Absolute Thresholds Is Something Out There

87
Sensory Systems
Energy contains
information
about the world.
1. Accessory
structure
modifies energy.
2. Receptor
transduces
energy into a
neural response.
FIGURE 3 .2
Elements of a Sensory System
Objects in the world generate energy that
is focused by accessory structures and detected by sensory receptors, which convert
the energy into neural signals. The signals
are then relayed through parts of the
brain, which processes them into perceptual experiences.
3. Sensory nerves
transfer the
coded activity
to the central
nervous system.
4. Thalamus
processes and
relays the
neural response.
5. Cerebral cortex
receives input
and produces
the sensation
and perception.
electromagnetic energy and transduces it into sounds. In much the same way, your
ears receive sound energy and transduce it into neural activity that you recognize as
voices and music. Transduction takes place at structures called receptors, which are
specialized cells that detect certain forms of energy. These receptors respond to incoming energy by firing an action potential and releasing neurotransmitters that send signals to neighboring cells. Sensory receptors respond best to changes in energy
(Graziano et al., 2002). A constant level of stimulation usually produces adaptation,
or a decreasing responsiveness to the stimulus over time. This is the reason that the
touch sensations you get from your glasses or wristwatch disappear shortly after you
have put them on.
Sensory nerves carry information from receptors to the brain (Step 3 in Figure 3.2).
For all the senses except smell (Shepherd, 2005), this information goes first to the thalamus, which does some initial processing before sending it on to the cerebral cortex
(Step 4). The most complex processing occurs in the cortex (Step 5). (For a reminder
of the location of these brain structures, see Figure 2.9.)
Coding Sensations: Did You Feel That?
When receptors transduce, or convert, energy, they must somehow code the physical
properties of the stimulus into patterns of neural activity. When organized by the brain,
those neural patterns allow you to make sense of the stimulus. This processing lets you
determine whether you are looking at a cat, a dog, or your next-door neighbor.
Each psychological dimension of a sensation, such as brightness or color, has a corresponding physical dimension that is encoded by the sensory receptors. In other words,
coding translates the physical properties of a stimulus, such as the loudness of sound,
into a pattern of neural activity that tells us what those physical properties are.
Absolute Thresholds: Is Something Out There?
receptors Cells specialized to detect
certain types of energy and convert it
into neural activity.
adaptation Decreasing responsiveness
to an unchanging stimulus.
coding Translation of the physical
properties of a stimulus into a specific
pattern of neural activity.
absolute threshold The minimum
amount of stimulus energy that can be
detected 50 percent of the time.
How much stimulus energy does it take to trigger a conscious perceptual experience?
Not much at all. Normal human vision can detect the light equivalent to a candle flame
burning in the dark thirty miles away. The minimum detectable amount of light, sound,
pressure, or other physical energy is called the absolute threshold. Table 3.1 lists absolute
thresholds for human vision, hearing, taste, smell, and touch.
Psychologists discovered these thresholds by exploring psychophysics, the relationship
between physical energy in the environment and your psychological experience of that
energy. In a typical absolute threshold experiment, you would be seated in a darkened
laboratory. After your eyes got used to the darkness, the researcher would show you
brief flashes of light. These flashes would differ in brightness, or stimulus intensity.
Each time, you’d be asked if you saw the light. Averaged over a large number of trials,
your responses would probably form a curve like the one shown in Figure 3.3. As you
can see, the absolute threshold is not an all-or-nothing affair. A stimulus at an intensity of 3, which is below the absolute threshold in the figure, will still be detected 20
percent of the time it occurs. Because of such variability, psychophysicists redefined the
absolute threshold as the smallest amount of energy that can be detected 50 percent
88
Chapter 3 Sensation and Perception
3.1
TA B L E
Some Absolute Thresholds
Absolute thresholds can be
amazingly low. Here are examples of the stimulus equivalents
at the absolute threshold for the five primary senses. Set up the conditions for testing the absolute threshold for sound, and
see if you can detect this minimal amount
of auditory stimulation. If you can’t hear it,
the signal-detection theory we discuss in
this section may help explain why.
doing
2
learn
by
Human
Sense
Absolute Threshold Is Equivalent to:
Vision
A candle flame seen at 30 miles on a clear night
Hearing
The tick of a watch under quiet conditions at 20 feet
Taste
One teaspoon of sugar in 2 gallons of water
Smell
One drop of perfume diffused into the entire volume of air in a
6-room apartment
Touch
The wing of a fly falling on your cheek from a distance of
1 centimeter
Percentage of presentations detected
Source: Galanter (1962).
100
90
80
Actual
responses
70
60
50
40
30
20
Absolute
threshold
10
1
2
3
4
5
6
7
8
of the time. Why does a supposedly “absolute” threshold vary? The two most important reasons have to do with internal noise and our response criterion.
Internal noise is the random firing of cells in the nervous system that continues in varying amounts whether or not you are stimulated by physical energy. This
ongoing neural activity is a little like “snow” on a television screen or static between
radio stations. If the amount of internal noise happens to be high at a particular
moment, your sensory systems might mistakenly interpret the noise as an external
stimulus.
The second source of variation in absolute threshold, the response criterion,
reflects a person’s willingness to respond to a stimulus. A person’s motivation—wants
and needs—as well as expectations affect the response criterion. For example, if you
were punished for reporting that a faint light appeared when it did not, then you
might be motivated to raise your response criterion. That is, you would report the
light only when you were quite sure you saw it. Similarly, expecting a faint stimulus
to occur lowers the response criterion. Suppose, for example, that you worked at an
airport security checkpoint, examining x-ray images of people’s handbags, briefcases,
and luggage. The signal to be detected in this situation is a weapon. If there has been
a recent terrorist attack, or if the threat level has just been elevated, your airport will
be on high alert. So your response criterion for saying that some questionable object
on the x-ray image might be a weapon will be much lower than if terrorism were not
so likely.
9
Stimulus intensity
FIGURE 3 .3
The Absolute Threshold
The curved line shows the relationship between the physical intensity of a signal
and the chance that it will be detected. If
the absolute threshold were truly absolute, or exact, all signals at or above a
particular intensity would always be detected, and no signals below that intensity
would ever be detected (as shown by the
red line). But this response pattern almost
never occurs, so the “absolute” threshold
is defined as the intensity at which the
signal is detected 50 percent of the time.
Once researchers understood that the detection of a
stimulus depends on the combination of its physical energy, the effects of internal
noise, and the response criterion, they realized that measurement of absolute thresholds could never be more precise than the 50 percent rule mentioned earlier. So they
abandoned the effort to pinpoint absolute thresholds and turned instead to signaldetection theory.
Signal-detection theory presents a mathematical model of how your personal sensitivity and response criterion combine to determine your decision about whether or
not a near-threshold stimulus occurred (Green & Swets, 1966). Sensitivity refers to
your ability to discriminate a stimulus from its background. It is influenced by internal noise, the intensity of the stimulus, and the capacity of your sensory systems. As
already mentioned, the response criterion is the internal rule, also known as bias, that
you use in deciding whether to report a signal. How likely is it that an airport security
guard will spot a weapon in a passenger’s x-rayed luggage? Signal-detection theory provides a way to understand and predict such responses, because it allows precise measurement of sensitivity to stimuli of any kind (MacMillan & Creelman, 2004; Swets,
1992, 1996).
Signal-Detection Theory
89
Sensory Systems
According to signal-detection theory, the likelihood that security screeners will detect
the outline of a bomb or other weapon in
a passenger’s luggage depends partly on
the sensitivity of their visual systems as
they look at x-ray images and partly on
their response criteria. Those criteria are
affected by their expectations that
weapons might appear, as well as by how
motivated they are to look carefully for
them. To help keep inspectors’ response
criteria sufficiently low, airport security
officials occasionally attempt to smuggle
a simulated weapon through a checkpoint. This procedure evaluates the inspectors’ performance but also helps
improve it by keeping inspectors more
focused on their vital task (McCarley
et al., 2004).
DETECTING VITAL SIGNALS
Sometimes our task is not to detect a faint
stimulus but to notice small changes in a stimulus or to decide whether two stimuli are
the same or different. When tuning up for a concert, musicians must discern whether
notes played by two instruments are the same. When repainting part of a wall, you have
to judge whether the new paint matches the old. And when cooking, you have to decide
whether your soup tastes any spicier after you added some pepper.
Your ability to judge differences between stimuli depends on the strength of the
stimuli you are dealing with. The weaker the stimuli are, the easier it is to detect small
differences between them. For example, if you are comparing the weight of two oranges,
you will be able to detect a difference of as little as a fraction of an ounce. But if you
are comparing two boxes weighing around fifty pounds each, you may not notice a difference unless it is a pound or more.
One of the oldest laws in psychology, named after German physiologist Ernst Weber
(pronounced “VAY-ber”), describes the role of stimulus strength in detecting differences. Weber’s law states that the smallest detectable difference in stimulus energy is
a constant fraction of the intensity of the stimulus. This smallest detectable difference
is called the difference threshold or just-noticeable difference (JND). According to
Weber’s law, if an object weighs twenty-five pounds, the JND is only half a pound. So,
if you added a container of yogurt to a grocery bag with three gallons of milk in it,
you would not be able to tell the difference in weight. But candy snatchers beware: It
takes a change of only two-thirds of an ounce to determine that someone has been into
a two-pound box of chocolates! The size of the just-noticeable difference differs from
one sense to the next. The human visual system, for example, is more sensitive than
the human taste system, so we will notice smaller differences in the brightness of a light
than in, say, the saltiness of a salad.
Judging Differences Between Stimuli
internal noise The spontaneous, random firing of nerve cells that occurs
because the nervous system is always
active.
response criterion The internal rule a
person uses to decide whether or not to
report a stimulus.
signal-detection theory A mathematical model of what determines a person’s
report of a near-threshold stimulus.
sensitivity The ability to detect a
stimulus.
Weber’s law A law stating that the
smallest detectable difference in stimulus energy (just-noticeable difference) is
a constant fraction of the intensity of
the stimulus.
just-noticeable difference (JND) The
smallest detectable difference in stimulus energy. Also called difference
threshold.
wavelength The distance between
peaks in a wave of light or sound.
frequency
The number of complete
waves, or cycles, that pass a given point
per unit of time.
Sensory Energy The sensory energies of light and sound vibrate as waves passing
through space. These waves result from reflected light or from changes in air pressure
caused when vocal cords and other objects move. The eye and ear detect the waves as
light and sound. Waves of light and sound can be described in terms of wavelength,
frequency, and amplitude, and it is these properties that determine what is sensed and
perceived. Wavelength is the distance from one peak of the wave to the next. Wave
frequency is the number of complete waves, or cycles, that pass a given point in a