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Chapter Summary
Measuring Responses to Low-Intensity Stimuli
1. Psychophysics is the study of the relationship between physical stimuli and the psychological
reactions to them.
2. The classical psychophysics methods measure absolute thresholds, the smallest amount of
energy required for the stimulus to be reported 50% of the time. The transition between
nondetection and detection is gradual rather than abrupt.
3. The method of limits involves presenting systematically increasing or decreasing amounts of a
stimulus. It provides a fairly reliable threshold without too much time investment.
4. The method of limits can use both ascending and descending series to correct for errors of
habituation and errors of anticipation. The series do not begin with the same starting point
every time; this precaution guards against observers’ merely counting trials before reporting
detection.
5. The method of adjustment involves the observer’s adjusting the intensity of the stimulus until
it is barely detectable. It provides a threshold very rapidly, but errors are more likely than
with the other two methods.
6. The method of constant stimuli involves presenting near-threshold stimuli in random order. It
provides a highly accurate threshold, but it is time consuming and requires that the researcher
already have a good idea of where the threshold lies.
7. Signal detection theory disputes the very notion of a threshold, because classical
psychophysical techniques hopelessly entangle two factors—sensitivity and criterion.
Sensitivity depends on stimulus intensity and the sensitivity of the observer. The criterion,
which measures willingness to report the stimulus, depends on factors such as probability of
stimulus occurrence and payoff (rewards and punishments).
8. The outcome of a signal detection trial can be a hit, a correct rejection, a false alarm, or a
miss. The probability of each of these four outcomes depends on the sensitivity measure, d′,
and the observer’s criterion.
9. Signal detection theory can also be represented in terms of probability distributions for trials in
which signal + noise occurred and for trials in which only noise occurred.
10. An observer who is very sensitive would produce a large d′, but that person’s criterion also
influences responses. The criterion is determined by several factors, including payoffs for
particular responses.
11. ROC curves can be used to plot the proportion of hits and false alarms. Each separate ROC
curve represents a different d′.
12. Signal detection theory has been used in research on eyewitness identification and lie
detection. Research suggests that eyewitness identification is not particularly accurate, but it
is worse under some condition (e.g., identifying suspects whose race differs from that of the
eyewitness). Due to experience, some people are better at detecting lies. However, those
people may also adopt more liberal criteria.
13. The two-alternative forced choice procedure (2AFC) is often used because it minimizes the
influence of the observer’s criterion.
Measuring Responses to More Intense Stimuli
1. Discrimination studies examine how much to change a stimulus in order for it to be perceived
as just noticeably different.
2. Discrimination studies ask people to compare a standard stimulus with a comparison stimulus.
These studies typically calculate the just noticeable difference.
3. In the method of limits for measuring discrimination, the comparison stimulus is
systematically increased or decreased.
4. In the method of adjustment for measuring discrimination, the observer adjusts the
comparison stimulus.
5. In the method of constant stimuli for measuring discrimination, comparison stimuli are
presented in random order.
6. Three researchers—Weber, Fechner, and Stevens—have been primarily responsible for
formulating equations to describe the relationship between physical stimuli and psychological
reactions.
7. Ernst Weber found that observers require larger changes in the stimulus to notice a difference
when they are discriminating between intense, rather than weak, stimuli.
8. Gustav Fechner proposed that, as stimulus intensity increases, the magnitude of the
psychological response increases, but not as dramatically. Fechner’s law states that the
magnitude of the psychological response is related to the logarithm of the intensity of the
physical stimulus.
9. Using magnitude estimation and cross-modality matching techniques, S. S. Stevens proposed
that the magnitude of the psychological response is related to the intensity of the stimulus,
raised to a certain power, n. In general, Stevens’s predictions are more accurate than
Fechner’s.
Measuring Brain Activity Due to Perceptual Stimuli
1. Changes in the rate at which neurons generate action potentials (excitation or inhibition) are
crucial neural signals. Researchers use single-cell recording to study the activity of individual
neurons.
2. Electroencephalography (EEG) allows a researcher to study the massed activity of many
neurons. However, it’s difficult to determine the specific location of the activity.
3. Two noninvasive methods for mapping brain activity are positron emission tomography (PET)
and functional magnetic resonance imaging (fMRI).