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PS262 Chapter Notes -Detection Theory, Spectral Sensitivity

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Phillip Servos

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Appendix Signal Detection Theory
A Signal Detection Experiment
In a signal detection experiment studying the detection of tones, we use only a single low-intensity tone
that is difficult to hear, and we present this tone on some of the trials and present no tone at all on the
rest of the trials
o Thus, a signal detection experiment differs from a classical psychophysical experiment in two
ways: in a signal detection experiment,
1) only one stimulus intensity is presented, and
2) on some of the trials, no stimulus is presented
Experiment, when the tone is presented
o Says “yes” on 90 trials. This correct response – saying “yes” when a stimulus is present – is
called a hit in signal detection terminology
o Says “no” on 10 trials. This incorrect response – saying “no” when a stimulus is present – is
called a miss
When no tone is presented
o Says “yes” on 40 trials. This incorrect response – saying “yes” when there is no stimulus is
called a false alarm
o Says “no” on 60 trials. This correct response – saying “no” when there is no stimulus is called
a correct rejection
Payoffs: a system of rewards and punishments used to influence a participants motivation in a signal
detection experiment
o Add some financial inducements to the experiment
o Tell the participant that we are going to reward them for making correct responses and are going
to penalize her for making incorrect responses by using payoffs
Receiver operating characteristic (ROC) curve: a graph in which the results of a signal detection
experiment are plotted as the proportion of hits versus the proportion of false alarms for a number of
different response criteria
Other than demonstrating that people will change how they respond to an unchanging stimulus, what
does the ROC curve tell us?
o The beauty of signal detection theory is that the person’s sensitivity is indicated by the shape of
the ROC curve, so if the experiments on two people result in identical ROC curves, their
sensitivities must be equal
What does signal detection theory tell us about functions such as the spectral sensitivity curve and the
audibility function, which are usually determined using one of the classical psychophysical methods?
o When the classical methods are used to determine these functions, it is usually assumed that the
persons criterion remains constant throughout the experiment, so that the function measured is
due not to changes in response criterion but to changes in the wavelength or some other physical
property of the stimulus
o This is a good assumption because changing the wavelength of the stimulus probably has little or
no effect on factors such a motivation, which would shift the persons criterion
Signal Detection Theory
Our purpose is to explain the theoretical bases underlying two ideas:
o 1) the percentage of hits and false alarms depends on a persons criterion
o 2) a person’s sensitivity to a stimulus is indicated by the shape of the persons ROC curve
Signal and Noise
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