Chapter 8: Organization of the Sensory System
General Principles of SensorySystem Function
Our sensory systems are extremely diverse, and, at first blush, vision, audition,
body senses, taste, and olfaction appear to have little in common. But, although
our perceptions and behavior in relation to these senses are very different, each
sensory system is organized on a similar, hierarchical plan. In this section, we
consider the features common to the sensory systems, including their receptors,
neural relays between the receptor and the neocortex, and central representations
within the neocortex.
Sensory receptors are specialized cells that transduce, or convert, sensory energy
(for example, light photons) into neural activity. The next six subsections
deal with properties that our wide range of sensory receptors have in common,
properties that allow them to provide us with a rich array of information about
Receptors are Energy Filters
If we put flour into a sieve and shake it, the more finely ground particles will fall
through the holes, whereas the coarser particles and lumps will not. Similarly,
sensory receptors are designed to respond only to a narrow band of energy—
analogous to particles of certain sizes—within each modality’s energy spectrum.
Figure 8.1 illustrates the entire electromagnetic spectrum, for example, and indicates
the small part of it that our visual system can detect. Were our visual receptors
somewhat different, we would be able to see in the ultraviolet or infrared
parts of the electromagnetic spectrum, as some other animals can.
We refer to people who lack receptors for parts of the usual visual spectrum
as being color deficient or colorblind. There are also differences in the visual receptors
of individual people who see the usual range of color. Joris Winderickx
and his colleagues report that about 60% of men have one form of the red receptor
and 40% have another form. Many females may have both forms. Hence,
different people may see different “reds.”
For audition, the receptors of the human ear respond to sound waves between
20 and 20,000 hertz (Hz, cycles per second), but elephants can hear
and produce sounds below 20 Hz, and bats can hear and produce sounds as
high as 120,000 Hz. In fact, in comparison with those of other animals,
human sensory abilities are rather average. Even our pet dogs have “superhuman”
powers: they can detect odors that we cannot detect, they can hear
the ultrasounds emitted by rodents and bats, they can hear the lowrange
sounds of elephants, and they can see in the dark. We can hold up only our
superior color vision. Thus, for each species and individual, sensory systems
filter the possible sensory world to produce an idiosyncratic representation of reality.
Receptors Transduce Energy
Each sensory system’s receptors are specialized to filter a different form of
■For vision, light energy is converted into chemical energy in the
photoreceptors of the retina, and this chemical energy is in turn converted
into action potentials.
■In the auditory system, airpressure waves are converted into a number of
forms of mechanical energy, the last of which eventually activates the
auditory receptors, which then produce action potentials.
■In the somatosensory system, mechanical energy activates
mechanoreceptors, cells that are sensitive, say, to touch or pain.
Somatosensory receptors in turn generate action potentials. ■For taste and olfaction, various chemical molecules
by the air or contained in food fit themselves into receptors of
various shapes to activate action potentials.
■For pain sensation, tissue damage releases a chemical that
acts like a neurotransmitter to activate pain fibers and thus
produce action potentials.
Thus, each type of sensory receptor transduces the physical
or chemical energy that it receives into action potentials. Figure
8.2 illustrates how the displacement of a single hair on the arm
results in an action potential that we interpret as touch. The dendrite
of a somatosensory neuron is wrapped around the base of
the hair. When the hair is displaced, the dendrite is stretched by
The dendrite has Na _ channels that are “stretch sensitive”
and open in response to the stretching of the dendrite’s membrane.
If the influx of sodium ions in the stretchsensitive Na _
channels is sufficient to depolarize the dendrite to its threshold
for an action potential, the voltagesensitive K _and Na _ channels
will open, resulting in a nerve impulse heading to the brain.
Receptive Fields Locate Sensory Events
Every receptor organ and cell has a receptive field, a specific
part of the world to which it responds. For example, if you fix
your eyes on a point directly in front of you, what you see of the
world is the scope of your eyes’ receptive field. If you close one
eye, the visual world shrinks, and what the remaining eye sees is
the receptive field for that eye.
Within the eye is a cupshaped retina that contains thousands of receptor cells
called rods and cones. Each photoreceptor points in a slightly different direction
and so has a unique receptive field. You can appreciate the conceptual utility of
the receptive field by considering that the brain uses information from the receptive
field of each sensory receptor not only to identify sensory information but
also to contrast the information that each receptor field is providing.
For each of the sensory systems, its receptors’ unique “view” of the world is
its receptive field. Receptive fields not only sample sensory information but
also help locate sensory events in space. Because the receptive fields of adjacent
sensory receptors may overlap, their relatively different responses to events
help in localizing sensations. The spatial dimensions of sensory information
produce cortical patterns and maps of the sensory world that form, for each of
us, our sensory reality.
Receptors Identify Change and Constancy
Each sensory system answers questions such as, Is something there? And is it
still there? Sensory receptors differ in sensitivity. They may adapt rapidly or slowly to stimulation or react only to a specific type of energy.
Rapidly adapting receptors detect whether something is there. They are
easy to activate but stop responding after a very short time. If you touch yourarm very lightly with a finger, for
example, you will immediately detect the
touch, but, if you then keep your finger still, the sensation will fade as the receptors
adapt. It fades because the rapidly adapting hair receptors on the skin
are designed to detect the movement of objects on the skin.
If you push a little harder when you first touch your arm, you will feel the
touch much longer because many of the body’s pressuresensitive receptors
are slowly adapting receptors that adapt more slowly to stimulation. In the
visual system, the rapidly adapting rodshaped receptors in the eye respond to
visible light of any wavelength and have lower response thresholds than do the
slowly adapting coneshaped receptors, which are sensitive to color and position.
A dog, having mainly black–white vision, is thus very sensitive to moving
objects but has more difficulty detecting objects when they are still.
Receptors Distinguish Self from Other
Our sensory systems are organized to tell us both what is happening in the world
around us and what we ourselves are doing. Receptors that respond to external
stimuli are called exteroceptive; receptors that respond to our own activity are
called interoceptive. For example, objects in the world that we see, that touch us,
or that are touched by us and objects that we smell or taste act on exteroceptive receptors,
and we know that they are produced by an external agent.
When we move, however, we ourselves change the perceived properties of
objects in the world, and we experience sensations that have little to do with
the external world. When we run, visual stimuli appear to stream by us, a stimulus
configuration called optic flow. When we move past a sound source, we
hear auditory flow, changes in the intensity of the sound that take place because
of our changing location.
Some of the information about these changes comes to us through our exteroceptive
receptors, but we also learn about them from interoceptive receptors
in our muscles and joints and in the vestibular organs of the inner ear.
These interoceptive receptors tell us about the position and movement of our
bodies, the awareness that Ian Waterman lost (see the Portrait at the beginning
of this chapter).
Not only do interoceptive receptors play an important role in helping to distinguish
what we ourselves do from what is done to us, they also help us to interpret
the meaning of external stimuli. For example, optic or auditory flow is
useful in telling us how fast we are going, whether we are going in a straight
line or up or down, and whether it is we who are moving or an object in the
world that is moving.
Try this experiment. Slowly move your hand back and forth before your eyes
and gradually increase the speed of the movement. Your hand will eventually
get a little blurry because your eye movements are not quick enough to follow
its movement. Now keep your hand still and move your head back and forth.
The image of the hand remains clear. When the interoceptive receptors in the
inner ear inform your visual system that your head is moving, the vis