Invitation to Psychology

188 Chapter 6 Sensation and Perception

(dendrites) of sensory neurons (see Chapter  4).

The receptors for vision, hearing, and taste are

specialized cells separated from sensory neurons

by synapses.

When the sense receptors detect an appro-

priate stimulus—light, mechanical pressure, or

chemical molecules—they convert the energy of

the stimulus into electrical impulses that travel

along nerves to the brain. Sense receptors are like

military scouts who scan the terrain for signs of

activity. These scouts cannot make many decisions

on their own; they must transmit what they learn

to “field officers,” sensory neurons in the nerves of

the peripheral nervous system. The field officers

in turn must report to generals at a command

center, the cells of the brain. The generals are

responsible for analyzing the reports, combining

information brought in by different scouts, and

deciding what it all means.

The sensory-neuron “field officers” all use the

same form of communication, a neural impulse.

It is as if they must all send their messages on a

bongo drum and can only go “boom.” How, then,

are we able to experience so many different kinds

of sensations? The answer is that the nervous

system encodes the messages. One kind of code is

anatomical, first described in 1826 by the German

physiologist Johannes Müller in his doctrine of

specific nerve energies. According to this doctrine,

different sensory modalities (such as vision and

hearing) exist because signals received by the

sense organs stimulate different nerve pathways

leading to different areas of the brain. Signals

from the eye cause impulses to travel along the

optic nerve to the visual cortex. Signals from the

ear cause impulses to travel along the auditory

nerve to the auditory cortex. Light and sound

waves produce different sensations because of

these anatomical differences.

The doctrine of specific nerve energies im-

plies that what we know about the world ulti-

mately reduces to what we know about the state

of our own nervous system: We see with the brain,

not the eyes, and we hear with the brain, not the

ears. It follows that if sound waves could stimulate

nerves that end in the visual part of the brain,

we would “see” sound. In fact, a similar sort of

crossover does occur if you close your right eye

and press lightly on the right side of the lid: You

will see a flash of light seemingly coming from the

left. The pressure produces an impulse that travels

up the optic nerve to the visual area in the right

side of the brain, where it is interpreted as com-

ing from the left side of the visual field. By taking

advantage of such crossover from one sense to an-

other, researchers hope to enable blind people to

see by teaching them to interpret impulses from

doctrine of specific
nerve energies The
principle that different
sensory modalities exist
because signals received
by the sense organs
stimulate different nerve
pathways leading to dif-
ferent areas of the brain.

learn why people sometimes perceive things

that are not there and, conversely, why they

sometimes miss things that are there—why

they can look without seeing, listen without

hearing.

You are about to learn...

•    why we experience separate sensations even

though they all rely on similar neural signals.

•    what kind of code in the nervous system helps

explain why a pinprick and a kiss feel different.

•    how psychologists measure the sensitivity of our

senses.

•    the bias that influences whether or not you think

you hear the phone ringing when you are in the

shower.

•    what happens when people are deprived of

external sensory stimulation.

•    why we sometimes fail to see an object that

we’re looking straight at.

Our Sensational Senses

Lo 6.1

At some point, you probably learned that there

are five senses: vision, hearing, taste, touch, and

smell. Actually, there are more than five senses.

The skin, which is the organ of touch or pressure,

also senses heat, cold, and pain, not to mention

itching and tickling. The ear, which is the organ

of hearing, also contains receptors that account

for a sense of balance. The skeletal muscles con-

tain receptors responsible for a sense of bodily

movement.

Watch the Video The Basics: In Full Appreciation

of the Cookie at MyPsychLab

All of our senses evolved to help us survive.

Even pain, which causes so much human misery,

is an indispensable part of our evolutionary heri-

tage because it alerts us to illness and injury. Some

people are born with a rare condition that pre-

vents them from feeling the usual hurts and aches

of life, but you shouldn’t envy them: They are sus-

ceptible to burns, bruises, and broken bones, and

they often die at an early age because they can’t

take advantage of pain’s warning signals.

The riddle of Separate

Sensations Lo 6.2

Sensation begins with the sense receptors, cells lo-

cated in the sense organs. The receptors for smell,

pressure, pain, and temperature are extensions

sense receptors
Specialized cells that
convert physical energy
in the environment or the
body to electrical energy
that can be transmitted
as nerve impulses to the
brain.

If you stare at the cube,
the colored panel will
alternate from being at
the back to being at the
front because your brain
can interpret the sensory
image in two different
ways. The other drawing
can also be perceived in
two ways. Do you see the
word?