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This outline summarizes major points covered in lecture. It is not intended to replace your own lecture notes. TSD & Response Bias Summary  Changes in listener sensitivity must be measured under similar conditions of response bias (i.e. under conditions of constant response bias). o Such changes are observed as an increase (decrease) in hits and decrease (increase) in false alarms.  Changes in listener response bias must be measured under conditions of similar listener sensitivity (i.e. under conditions of constant listener sensitivity). o Such changes are observed as an increase (decrease) in hits and an increase (decrease) in false alarms – hits and false alarms go together (change same direction). Receiver Operator Characteristic (ROC)  Changes in sensitivity, when response bias does not change, are measured and observed as an increase (decrease) in hits and decrease (increase) in false alarms.  Changes in response bias, when sensitivity does not change, are measured and observed as increase (decrease) in hits and increase (decrease) in false alarms – hits and false alarms go together.  Area under ROC curve is a measure of response sensitivity unaffected by listener response bias.  Psychometric function derived from values of the area under the ROC curve (integrate area). o Calculate area under ROC curve for each stimulus condition. o As stimulus SPL changes, area under ROC curve will vary between 50%-100%. o Area under ROC curve increases as S’s performance increases. o Area under ROC curve indicates how good S’s are at detecting signal. o Derive psychometric function using the ROC and notice that the mid-point is at 75%. Alternative Sensitivity Measure: % Correct  An alternative measure of a S sensitivity is percent correct trials for each stimulus condition: o P(C) = p(hits) x p(signal) + ([1-p(falsealarms]) x p(nosignal)) o P(C) = p(hits) x p(signal) + [p(correctrejection) x p(nosignal)]  If signal is present in 50% of trials (i.e. 50% catch trials), then the above relationship reduces to: o P(C) = ( p(hits) + (1-p(falsealarms)) ) / 2 o P(C) = ( p(hits) + p(correctrejection) ) / 2 Matching Procedures  Typically, 2 stimuli are presented to a listener.  Listener is asked to adjust physical dimension of Stimulus #1 to match physical dimension of stimulus #2.  E.g. Equal loudness contour: change the loudness of a 500 Hz tone until you perceive it to be equally loud to a 2000 Hz tone.  Having Ss vary and match different physical dimensions between stimuli allows the experimenter to determine which aspects of a stimulus make it appear similar (equal) to another stimulus. Differential Sensitivity  Stimulus thresholds define the ranges of detection for parameters of acoustic signals (e.g. frequency, intensity, phase, etc.)  Thresholds do not capture a Subject’s sensitivity to changes in a stimulus parameter.  Psychophysicists also interested in measuring the smallest difference between physical stimuli that will just barely cause a difference in sensation.  How does perception scale with changes in stimulus modality and magnitude?  2 famous psychologists looked at the question of how differences in stimulus intensity (magnitude) map onto perception. Ernst Heinrich Weber  Devised method for measuring the existence of internal (mental) events; formulated the theory of signs; with his brother he studied acoustics and wave motion.  Discovered psychology’s 1 empirical constant – Weber Fraction. This concept uses the awareness of a difference in sensation to measure a difference in sensation. Psych 3A03 08 November 2012 Week 10 Dr. Paul A. Faure  Weber Fraction: the increase in stimulus intensity necessary to produce an increase in sensation is not a fixed quantity but is a constant proportion of the smaller intensity.  That is, the JND (just noticeable difference) between two stimuli varies proportionally to the smaller of the two stimuli. Gustav Theodor Fechner  Studied anatomy under E.H. Weber  Developed first theory of hypothesis testing; technical foundations for descriptive statistics; ideas about consciousness; first measurement of a features of the mind.  His book Elemente der Psychophysik (1860) describes methods for quantifying the relationships between sensory stimuli and perception.  Father of modern psychophysics  Research on the accuracy of perceptual representations of physical stimuli considered to be the bedrock of modern psychophysics.  Realized that the relationships between stimulus events and mental events might be reducible to simple laws.  Fechner termed the perceptual difference threshold as Just Noticable Difference (JND) threshold.  JND is a psychological entity expressed in units of stimulus magnitude.  JND scale: equal sensation intervals.  Fechner’s Law: magnitude of sensory response (R) increases with logarithm of stimulus intensity (S) above stimulus threshold (S0). R=k log S Weber-Fechner Law Incomplete  To a first approximation it describes how changes in perception are related to changes in stimulus magnitude.  However, the WF Law is incomplete (the so called near misses to Weber’s Law) and is too inflexible across different sensory modalities.  WF Law now superseded by Steven’s Power Law first formulated by the famous psychologist Stanley Smith Stevens.  Stevens’ Power Law: subjective scaling of stimulus magnitude relates to physical intensity of stimulus as an exponential function, not as a logarithmic function. Scaling Procedures  Scaling procedures designed to obtain information about subjective perception of stimulus.  Scaling procedures attempt to quantify magnitude of subjective experience (loudness, pitch, location) to actual physical changes in stimulus (SPL, frequency, phase).  Scaling measures are reliable and valid.  Goal of scaling procedures: obtain scale relating perceived magnitude with physical magnitude.  Various types of scaling procedures.  Magnitude Estimation o Ratio Compression o Cross-Modality Matching o etc Stevens’ Power Law  P = kS where: o P = perceived magnitude of stimulus o S = physical magnitude of stimulus (physical units) o k = constant o n = exponent (varies with sensory modality)  When you take the log of both sides of this equation, you get a straight line with a slope of n.  That the data have a linear fit on a log-log plot means that the magnitude of subjective experience is related to a magnitude of physical stimulus by a power function.  Knowing power function allows experimenter to compute stimulus magnitude for achieving a desired subjective experience (e.g. doubling loudness). Measuring Auditory Sensitivity Psych 3A03 08 N
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