In many ways, the study of sensation, and perception, represents the origins of
psychology. Most of the earliest psychologists were interested in issues of sensation
Sensation and perception are related, but are, crucially, different concepts.
Sensation is the process by which our sensory systems gather information about
Perception is the selection, organization and interpretation of sensations. For
example, the sensation of a light pressure on skin might be perceived as a gentle,
The distinction between sensation and perception can be “fuzzy”, but perception is
more closely related to cognitive processes of thought, language, attention, and
memory, whereas sensation is closely related to brain processes (it is a major topic
Scientific investigation of sensation and perception began in the 19th century
Von Helmholtz (1821-1894) was instrumental in developing the science of
physiology in directions that answered psychological questions about perception.
His approach led to the acceptance of physiological explanations for mental
phenomena. His work, and the work of his students, laid the foundation of our
current understanding of hearing and vision, via research on the physiology of the
ear and eye. Through such roots in physiology, investigation of psychological
phenomena became respectable, more like the natural sciences, and could be
funded. Ernst Weber (1795-1878) and Gustav Fechner (life) were contemporaries of
von Helmholtz. They developed methods for measuring the magnitude of human
sensations. Their work led to the field of psychophysics, which we will cover in
more detail in next week’s lesson.
Johannes Müller (1835)
The Doctrine of Specific Nerve Energies
The sensory quality experienced, such as from a sound, touch or light, depends on
which nerve is stimulated, not on HOW it is stimulated.
It is not the form of physical energy that determines the nature of the sensation but
rather the specificity of the neurons, receptors and nerves activated by the stimulus.
Our brain has no access to the physical stimulus itself. (!!!)
Johannes Müller proposed The Doctrine of Specific Nerve Energies in 1835.
All sensory receptors are uniquely sensitive to particular types of energy – for
example, the eye to light (but not to sound) the skin to mechanical pressure (but not
to light). The eye is sensitive to mechanical pressure too, but only a little bit. If you
close your eye and push on your eyelid with your finger, you might see see sparkles
(phosphenes). Thus, this mechanical pressure is registered by the receptor cells in
your eye as light, not touch. Every sensory domain has an ‘adequate stimulus’ – a
type of physical energy to which is it more-or-less uniquely sensitive. An implication
of this Doctrine is that although sensation ‘feels’ direct and immediate – it is not. You
perceive light because light energy is detected by the eye, and sensory receptors
transduce this energy into electrical signals which are carried along axons, cross
synapses, and are ultimately “interpreted by your brain”. Many types of physical
energy are NOT
detectable by humans.
Our vision is
limited to a
narrow range of the
spectrum, and our
limited to a
narrow range of sounds.
Many other animals can
see and hear things
we cannot. For example,
bats, rats, and mice
communicate with very
that we cannot hear.
Elephants communicate with low sound
frequencies that we cannot hear. Many insects can see very short wavelength
(ultraviolet light) and snakes can image long wavelength light (infrared light – heat
– given off by warm blooded animals). This lack of sensitivity to other types
physical energy means that we are
‘blind’ to some sources of information.
We cannot sense ionizing radiation,
even though it is potentially fatal. Likewise, we
cannot sense strong magnetic fields, like those in magnetic resonance imaging (MRI)
That is probably why accidents like this can happen. Here, a cleaner was
washing the floor and put an office
chair (with metal on the base) on the
bed of an
MRI system. This was close enough to the strong field that it flew
into the bore of
the magnet and got stuck.
The senses we have do not give us all the information in the world. They act like
gates. Like gates, they allow some information in; and, like gates, they do not admit
other information. Why do you think that we’ve evolved in such a way that we are
NOT able to detect all the information there is “out there”? Surely, more information
would be better than less, and confer a survival advantage?
The sensory systems, and the type of energy to which they are most sensitive, are
given in table 5.1 of your textbook (p 130). All sensory systems have specialized
receptor cells that detect the energy, and convey signals to the brain about the
presence of environmental stimuli through neural firing. Receptor cells can convey
different kinds of information. If you see a spot of light, you know what colour it is,
where it is, and how intense it is. How can neurons convey these different kinds of
information? We’re not entirely sure, but we know that there are different codes
that neurons can use.
Place (or labeled-line) code. Neurons in difference places in the body signal different qualitative features. For example, where the nerve cell is located in the
retina says something about where in the visual field the stimulus must be since
light travels in a straight line – if the eyes are straight ahead, the more off to the side
an object is, the more off to the side the image of it will be on the retina. The retina is
a bit like a page scanner, in that every place on the scanner bed is sensitive to a
different place on the page being scanned. The particular cells in the retina that are
activated by a stimulus tell the organism where the stimulus is ‘out there’.
Population (or pattern) code
Instead of information being conveyed by single nerve cells, or a small group of cells,
it is conveyed in the activity across a whole population – a lot of cells. As an analogy,
think of the children’s toy, Lite Brite. This toy is a
dense rectangular array of holes,
which you stick lights. You can arrange
the lights to make a pattern. Think of
dense array of holes as a whole nerve
bundle, with thousands and thousands
nerve fibers (axons). The lights are the
nerve fibers that are firing. The image
Mr Potato Head isn’t in any one axon,
but depends a pattern of activity
the whole array. This seems to be the
way that the olfactory system in
your nose works. Specific smells (coffee, banana, vanilla, lavender) are not coded by
a particular set of cells firing (there are no banana cells or coffee cells). Instead, the
pattern of activity across a whole array of olfactory cells codes the particular odor.
Neurons can fire quickly, or they can fire slowly. There is an upper limit on how fast
they can go: different neurons fire at different speeds, but a rough estimate is that a
neuron can fire once every 5 milliseconds, or about 200 times a second. The
frequency of a sound – perceived as pitch, can be coded in the firing rate of a group
of neurons, as you’ll learn next week. Loudness, the psychological correlate of a
sound’s intensity, is also coded in firing rate, as is brightness, the psychological
correlate of the intensity of light.
Neurons do fatigue though – like anything going as fast as it can go, eventually it
gets tired, and stops. It takes only a few seconds of steady, continuous stimulation
for a neuron to fatigue. This is a very important feature of sensory systems that we’ll
come back to – the neural mechanism underlying the phenomenon of adaptation.
There’s also a minimum rate of neuronal firing. When there is no stimulus present,
nerve cells still fire randomly, at some spontaneous rate.
If a neuron isn’t fatigued,
then the rate of firing indicates the intensity of a stimulus – how strong (e.g., bright;
loud) it is. Weak stimuli will increase the firing rate a little bit; strong stimuli will
increase it a lot. A stimulus will be detectable if it causes the rate of firing to increase
beyond what is the typical spontaneous rate.
Sensory adaptation: is the term for what happens to sensation (and hence
perception, which is the interpretation of sensation) when you fatigue sensory
It is such an important concept, we’re going to say a little more about it. It happens
in all sensory domains, and is defined as a change in sensitivity that occurs when a
sensory system is repeatedly stimulated in exactly the same way. When the same
stimulus is repeatedly presented, the system usually becomes less responsive—i.e.,
the receptor neurons remain in a kind of “refractory period”(temporarily, it is
harder than normal to elicit an action potential from the cells).
Sensory adaptation allows the organism to ignore stimuli that remain unchanged
(may be irrelevant to survival) and to react to a sudden change in stimulation, which
may be very important for survival (e.g., the sudden appearance of a predator). Adaptation is also efficient –our brains are only a fixed size, and so we need
something like a data compression algorithm (data compression algorithms are
used to reduce the size of songs and movies so that you can play them on your
portable devices). Adaptation is one way to reduce the amount of stuff that needs to
be processed by the brain, without actually reducing the amount of information very
Next, we will look at specific sensory processes. You may remember that you have 5
senses. What are they? Psychologists think of them a little differently to the
traditional view. Seeing (vision), hearing (audition), are two that you will remember.
Taste (gustation) and smell (olfaction) are typically considered as the chemical
senses and are often treated together, since they are similar in many ways. In fact,
the conventional use of the term ‘taste’ (as in “Gee, this tastes yummy!” is a
misnomer – what we “taste” in food and drink is actually a combination of gustation
and olfaction. Finally, what laypeople might call ‘touch’ psychologists break down
into the somatosenses, which include the skin senses of touch, temperature and
pain, the internal senses (how you know the position of your limbs, trunk and head
in space) and the vestibular senses (your sense of balance and acceleration).
Although we will cover these senses one at a time, please remember that, in the real
world, your senses work together – we’ve already mentioned taste as an example of
this. As another example, your vestibular senses and your vision work together to
help you maintain balance. If you are standing on one leg, you’ll be able to maintain
your balance longer with your eyes open than with your eyes closed (try it!). If you
are petting a contented cat on your lap, you see it, you feel its warm, soft fur, and
you can hear and feel its purr. In other words, most perception is multisensory.
Each one of the senses covered in the textbook has a complex
process by which electrical potentials in receptor cells develop in response to
physical stimuli. Each receptor varies its responses to stimuli that differ
quantitatively (energy level of stimulation—e.g., sound intensity; brightness of a
light) and qualitatively (type of stimulus energy—e.g., sound frequency; colour of a
light). In all cases, the stimulus characteristics must be coded in some way (please
refer to the earlier discussion on the possible types of codes). Using these codes,
each sensory system passes the information along specific anatomical pathways to
the cortex of the brain, where further processing occurs so that sensations are
organized and interpreted as perceptions
For many of the senses, information travels along cranial nerves to the brain. The
skull is a protective bony case with only a few holes in it: information has to pass
through one of the holes to get into and out of the brain. There are 12 cranial nerves
that pass through these holes -- you can think of them like channels into and out of
the skull. Some nerves are all sensory (afferent), some are exclusively motor
(efferent), and some serve both sensory and motor functions. Sensory information
for vision, audition, gustation, olfaction, vestibular sensation, and the skin and
internal senses related to the head is all carried via various cranial nerves. For an
engaging and amusing interactive look at the cranial nerves and the information
they carry, visit:
(p 131-138 of Carlson & Heth)
We will begin with a discussion of sensory processes in vision, including the nature of the stimulus, the anatomy and physiology of the receptor system, and a number of
sensory phenomena that psychologists relate to the sensory transduction of visual
The adequate stimulus
The stimulus for vision is light. So, to understand vision, one must first understand
the physical characteristics of light. Light is a form of electromagnetic energy (as are
X-rays, ultraviolet and infrared radiation). The electromagnetic radiation to which
we are sensitive is a very small part of the electromagnetic spectrum (see Fig. 5.6, p.
135). As mentioned earlier, other creatures can see beyond the range that we are
capable of seeing.
Light is made up of particles, called photons. These particles travel in waves. Light
waves have three characteristics:
distance from crest of one wave to crest of next
The wavelength of visible spectrum covers a range from 400 to 700 nanometers,
where 1 nm is a billionth of a meter and the wavelength is the number of nms
from one crest to the next of a light wave.
• The wavelength of light is related to
the psychological property of colour (or hue) – 400 nm appears to us as violet;
700nm appears to us as red—if we have “normal” colour vision of course.
2. Amplitude or Intensity (Intensity is proportional to the square of amplitude):
Amplitude is the height from a crest to a trough of the light wave.
Intensity corresponds to the psychological property of brightness.
Higher amplitude waves are perceived as brighter.
Very few light waves we see are pure - actually they are made up of a combination of
a number of different light waves.
The purity of light corresponds to the psychological dimension of saturation and
also colour (hue). For example, a light of 650nm appears red, and adding other
wavelengths will make it appear to be a "washed out" pinkish or purplish colour, or
less saturated. If enough of another colour is added, the colour will change – adding
enough green light to a red light will cause it to appear yellow.
The following table (and table 5.2,on p. 141) defines the physical characteristics of
light waves, and the psychological properties that correspond to these physical
characteristics (remember: psychophysical methods could be used to show the
The Eye. The next step in understanding vision is to examine the sensory receptor, the eye.
The eye has two functions: to focus light onto the back of the eye (retina) and to transduce
(convert) the light energy into neural impulses.
Focusing light. The cornea, pupil and lens focus light onto the retina (the structure at the
back of the eye). These structures are shown in Fig. 5.7, p. 136.
Cornea: transparent covering of eye
• Primarily responsible for bringing light into eye
Acts as a fixed focus lens to give general focus to light
Pupil: opening in the iris. The pupil is the black circle in the middle of your iris (the part
coloured brown, or green or blue). The iris changes its size to increase or decrease the
amount of light entering the eye.
Lens: transparent structure located behind pupil.
o It changes its shape through the action of muscles.
o The lens becomes short and fat to focus on close items; long and skinny to focus
on far items. This is called accommodation. The ability of the lens to change
shape changes with age –it stiffens so that it cannot become as short and fat as it
needs to for close objects. This is why a lot of people in their 40s suddenly need
glasses, when they didn’t before.
o The lens and cornea work together to collect and focus light rays reflected from
an object. They bring the rays together to form an inverted image of the object
on the retina.
The inner surface of the back of the eye. This is where the light- sensitive receptor
cells (photoreceptors) are located. Photoreceptors fall into two general classes: the
rods and the cones, so named because of their shape. Rods and cones transduce the
light energy into neural impulses.The retina consists of three layers.
The 125 million rods and 6 million cones are located at the very back of the eye.
They feed into a layer of cells called the bipolar and amacrine cells which, in turn,
feed into a layer of cells called the ganglion cells. The axons of the ganglion cells
form the optic nerve (one of the cranial nerves) that leaves the eye and goes to the
thalamus. If you think about this for a minute, you’ll realize that this is awfully
strange: the light rays have to pass through two cell layers – including all the
supporting stuff like blood vessels and other cells - in order to get to the receptors at
the very back of the eye (see Figure 5.11)
The point where the optic nerve leaves the eye is the optic disk. This place is devoid
of photoreceptors and, thus, is actually blind (the optic disk is shown in Fig. 5.9, p.
265). As you have probably figured out, we usually aren't aware of this "blind spot" -
you should demonstrate it for yourself using Fig. 5.10, p. 138 in the textbook.
The optic nerves from each eye exchange some fibers at the optic chiasm so that
information from the left visual field (imaged on the right halves of each retina; red)
goes to the right visual cortex, and information from the right visual field (imaged
on the left halves of each retina; green) goes to the left visual cortex. Visual
information is passed through the thalamus, to primary visual cortex in the occipital
lobe, and to other visual areas for further processing.
The two types of photoreceptors: rods and cones, have different response properties.
The existence of 2 types of photoreceptors was theorized before technology was
available to prove it. Psychophysicists in the 19th century didn’t yet know about the
physiology (the receptors) but they provided three pieces of behavioural, perceptual
evidence in favour of this theory. They studied dark adaptation, colour processing in
dim light, and the Purkinje shift. Dark adaptation.
When we enter a building from outside on a bright, sunny day, it is initially very
difficult to see. Gradually we can see: the light becomes sufficient. We are actually
becoming more sensitive to light when we're in a darker environment. This process
is called dark adaptation - an increase in sensitivity with exposure to darkness.
You can also experience this when you enter a dark movie theatre. At first you can’t
see anything, but gradually you can make out the chairs and the other people...
You can easily measure the change in your own threshold by using a small white
light behind a piece of white paper to create a dim spot of light. You have to have a
light source with adjustable intensity, like the dimmer switches you find in some
houses. You simply adjust the light’s intensity until you can just barely see it, record
the position of the dimmer switch (this, your dependent variable, is a measure of
threshold intensity), and re-adjust it again after each minute in the dark. If you do
the experiment, you would get something like the function shown below on the left.
The graph in this figure shows how the absolute threshold decreases as we become
more sensitive to light for the first 5 minutes in the dark, and then there’s not much
change for 5 min or so, and then suddenly we become much more sensitive over the
next 10 minutes, that by 20 min after coming into the dark we’re pretty much as
sensitive as we’re going to be.
The fact that the dark adaptation curve is not a simple curve (i.e., a continuous
smooth drop in threshold) suggested to researchers in the 19th century that there
were two underlying physiological mechanisms that created this "bump" (at ‘what’s
going on here? above left) in the dark adaptation curve. We now know that the
activity of the cones make up the top part of the curve, while the activity of the rods
make up the bottom part of the curve (shown above, right).
Colour Processing in Dim Light.
In bright lighting conditions we see in colour. This is because our cones allow us to
perceive colour (as we'll discuss later). In dark conditions we see in shades of grey.
Our rods, which work best in dim-light conditions, cannot perceive colour, for
reasons we’ll get to a bit later. The loss of colour vision in dim light is the second
piece of evidence suggesting that there are two types of photoreceptors. Purkinje Shift.
As twilight approaches, our sensitivity to different colours changes. During bright
(midday) conditions, we find yellows and reds to be most brilliant. When twilight
comes, blues and greens become more brilliant. This was first described by a
physiologist named Purkinje who noted it when he was gardening one evening. This
is the third piece of evidence supporting the idea that there are two photoreceptor
We now know that in bright sunlight, our cones are active and our rods are inactive.
Our cones are most sensitive to the yellows and reds. In twilight, our rods become
active as our cones lose their sensitivity. Rods cannot process colour, but they are
most sensitive to light with a wavelength that we perceive as yellow-green. You can
easily demonstrate this effect yourself by observing the brightness of red, and
yellow or green flowers in the daylight and then again at dusk. As twilight
progresses, red flowers darken (because only your cones are sensitive to red) while
yellow and green flowers appear brighter (because cones AND rods are sensitive to
Rods and Cones.
We now know that 2 types of photoreceptors exist: the rods and cones.
They differ not only in shape (see Fig. 5.11), but also in the chemicals they contain
that respond to light (the photopigments described in your textbook on p. 139).
When exposed to light, photopigments change their chemical structure, gradually
becoming white (bleaching). This bleaching has two effects - it generates a neural
impulse and it causes the photoreceptor to become less receptive to light (because
white reflects more light—so less light is absorbed by the photoreceptor). If
sufficiently bleached, the photoreceptor bounces all the light away, and thus stops
processing. In very bright light conditions, the photopigment in rods is completely
bleached. Thus, rods are inactive in bright light conditions.
If you put the photopigment back into the dark, it gradually "unbleaches" (returns to
its original colour). This unbleaching follows the same time-function as the dark
adaptation curve—showing that the dark adaptation curve is mediated by the
change in the photopigments. The cone photopigment unbleaches first, then the rod
photopigment. So, rods are sensitive to dim light and cones are sensitive to colour in
There are four different kinds of photopigment in a normal human retina – all four
are differentially sensitive to different wavelengths. Three of these are found in
cones (so there are three types of cones) and one photopigment is found in rods. All
three types of cone photopigment are sensitive to a broad range of wavelengths, but one is most sensitive to long wavelengths (most people call these ‘long-wavelength
cones, but your book calls them ‘red cones’ so I will, too). One is most sensitive to
medium wavelengths (medium-wavelength, or green cones), and one is most
sensitive to short wavelengths (short-wavelength, or blue, cones). The rod
photopigment, as I said earlier, is most sensitive to medium wavelengths, but has a
slightly different sensitivity than the green cones.
Sensitivity and Convergence
Sensitivity to light is not only a function of photopigment; it also is a function of the
distribution of photoreceptors on the retina and their neural connections or wiring.
Let’s take convergence first. Rods and cones are wired differently. Many rods, over
a relatively wide expanse of retina, converge their outputs onto a few ganglion cells.
Such ganglion cells are sensitive to dim light, since the activity of individual rods is
added together to generate a signal in the ganglion cell.
The problem with this is that it reduces the ability of the rod system to see fine
spatial detail, since this ganglion cell only registers the information that there was
activity somewhere in the area of retina it gets input from – it doesn’t record
WHERE in the rod array the information came from – so there’s no difference in its
response if a light activates rod X, or 3 rods over in a region of the retina sensitive to
a slightly different part of the visual field. Cones, on the other hand, have little
convergence; there is almost a 1:1 relationship between cones and ganglion cells.
This reduces the ability of cones to respond to dim light but increases the ability of
the cone system to register fine spatial detail.
Distribution of Rods and Cones
Rods and cones are distributed differently across the retina. Recall that the lens
focuses light. Most of the light is focused onto an area directly behind the pupil
called the fovea. Whatever you look at directly (that is, whatever you are fixating
on), is being projected onto your fovea. The fovea has a lot of cones, packed very
closely together, but no rods. In contrast, outside the fovea there are many rods but
relatively few cones. So the fovea is where we see fine detail, since the density of
cones means that every point in the visual field has receptor cell, with a dedicated
ganglion cell, to sense it.
The periphery (outside the fovea) doesn't see fine detail (because many points in
the visual field are sensed by individual cells but these all converge onto single
ganglion cells) but is more sensitive to light. If you want to see a dim star, it is best
to look slightly away from it as this allows the light to hit the peripheral, rod- rich
regions of the retina. Of course, when you do so, you lose colour vision because the
rods are colour blind (i.e., they only have one type of photopigment so there is no
separate signal for lights of different wavelengths).
Consider your kitchen tap. You probably have a mixer tap that changes both the
amount of water coming out, and its temperature. You move the tap up and down to
change the amount, and right and left to change the temperature. You need these
two dimensions to change two qualities of the water (amount, and temperature).
Photoreceptors are the same. If you only have one type, it’s like only being able to
move in one dimension on a mixer tap, say up and down. There is only one kind of
rod photopigment – hence rods can only signal amount of light (i.e., brightness), not
colour (similar to temperature). If you have different types of photopigments, that respond to different wavelengths in different ways, you can signal colour as well as
In the early 19th century, Thomas Young demonstrated that any single wavelength
can be matched by mixing together different amounts of precisely three other lights
of different wavelengths. People were told to play around and change the amount of
each of three lights contributing to the mix, until they just matched the sample.
Turned out that having just two lights to make the match wasn’t enough to get the
match correct, and four was too many – it could be done with three. Young’s
conclusions found on pp 142- 143 of your book.
There are two ways to vary the colour of a stimulus. Young's colour matching
experiment is an example of additive colour mixing—the creation of new colours
by adding together coloured lights as shown in Fig. 5.16, p. 142. There are more light
wavelengths in the mixture than there are in any single component. White light is
obtained when lights of many different wavelengths are mixed together – the more
wavelengths are present, the ‘whiter’ the light.
Subtractive colour mixing occurs when you mix different paint pigments together.
A paint pigment appears to us to be a particular colour because it reflects
wavelengths of that colour while absorbing other wavelengths. So, by adding
pigments you actually increase the absorbing quality of the mixture, which increases
the number of wavelengths that are subtracted from the reflected light; Fig. 5.16, p.
142. A black surface absorbs all wavelengths. What about a white surface?
Trichromatic Theory of Colour Vision
Based on his findings, Young, and later Helmholtz, suggested that colour vision is the
result of the activity of three different colour receptors in the retina. In the 1940s
Wald found that there were, in fact, three different types of cones. Each one
responds to a range of wavelengths, but is most responsive to a different
wavelength (what we see as yellow-green, green, and blue-violet hues – see Fig.
5.17, p. 143. As mentioned earlier these three cone types are called ‘red’, ‘green’ and
‘blue’ respectively by your book, although you might now appreciate why ‘long-
wavelength’, ‘medium- wavelength’, and ‘short-wavelength’ are more accurate
names. The different responsiveness of each cone type depends on how that cone
types photopigment absorbs light.
Additional evidence supporting the
trichromatic theory comes from
matching studies of people with defective colour vision (‘colour blindness’).
People with defects in colour vision (see p. 145-146 of your text) are generally
missing all, or some of, one or more types of cone.
We don’t use the term colour blindness any more since if only one population of
cones is defective, peop