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Principles of Perceptual Measurement

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McGill University
PSYC 212
Evan Balaban

Principles of Perceptual Measurement - we can measure the energy from physical stimuli, but we cannot measure someone’s psychological experience of the stimuli Quantitative Relationships - provides an estimate of the perceptual quality of a stimulus in numbers - allows comparisons among individuals and species - allows comparisons across senses Types of Relationships - linear - exponential - logarithmic Absolute Threshold: Minimum intensity level of a stimulus before it is registered in the brain as a perceptual event Subthreshold: Stimuli below absolute threshold that are not detectable by the sensory system Suprathreshold: Stimuli above the absolute threshold where sensation takes place Difference Threshold: The smallest change in stimulus intensity required to produce a discriminable change in sensation Gustav Fechner - 1860, set out to establish experimental methods for psychophysics - psychophysics is the study of quantitative relationships between stimuli and psycholog- ical evens Methods - method of adjustment: participant adjusts intensity of a stimulus until it is barely de- tectable - method of limits: stimulus intensity is changed in the same direction by a fixed amount until the participant reports that it is perceived - method of constant stimuli: stimulus intensity values are randomly chose from a preset range and the participant repots whether it is detectable Thresholds - humans are not ideal detectors due to many sources of detection variability (stimulus is a factor, sensory system adds noise, emotions & cognitive factors influence perception) Absolute Threshold - Absolute threshold graphed function with no noise: looks like a step function, but doesn’t ever occur - Absolute threshold graphed function with noise: ogive function (psychometric function), an S curve, results from the cumulative probabilities at progressively greater values of sensation magnitude How to determine absolute threshold - the level of stimulus intensity that is detected 50% of the time Difference Thresholds - ∆l is the extra bit of physical intensity that is needed to make a just noticeable differ- ence - a is point of perceptual equivalence - b is conventional response level as the measure of JND in the stimulus ∆l = b - a Decrement Threshold - difference threshold for a reduction in sensation uses the 25% brighter response level, point c ∆l = a-c - increment and decrement thresholds are averaged to provide a composite value of the difference threshold Ernst Weber - first person to address how difference thresholds change as a function of the reference intensity of stimuli - ex) with light, shape of ogive function changes with intensity, increment threshold (b-a) increases as intensity increases Weber’s Law: the difference threshold increases in a linear way with stimulus intensity ∆l = k x l Where ∆l = difference threshold, k = Weber’s fraction, l = stimulus intensity - the difference threshold is some proportion of the stimulus intensity - you can calculate the proportion (k) by plotting th
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