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PSYC212- Lectures 1-14.docx

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McGill University
PSYC 212
Kevin Francis Casey

Classical Psychophysics: How to Do It  Psychophysical methods:  Method of adjustment: participants adjusts the physical intensity of a stimulus themselves until a change is barely detectable  Methods of limits: stimulus intensity is increased or decreased by a fixed amount on successive trials until the participant reports that a change is perceived  Method of constant stimuli: stimulus intensity values are randomly chosen from a preset range and the participant reports whether a change is detectable Classical Psychophysics: Thresholds  Considerations for absolute thresholds:  Humans are not ideal detectors, as there are many different sources of detection variability  The nature of the stimulus together with the context it is presented in  Sensory systems have their own „internal‟ noise because the nervous system has spontaneous activity  Judgments of stimulus perception can be and are influenced by a number of physical, emotional, and cognitive factors  Factors may vary with time and within an individual subject Classical Psychophysics: Absolute Threshold  If humans were ideal detectors a step function would show perception of stimuli. Step functions provide a potential response curve in which stimulus intensities below the absolute threshold are never perceived and stimulus intensities above the absolute threshold are always perceived. However, humans are not ideal detectors!  What actually happens when you put real people into this type of detection experiment is that you don‟t get a step function anymore. You get an Ogive/ S-shaped curve. Ogive functions show stimulus intensities around threshold cause some uncertainty as to whether or not a sensory event occurred  High reliability occurs at very high or low intensities, this means that our threshold for detection lies somewhere between those 2 values
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