PSYCH – Week 8 Online Readings
Week 8: Sensation
Focus Question: What is real? How do you define ‘real’? If you’re talking about what
you can feel, what you can smell, what you can taste and see, then real is simply
electrical signals interpreted by your brain.
Physiological process that underlies transformation of the chemical, mechanical, light,
and sound energy in the world into electrical activity in the brain.
Psychological process involved in the organization and interpretation of sensations.
The distinction between sensation and perception can be “fuzzy”, but “sensation” refers
to the processes that turn chemical, mechanical, sound and light energy in the world into
electrical signals in the brain, whereas “perception” is more related to extracting meaning
from what you sense, tying it to what you know and understand about the world through
past experiences and knowledge.
The Five Senses
Psychologists think of the five senses a little differently from the traditional view.
Seeing (vision) and hearing (audition) are two that you will remember. Taste (gustation)
and smell (olfaction) are typically considered the chemical senses and are often treated
together because 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 people might call ‘touch’, psychologists break down into the
somatosenses, which include the skin senses of touch, temperature and pain, the internal
or kinaesthetic senses (how you know the position of your limbs, trunk and head in
space) and the vestibular senses (your sense of balance and acceleration).
Justnoticeable difference: Minimum level of detectable difference in the magnitude of
a stimulus. The smaller your justnoticeable difference is, the more sensitive you are.
Fatigue: Neurons that are subjected to steady, continuous stimulation become unable to
send signals, probably because of a temporary depletion in the neurotransmitters that
send signals across synapses.
Sensory adaptation: A change, usually a decrease, in sensitivity that occurs when a
sensory system is repeatedly stimulated in exactly the same way.
See Table 5.1 in textbook (pg 130). All sensory systems have specialized receptive cells that detect energy and convey
signals to the brain (through neural firing) about the presence of environmental stimuli.
Neurons use different codes to tell us the information.
Place (or labeledline) 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 its image 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 across a whole population – a lot of cells.
Temporal code: Neurons can fire quickly or they can fire slowly. There is an upper limit
to 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.
These codes, combined with the Doctrine of Specific Nerve Energies, form the basis of
When no stimulus is 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 or loud) it is.
If a neuron becomes fatigued as a result of adaptation, its rate of signalling may fall
below the spontaneous rate, or minimum detectable level, and your body no longer
notices the stimulus.
Multisensory: Relating to or involving more than one physiological sense.
Transduction: Conversion of physical energy into electrical potentials – happens in
sensory receptor cells, which are specialized neurons
Cranial nerves: Twelve pairs of nerve fibers that travel into and out of the skull and
carry all sensory information (except for somatosenses) from parts of the body below the
neck to the brain. The Visual System
What is light? Light is made up of
photons, particles that transmit
Light waves have three
characteristics: Wavelength (distance from crest of one wave to
crest of next); Amplitude or intensity (amplitude is the height of each wave. Intensity
depends directly on amplitude); and purity (The extent to which a light source is
composed of a single wavelength).
Adequate stimulus: Type of physical energy to which a sensory receptor is especially
tuned. The adequate stimulus for vision is light.
Physical Characteristic of Light Psychological Property
Wavelength Colour (Hue)
Amplitude or Intensity Brightness
Purity Colour (Hue) and Saturation
Anatomy of the Eye
The cornea is the transparent covering of the eye. It is primarily responsible for
bringing light into eye and also acts as a fixedfocus lens to give general focus to light.
The pupil is an opening (the black circle) in the middle of your iris (the part coloured
brown, or green or blue). The iris changes size to increase or decrease the amount of light
entering the eye.
The lens is a transparent structure located behind pupil. It changes its shape through
the action of muscles. The lens becomes short and fat to focus on close items; long and
skinny to focus on far items. This change is called accommodation. The ability of the lens
to change shape declines with age – it stiffens so that it cannot become as short and fat as
it needs to for close objects. This is why some people in their 40s suddenly need glasses
when they didn’t before.
The lens and cornea work together to collect and focus light rays reflected from an
object. They bring the rays together to form an upsidedown image of the object on the
retina, the location at the back of the eye where light energy is transduced into neural
The retina is the inner surface of the back of the eye. This is where the lightsensitive
receptor cells (photoreceptors) are located. Two types of photoreceptors: rods and cones.
Rods detect greenyellow in low light conditions easily but don’t really give us much
colour information while cones do, and are particularly attentive to reds. Both rods and
cones contain photopigments. When exposed to light, these pigments change their
chemical structure, gradually becoming white (a process called bleaching.) 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). 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 cone
photopigments are sensitive to a broad range of wavelengths, but one is most sensitive to
long wavelengths. (Most people call these ‘longwavelength cones’, but your book calls
them ‘red cones’ so this lesson will, too.) One is most sensitive to medium wavelengths
(mediumwavelength, or green cones) and one is most sensitive to short wavelengths
(shortwavelength, or blue, cones). The rod photopigment is most sensitive to medium
wavelengths, but has a slightly different sensitivity than the green cones.
Conclusion: Rods are sensitive to dim light and cones are sensitive to
Most of the cones are concentrated near the centre of the retina. The
rods are distributed around the edges of the retina.
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. 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.
The fovea (near the centre of the retina) has lots of cones, packed very closely together,
but no rods. In contrast, outside the fovea 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.
People with defects in colour vision are generally missing some or all of one or more
types of cone.
The coding of colour or brightness by the relative activity in two photoreceptor
populations: red photoreceptors compared to green signals red/green; and (red and green
together) compared to blue signals (yellow/blue)
Eye movements: Human make two to four eye movements every second, or more than
100,000 every day. These eye movements are necessary because of the design of the
human eye – the eye's acuity varies across the visual field. Resolution: The size of the smallest difference that can be identified. A higher resolution
means that a smaller difference in location, color, amplitude, or other attribute can be
detected or distinguished.
The Auditory System
Sound results from the vibration of air molecules. A source that is emitting a sound
vibrates. This vibration causes adjacent air molecules to become compressed into local
regions of increased pressure, and rarefied in local