Psychophysics and
Psychoacoustics
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Yes, there is a difference
• Psychophysics: The scientific study of
the relation between physical events
and the sensations they give rise to.
• Psychoacoustics: The area of
psychophysics that focus on the
perception of sound.
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Psychophysical Laws
Physical quantity
Perception
Intensity
Loudness
Frequency
Pitch
Sensitivity (or detectability) vs. Response Proclivity
The difference between what you hear and what you say you hear.
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Background
• Background
– Absolute threshold or absolute limen
• Smallest stimulus that a person can detect.
– Difference threshold or difference limen
• Smallest difference that a person can detect
between two stimuli
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Classic Measurement Methods
• Method of Limits
• Method of Adjustment
• Method of Constant Stimuli
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Method of Limits
• The experimenter has control of the stimulus
and the subject responds after each trial.
• The point between the yes-no responses is
known as the transition. You need to average
the transitions to determine threshold.
• Example on next slide is for absolute limen
(threshold).
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Method of Limits
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Method of Limits
• Method of Limits for Difference Limen.
• Goal of this method is to …
– match a probe or test stimulus with a
reference stimulus.
– Determine the least noticeable difference
between a probe and a reference.
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Method of Limits
• Need to determine…
– High transition point
– Low transition point
– Point of subjective equality
• See next slide for example
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Method of Limits
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Modified Method of Limits
• Uses bracketing (stairstep) to determine
threshold.
• Can be more efficient.
• Numerous applications.
– e.g., audiometric threshold.
• See next slide for example.
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Modified Method of Limits
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Method of Adjustment
• Similar to method of limits except that
the the stimulus is constantly varied and
the subject controls the stimulus.
• E.g., Bekesy Audiometry
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Method of Constant Stimuli
• Stimulus level is randomized.
• Psychometric function (PF) is generated
based on percent of time the stimulus is
correctly identified.
• Threshold is A predetermined point in
PF.
– usually 50% or 75%.
• Common Application is PI-PB function.
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Method of Constant Stimuli
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A Few Practice Problems
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Problems with classical methods
• Method of limits and adjustment are
susceptible to yes (aggressive) or no
(passive) bias.
• Method of constant stimuli can identify
yes bias by using catch trials, but is still
susceptible to no bias.
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Theory of Signal Detection (TSD)
• Three things can affect our ability to
detect a signal.
– External noise
– Internal noise
– Internal response
• Using TSD we can control for internal
response (bias).
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Theory of Signal Detection (TSD)
• TSD provides a measure of detectability
(d’) that is free from response bias.
• d’ is the measure of strength of signal
relative to noise.
• Response bias (β): individual influence
on response.
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Theory of Signal Detection (TSD)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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Theory of Signal Detection (TSD)
• Examples on next slide…
– High intensity yes bias
– High intensity no bias
– Low intensity yes bias
– Low intensity no bias
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Theory of Signal Detection (TSD)
• Whenever the hits exceed the number
of false alarms (FAs), the listener can
theoretically detect the signal.
• Threshold is the lowest intensity or level
where the hits exceed the FAs.
• When hits are about the same as the
FAs, the person is guessing and can not
detect the signal.
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Theory of Signal Detection (TSD)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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Theory of Signal Detection (TSD)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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Theory of Signal Detection (TSD)
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Theory of Signal Detection (TSD)
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Other Measurement Methods
• Magnitude Estimation
– Subject estimates magnitude of the stimulus
• Ratio Comparison
– Subject perceives a ratio between two stimuli
• Cross-modality matching
– Matches magnitude between two stimuli. e.g.,
visual, auditory, somatosensory, olfactory, or
shock.
• Both magnitude estimation and ratio
comparison have been used to measure the
growth of loudness and pitch.
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Summary
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