Chapter 6 Sensation and Perception 191Yea-sayers will have more hits than naysayers
because they are quick to say “it was there” when
it really was, but they will also have more false
alarms because they are also quick to say, “It was
there” when it wasn’t. Naysayers will have more
correct rejections than yea-sayers, but they will
also have more misses because they will often say,
“Nope, nothing was there” when in fact it was.
This information can be fed into a mathematical
formula that yields separate estimates of a person’s
response bias and sensory capacity. The person’s
true sensitivity to a signal of any particular inten-
sity can then be predicted.
The original method of measuring thresholds
assumed that a person’s ability to detect a stimulus
depended solely on the stimulus. Signal-detection
theory assumes that there is no single threshold,
because at any given moment a person’s sensitiv-
ity to a stimulus depends on a decision that he or
she actively makes. Signal-detection methods have
many real-world applications, from screening ap-
plicants for jobs that require keen hearing to train-
ing air-traffic controllers, whose decisions about
the presence or absence of a blip on a radar screen
may mean the difference between life and death.Sensory adaptation Lo 6.4
Variety, they say, is the spice of life. Variety is also
the essence of sensation because our senses are
designed to respond to change and contrast in
the environment. When a stimulus is unchangingSignal-Detection Theory. The procedures we
have described are useful but have a serious limi-
tation. Measurements for any given individual
may be affected by that person’s general tendency,
when uncertain, to respond, “Yes, I noticed a
signal (or a difference)” or “No, I didn’t notice
anything.” Some people are habitual yea-sayers,
willing to gamble that the signal was there. Others
are habitual naysayers, cautious and conservative.
In addition, alertness, motives, and expectations
can influence how a person responds on any given
occasion. If you are in the shower and you are ex-
pecting a call, you may think you heard your cell
ring when it did not. In laboratory studies, when
observers want to impress the experimenter, they
may lean toward a positive response.
Fortunately, these problems of response bias are
not insurmountable. According to signal-detection
theory, an observer’s response in a detection task
can be divided into a sensory process, which depends
on the intensity of the stimulus, and a decision pro-
cess, which is influenced by the observer’s response
bias. One way a researcher can separate these two
components is by including some trials in which
no stimulus is present and others in which a weak
stimulus is present. Under these conditions, four
kinds of responses are possible: The person (1) de-
tects a signal that was present (a “hit”), (2) says the
signal was there when it wasn’t (a “false alarm”),
(3) fails to detect the signal when it was present
(a “miss”), or (4) correctly says that the signal was
absent when it was absent (a “correct rejection”).
signal-detection
theory A psychophysical
theory that divides the
detection of a sensory
signal into a sensory
process and a decision
process.Get Involved! Now You See it, Now You Don’t
Sensation depends on change and contrast in the environment. Hold your hand over one eye and stare
at the dot in the middle of the circle on the right. You should have no trouble maintaining an image of the
circle. However, if you do the same with the circle on the left, the image will fade. The gradual change from
light to dark does not provide enough contrast to keep your visual receptors firing at a steady rate. The
circle reappears only if you close and reopen your eye or shift your gaze to the X.