Psychology Chapter 5: SENSATION
Transduction, Sensory Coding , Psychophysics
Function of the sense organs is to provide information that can help guide behaviour. The
translation of information about environmental events into neural activity is called transduction.
In the neural system information must be translated into two types of neural codes:
ANATOMICAL OR TEMPORAL
Psychophysics is the study of the relationship between the physical characteristics of stimuli and
the perceptions they produce.
Purpose: our senses are the means by which we experience the world; everything we learn is
detected by sense organs and transmitted into our brain by sensory nerves, otherwise we would
have no purpose.
The visual scene received by our eyes changes rapidly as we move our body, our head, and our
Sound isn’t as variable as vision. Its intensity changes as we move toward or away from the as
we move toward or away from its origin, on the whole these changes are more gradual than
those faced by the visual system. Furthermore, sounds carry around obstacles in ways that light
Our auditory sense, then, has more time to process signals. As we will see, it uses time
discriminate complex waveforms of the underlying sounds.
Speech is important for human culture, as well as audition is important for social behaviour.
With vision it provides information about distant events, as does the sense of smell, which can
tell us about sources of aromatic molecules for upwind.
Sensation: the detection of elementary properties of stimulus
(bright colours , warmth, colour, and sweetness)
Perception: the detection of more complex properties of a stimulus. It’s the detection of
objects (animate and inanimate) , their locations, their movements, and their backgrounds
This means that seeing the colour red is sensation. While seeing a RED apple (object) is
Seeing a movement is sensation, but seeing a soccer ball coming toward us and realizing that we
will have to move left to block it is perception.
It was believed that perception was dependent on learning but sensation meant innate,
psychological mechanisms proven to be false
EXPERIENCE is essential to the development of some of the most elementary features of the
Sensory mechanisms: visual, auditory, gustatory, olfactory, and somasensory systems
Somasensory systems have components that are able to detect touch, warmth , coolness,
vibration, physical damage (pain) , head tilt, head movement, limb movement, muscular
contraction, and various events occurring with our bodies.
The only sense receptors that the brain possesses detects such things as temperature and salt
concentration of the blood and these receptors cannot inform it about what is going on outside.
Sense organs detect stimuli provided by light, sound, odour, taste, or mechanical contact with
1. Information about these stimuli is transmitted to the brain through neural impulses- action
potentials carried by the axons in the sensory nerves.
2. The task of sensory organs is to transmit signals to the brain that are coded in such a way to
represent the events that have occurred in the environment.
3. The task of the brain is to analyze this information and reconstruct what has occurred.
Transduction: is the process by which sense organs convert energy from environmental events
into neural activity.
Each sense organ responds to a particular form of energy given off by an environmental stimulus
and translates that energy into a neural firing into which the brain can respond.
In most senses, specialized neurons called receptor cells release neurotransmitters that
stimulate other neurons, thus altering the rate of firing of their axons.
In the samotosesnses (body senses) dendrites of neurons respond directly to physical stimuli
without the intervention of specialized receptor cells.
TYPES OF TRANSMISSION accomplished by the sense organs
Location of the sense organ
Tilt and rotation
Recognition of molecular shape
Recognition of molecular shape
Skin, Internal organs
Sensory information must accurately represent the environment
Nerves are bundles of axons, each of which can do no more than transmit action potentials.
These action potentials are fixed in size and duration : they cannot be altered.
Different stimuli cannot be translated into different types of action potentials. Yet we can detect
an enormous number of different stimuli with each of our sense organs
The information from the sense organs must somehow be coded in the activity of axons carrying
information from the sense organs to the brain
Anatomical Coding: a means by which the nervous system represents information, different
features are coded by the activity of different neurons.
Sensory organs in different parts of the body send their information to the brain through
Because they brain has no direct information about the physical energy impinging on a given
sense organ, it uses anatomical coding to interpret the location and type of sensory stimulus,
according to which nerve fibers are active.
o If you rub your eye you will mechanically stimulate the light-sensitive receptors located
there. This stimulation produces action-potentials in the axons of the nerves that
connect the eyes with the brain (optic nerves) . The visual system of the brain has no
way of knowing that the light sensitive receptors have been activated by a non- visual
o So, the brain acts as if the neural activity in the optic nerves was produced by light- so
you see stars and flashes. Experiments performed during surgery have shown that
artificial stimulation of the nerves that convey taste produces a sensation of taste,
electrical stimulation of the auditory nerve produces a sensation of a buzzing noise and
Temporal coding is the coding of information in terms of time. The simplest form of temporal
code is rate. By firing at a faster or slower rate according to the intensity of a stimulus , an axon
can communicate quantitative information to the brain.
For example, a light tough to the skin can be encoded by a low rate of firing and a more forceful
touch equates to a high rate of firing.
o THE FIRING OF A PARTICUAL SET OF NEURONS (anatomical code) tells where the body is
being touched; the rate at which the neurons fire
o The rate at which the neurons fire ( a temporal code) tells how intense that touch is. As
far as we know, all sensory systems use rate of firing to encode the intensity of
Psychophysics- is the systematic study of the relationship between physical characteristics of
stimuli and thee sensations that they produce.
Scientists had to find reliable ways to measure people’s sensations. We will examine tow of
these methods- the just noticeable difference and the procedures of detection theory-
The principle of just noticeable difference
Just noticeable difference (jnd) – is the smallest difference between two similar stimuli that can
be distinguished; also called the difference threshold
He discovered that that the jnd is directly related to the magnitude of the stimulus. When he
presented participants with two metal objects and asked them to say whether they differed in
weight, the participants reported that the two weights felt the same unless they differed by a
factor of 1 in 40.
Psychologically, the difference between a 40 gram weight and a 41 gram weight is equivalent to
the difference between an 80 gram weight and an 82 gram weight: one jnd.
The difference in the brightness of white light is approximately 1 in 60. (weber fractions)
Gustav Fechner another German physiologist, used Weber’s concept to measure people’s
Assuming that the jnd was the basic unit of sensory experience, he measured the absolute
magnitude of a sensation in jnds. For example, suppose we want to measure the strength of a
person’s sensation of light of a particular intensity.
EXPERIMENT: the person is sent to a dark room facing two discs of frosted glass, each having a
light bulb behind it ; the brightness of the bulb is adjustable
One bulb is turned off completely and the other one is increased in brightness until the
participants can just detect the difference. This is the value of one jnd.
Then we set the sample stimulus to that one jnd, and increase the brightness of the comparison
stimulus just until the participant can tell them apart. This new value of the comparison
stimulus is two jnds. A graph is constructed, and adjacent points are one jnd apart.
The amount of physical energy necessary to produce a jnd increases with the magnitude of
S.S Stevens suggested a power function to relate the physical intensity to the magnitude of
sensation. S is the psychological magnitude of the sensation, and I is the intensity of the
physical stimulus, then, according to the Steven’s power function
S=KIb K is a mathematical constant that adjusts for the way physical intensity is measured. The
important change is that (I) is raised to the power B