Invitation to Psychology

Chapter 6 Sensation and Perception 189

feel of a kiss. An additional kind of code is there-

fore necessary. This second kind of code has been

called functional because it has to do with how cells

in the nervous system are functioning at any par-

ticular time. Functional codes rely on the fact that

sensory receptors and neurons fire, or are inhib-

ited from firing, only in the presence of specific

sorts of stimuli. At any particular moment, then,

some cells in the nervous system are firing and

some are not. Information about which cells are

firing, how many cells are firing, the rate at which

cells are firing, and the patterning of each cell’s fir-

ing forms a functional code. Functional encoding

may occur all along a sensory route, starting in the

sense organs and ending in the brain.

Measuring the Senses Lo 6.3

Just how sensitive are our senses? The answer

comes from the field of psychophysics, which is

concerned with how the physical properties of

stimuli are related to our psychological experience

of them. Drawing on principles from both phys-

ics and psychology, psychophysicists have studied

how the strength or intensity of a stimulus affects

the strength of sensation in an observer.

absolute Thresholds. One way to find out

how sensitive the senses are is to show people a

series of signals that vary in intensity and ask them

to say which signals they can detect. The smallest

amount of energy that a person can detect reliably

is known as the absolute threshold. However, the

word absolute is a bit misleading because people

detect borderline signals on some occasions and

miss them on others. Reliable detection is said to

occur when a person can detect a signal 50 per-

cent of the time.

If your absolute threshold for brightness were

being measured, you might be asked to sit in a

absolute threshold

The smallest quantity of

physical energy that can

be reliably detected by an

observer.

other senses that are then routed to the visual
areas of the brain. Neuroscientists have developed
a device that translates images from a camera
into a pattern of electronic pulses that is sent to
electrodes on the tongue, which in turn sends
information about the pattern to visual areas of
the brain that process images (Chebat et al., 2011;
Ptito et al., 2005). Using this device, congenitally
blind people have been able to make out shapes,
and their visual areas, long quiet, have suddenly
become active.
Sensory crossover also occurs in a rare condi-
tion called synesthesia, in which the stimulation
of one sense consistently evokes a sensation in
another. A person with synesthesia may say that the
color purple smells like a rose, the aroma of cin-
namon feels like velvet, or the sound of a note on
a clarinet tastes like cherries. Most synesthetes are
born with the condition; one leading theory holds
that they have a greater number of neural connec-
tions between different sensory brain areas than
other people do (e.g., Bargary & Mitchell, 2008;
Rouw & Scholte, 2007). But in some cases synes-
thesia may be learned through childhood experi-
ences. In a study of 11 people who experience color
when they see written letters or numbers, all re-
ported that they had acquired this ability through
playing with colored alphabet blocks—and 10 of
them still had those toys! (Witthoft & Winawer,
2013). Synesthesia can also result from damage to
the brain. One woman who had recovered from a
stroke experienced sounds as a tingling sensation
on the left side of her body (Ro et al., 2007).
Synesthesia, however, is an anomaly; for most
of us, the senses remain separate. Anatomical
encoding does not completely solve the riddle
of why this is so, nor does it explain variations of
experience within a particular sense—the sight of
pink versus red, the sound of a piccolo versus the
sound of a tuba, or the feel of a pinprick versus the

synesthesia A condition

in which stimulation of

one sense also evokes

another.

Different species sense the world differently. The flower on the left was photographed under normal light. The one
on the right, photographed under ultraviolet light, is what a butterfly might see, because butterflies have ultraviolet
receptors. The hundreds of tiny bright spots are sources of nectar.