PSYC 363 Chapter Notes - Chapter 1: Absolute Threshold, Psychometric Function, Psychophysics

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Evolution and perception:
Natural selection: The basic mechanism of biological evolution, whereby
advantageous traits are more likely to be passed on to offsprings through genetic
inheritance and to become increasingly prevalent in a population.
Psychophysics investigates the relationship between stimuli and experience in
two ways:
- Thresholds of perceptual experience
- Scaling of perceptual experience
Absolute Threshold: The minimum intensity of a physical stimulus that can just
be detected by an observer.
Behavioural methods used in psychophysical experiments; when used to
measure the absolute threshold
- Method of adjustment: The participant observes a stimulus and
manipulates a control that directly adjusts the intensity of the stimulus.
Final estimate of absolute threshold: entire procedure repeated several times,
and the average of the threshold estimates
- Method of constant stimuli: The participant is repeatedly presented with a
fixed set of stimuli, in random order, covering a range of intensities, and
the participant must indicate whether or not each stimulus was detected.
Each intensity of a stimulus are presented many times, and plotted. The
curves are used to estimate the absolute threshold for given stimulus.
Psychometric function: Curve that relates a measure of perceptual
experience to the intensity of a physical stimulus.
- Staircase method: The participant is presented with a stimulus and
indicates whether it was detected, and based on that response, the next
stimulus is either one step up or one step down in intensity.
Difference Threshold (or just noticeable difference): The minimum difference
between two stimuli that allows an observer to perceive that the two stimuli are
different.
Psychological Scaling: Process of measuring how changes in stimulus intensity
relate to changes in the perceived intensity.
- Fechner’s law: Statement of how the perceived intensity of a stimulus
changes as its physical intensity changes.
The validity of Fechner’s law depends on the validity of Weber’s law.
- Weber’s law is approximatively good for perceptual dimensions like
brightness and loudness, but not so good for others, such as electric
shock.
- Stevens developed the method of magnitude estimation, and rejected
Fechner.
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Document Summary

Natural selection: the basic mechanism of biological evolution, whereby advantageous traits are more likely to be passed on to offsprings through genetic inheritance and to become increasingly prevalent in a population. Psychophysics investigates the relationship between stimuli and experience in two ways: Absolute threshold: the minimum intensity of a physical stimulus that can just be detected by an observer. Behavioural methods used in psychophysical experiments; when used to measure the absolute threshold. Method of adjustment: the participant observes a stimulus and manipulates a control that directly adjusts the intensity of the stimulus. Final estimate of absolute threshold: entire procedure repeated several times, and the average of the threshold estimates. Method of constant stimuli: the participant is repeatedly presented with a fixed set of stimuli, in random order, covering a range of intensities, and the participant must indicate whether or not each stimulus was detected. Each intensity of a stimulus are presented many times, and plotted.

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