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Psychophysics Outline • Classical Psychophysics – definition – psychometric function – sensory scales • zero point = absolute threshold – step function versus ogive – sources of variability • units = difference threshold – jnd – psychophysical laws Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D. Outline • Signal Detection Theory – why an improvement – d’ and B – factor affecting d’ and B – ROC curves • comparing perceivers Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D. Classical Psychophysics • Purpose: To derive the mathematical relationship between the experiential aspect of perception and the physical characteristics of the stimulus • Psychometric Function – formula that relates subjective experience to the physical stimulus – cannot use verbal descriptions Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D. Sensory Scales • Zero point = absolute threshold – theoretically the minimal detectable energy – sensitivity is the inverse of threshold – should get a step function – really get an ogive Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D. Sources Of Variability That Produce The Ogive • Imprecise equipment • Perceiver – learning and adaptation – drugs, fatigue, boredom, hunger – responses are difficult • self-confidence • bias to say yes – transients (noise) in the nervous system Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D. Sensory Scales • Absolute threshold – stimulus that is detected on 50% of the trials • Units = difference threshold – smallest detectable change in perception – the change that is detected on 50% of the trials – also called the “jnd” just noticeable difference Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D. Psychophysical Laws • History – Bernoulli (1738), the psychological worth of money • Weber, Fechner, Stevens – variations on mathematical functions that all describe the same relationship – the amount of stimulus needed to notice a change depends on what you already have – the more stimulus already present the larger the change needed to notice a difference Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D.