Chapter 8: Organization of the Sensory System
General Principles of Sensory System Function
• Sensory receptors are specialized cells that transduce, or
convert, sensory energy (for example, light photons) into neural
Receptors are Energy Filters
• We refer to people who lack receptors for parts of the usual
visual spectrum as being color deficient or color- blind. 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.”
Receptors Transduce Energy
• Each sensory system’s receptors are specialized to filter a
different form of energy:
• 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, air- pressure waves are converted into a
number of forms of mechanical energy, the last of which
eventually activates the auditory receptors, which then produce
• 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 carried 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.
• 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 stretch- sensitive Na channels is sufficient to depolarize the dendrite to its threshold
for an action potential, the voltage- sensitive 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 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 Identity Change and Constancy
• 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
• 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
pressure- sensitive receptors are slowly adapting receptors that
adapt more slowly to stimulation. In the visual system, the
rapidly adapting rod- shaped receptors in the eye respond to
visible light of any wavelength and have lower response
thresholds than do the slowly adapting cone- shaped receptors,
which are sensitive to color and position.
Receptors Distinguish Self from Other
• 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 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 be-cause of our changing location. Receptor Density Determines Sensitivity
• The ability to recognize the presence of two pencil points close
together, a measure called two- point sensitivity or
discrimination, is highest on the parts of the body having the
most touch receptors.
• In the fovea (a small area of the retina where color
photoreceptors are concentrated), the receptors— all cone cells
— are small and densely packed to make sensitive color
discriminations in bright light. In the periphery of the retina, the
rod cells that are the receptors for black– white vision are larger
and more scattered, but their sensitivity to light (say, a lighted
match at a distance of 2 miles on a dark night) is truly
Relays Determine the Hierarchy of Motor Responses
• Some of the three to four relays in each sensory system are in
the spinal cord, others are in the brainstem, and still others are
in the neocortex. At each level, the relay allows a sensory system
to produce relevant actions that define the hierarchy of our
• The pain pathway also has relays in the brainstem, especially in
the midbrain periaqueductal gray matter (PAG) that surrounds
the cerebral aqueduct. This region is responsible for a number of
complex responses to pain stimuli, including behavioural
activation and emotional responses. Pain re-lays in the neocortex
not only localize pain in a part of the body, but also identify the
kind of pain that is felt, the external cause of the pain, and
• Recall that the superior colliculus is a major visual center of the
brainstem and the inferior colliculus is a major auditory center. In
animals without a neo-cortex, these brainstem regions are the
main perceptual systems. For animals with visual and auditory
areas in the neocortex, these subcortical regions still perform
their original functions of:
- Detecting stimuli and
- Locating them in space
Message modification Takes Place at Relays
• The messages carried by sensory systems can be modified at relays. For example, descending impulses from the cortex can
block or amplify pain signals at the level of the brainstem and at
the level of the spinal cord. Many of us have had the experience
when we are excited by an activity, as occurs when we are
playing a sport, that we may not notice an injury only to find
later that it is quite severe. This inhibition, or gating, of sensory
information can be produced by descending signals from the
cortex, through the periaqueductal gray matter, and on to lower
• Descending messages from the brain gate the transmission of a
pain stimulus from the spinal cord to the brain.
Relays allow Sensory Interactions
• Where relays take place in sensory pathways, systems can
interact with one an-other. For example, we often rub the area
around an injury to reduce the pain or shake a limb to reduce the
sensation of pain after an injury. These actions increase the
activity in fine touch and pressure pathways, and this activation
can block the transmission of information in spinal- cord relays of
the pain pathways.
• A dramatic effect of sensory interaction is the visual modification
of sound known as the McGurke effect. If a speech syllable such
as “ ba” is played by a recorder to a listener who at the same
time is observing someone whose lips are articulating the
syllable “ da,” the listener hears not the actual sound ba, but the
articulated sound da. The viewed lip movements modify the
auditory perception of the listener. The potency of the McGurke
effect highlights the fact that our perception of speech sounds is
influenced by the facial gestures of a speaker.
Central Organization of Sensory Systems
• The code sent from sensory receptors through neural relays is
interpreted and eventually translated into perception, memory,
and action in the brain, especially in the neocortex.
Sensory Information Is Coded
• After it has been transduced, all sensory information from all
sensory systems is encoded by action potentials that travel along
peripheral- system nerves until they enter the brain or spinal
cord and then on tracts within the central nervous system. Every
bundle carries the same kind of signal.
• The presence of a stimulus can be encoded by an increase or decrease in the discharge rate of a neuron, and the amount of
increase or decrease can encode the stimulus intensity.
• Some people hear in color or identify smells by how the smells
sound to them. This mixing of the senses is called synesthesia.
Anyone who has shivered when hearing certain notes of a piece
of music or at the noise that chalk or fingernails can make on a
blackboard has “ felt” sound.
Early Sensory System Is Composed of Subsystems
• The pathway from the eye to the suprachiasmatic nucleus
(number 1) of the hypothalamus controls the daily rhythms of
such behaviours as feeding and sleeping in response to light
changes. The pathway to the pretectum (2) in the midbrain
controls pupillary responses to light. The pathway to the pineal
gland (3) controls long- term circadian rhythms. The pathway to
the superior colliculus (4) in the midbrain controls head
orientation to objects. The pathway to the accessory optic
nucleus (5) moves the eyes to compensate for head movements.
The pathway to the visual cortex (6) controls pattern perception,
depth perception, color vision, and the tracking of moving
objects. The pathway to the frontal cortex (7) controls voluntary
Sensory Systems Have Multiple Representations
• Topographic organization is a neural– spatial representation of
the body or areas of the sensory world perceived by a sensory
• Rays of light enter the eye through the cornea, which bends
them slightly, and then through the lens, which bends them to a
much greater degree so that the visual image is focused on the
receptors at the back of the eye. The light then passes through
the photoreceptors to the sclera, which reflects the light back
into the photoreceptors.
• The light’s having to pass through the layer of retinal cells (to be bounced back at them by the sclera) poses little obstacle to our
visual acuity for two reasons. First, the cells are quite
transparent and the photoreceptors are extremely sensitive;
they can be excited by the absorption of a single photon.
Second, many of the fibers forming the optic nerve bend away
from the retina’s central part, or fovea, so as not to interfere with
the passage of light through the retina.
• The retina contains two types of photoreceptive cells that
transduce light energy into action potentials. Rods, which are
sensitive to dim light, are used mainly for night vision. Cones are
better able to transduce bright light and are used for daytime
vision. Three types of cones, each type maximally responsive to
a different set of wavelengths— either red or blue or yellow—
mediate color vision.
• Rods and cones differ in their distribution across the
retina: cones are packed together densely in the
foveal region, whereas rods are absent from the
fovea entirely and more sparsely distributed in the
rest of the retina. Thus, to see in bright light, acuity is
best when looking directly at things and, to see in
dim light, acuity is best when looking slightly away.
• The photoreceptive cells synapse with a simple type
of neuron called a bipolar cell and induce graded
potentials in such cells. Bipolar cells, in turn, induce
action potentials in ganglion cells. Retinal ganglion
cells send axons into the brain proper (remember
that the retina is a part of the brain).
• Just before entering the brain, the two optic nerves (one from
each eye) meet and form the optic chiasm.
• Optic Chiasm: Point at which the optic nerve from one eye partly
crosses to join the other, forming a junction at the base of the
• So the right half of each eye’s visual field is represented in the
left hemisphere of the brain, and the left half of each eye’s visual
field is represented in the right hemi-sphere of the brain.
• Having entered the brain proper, the optic tract, still consisting of
the axons of retinal ganglion cells, diverges to form two main
pathways to the visual cortex. Both pathways relay through the
thalamus. The larger projection synapses in the lateral
geniculate nucleus (LGN) of the thalamus, on neurons that then
project to the primary visual cortex, V1. • The LGN has six well- defined layers: layers
2, 3, and 5 receive fibers from the ipsilateral
eye, and layers 1, 4, and 6 receive fibers
from the contralateral eye. The topography
of the visual field is reproduced in the LGN:
the central parts of each layer represent the
central visual field, and the peripheral parts
represent the peripheral visual field.
• The LGN cells project mainly to layer IV of
the primary visual cortex. This layer is very
large in primates and has the appearance of
a stripe across the visual cortex; hence the
name striate (striped) cortex.
• The major visual pathway from the retina to
the LGN to the striate cortex is the
geniculostriate pathway, bridging the
thalamus (geniculate) and the striate
• The geniculostriate pathway takes part in pattern recognition
and conscious visual functions. The second main visual
pathway takes part in detecting and orienting to visual
stimulation. This tectopulvinar pathway relays from the eye to
the superior colliculus in the midbrain tectum and reaches the
visual areas in the temporal and parietal lobes through relays
in the lateral posterior-pulvinar complex of the thalamus.
• Hearing is the ability to construct perceptual representations
from pressure waves in the air. Hearing abilities include sound
localization— identifying the source of pressure waves— and
echo localization— detecting pressure waves reflected from
objects— as well as the ability to detect the complexity of
pressure waves, through which we hear speech and music.
Auditory Receptors • Sounds are changes in air- pressure waves. The frequency,
amplitude, and complexity of these changes determine what we
hear. We hear the frequency, or speed, of pressure changes as
changes in pitch; we hear the amplitude, or intensity, of pressure
changes as loudness; and we hear the complexity of pressure
changes as timbre, the perceived uniqueness of a sound.
• The human ear has three major anatomical divisions: outer ear,
middle ear, and inner ear. The outer ear consists of the pinna
and the external ear canal. The pinna catches waves of air
pressure and directs them into the external ear canal, which
amplifies them somewhat and directs them to the eardrum. The
middle ear consists of the eardrum and, connected to it, the
hammer, anvil, and stirrup, a series of three little bones (the
ossicles). The ossicles in turn connect to the oval window of the
• In short, pressure waves in the air are amplified and transformed
a number of times in the ear: by deflection in the pinna, by
oscillation as they travel through the external ear canal, and by
the movement of the bones of the middle ear.
• In the inner ear is the cochlea, which contains auditory sensory
receptors called hair cells. The cochlea is rolled up into the shape
of a snail. It is filled with fluid, and floating in the middle of this
fluid is the basilar membrane. The hair cells are embedded in a
part of the basilar membrane called the organ of Corti.
• When the oval window vibrates, a second membrane within the
cochlea, the round window, bulges, sending waves through the
cochlear fluid that cause the basilar membrane to bend and thus
to stimulate the hair cells to produce action potentials. The larger
the air- pressure changes, the more the basilar membran