Sensation and Perception
Sensation –> the stimulusdetection process by which our sense organs respond
to and translate environmental stimuli into nerve impulses that are sent to
Perception> the active process of organizing this stimulus input and giving it
meaning i.e making “sense” of what our senses tell us.
In the diagram, Perception allows us to realize the difference between ‘B’ and ‘13’.
Perception of the characters is influenced by their context > when we were younger, we
learnt that in the alphabet B is after A and before C, and numerically 12 is followed by
Synaesthesia > “mixing of the senses” … a rare and mysterious condition where some
people may experience sounds as colours or tastes are touch sensations that have different
We know the 5 classical senses:
• Audition (hearing)
• Gustation (taste)
• Olfaction (smell)
But are these our only senses? Other ‘senses’ in our body provide information to our
brain about balance, body position, touch – can be broken down into degree of pressure,
pain and temperature for example.
Psychophysics is a scientific area which studies relations between physical
characteristics of stimuli and sensory capabilities. Psychophysics is concerned with two
types of sensitivity:
1. Absolute limits of sensitivity – what is the softest sound humans can hear?
2. Differences between stimuli What is the smallest difference in brightness we can
Stimulus Detection: The Absolute Threshold Absolute Threshold is the lowest intensity at which a stimulus can be detected correctly
50% of the time. The lower the abs. threshold, the greater the sensitivity.
Signal Detection Theory
concerned with the factors that influence sensory judgments.
Decision Criterion > a standard of how certain one must be that a stimulus is present
before they will say they detect it.
Not everyone has the same decision criterion. Factors affecting it can be fatigue,
expectation, and potential significance.
In a signal detection experiment, participants are told they may or may not hear a warning
tone. This makes persons expect to hear a tone, and may therefore pay higher attention to
listening for it.
The Difference Threshold
Difference Threshold is defined as the smallest difference between two stimuli that
people can perceive 50% of the time (Sometimes called just noticeable difference or
For example, a slight difference in the taste of food can signal that it has gone bad. Wine
tasters and piano tuners earn their living by being able to make very slight
discriminations between stimuli.
Weber’s Law > states that the jnd is directly proportional to the magnitude of the
stimulus with which the comparison is being made, and can be expressed as a Weber
Fraction. The jnd value for weights is a Weber fraction of 1/50.
e.g If you lift a weight of 500 grams, another comparison object must weigh 510 grams
for you to discriminate between them.
Sensory Adaptation* > the diminishing sensitivity to an unchanging stimulus. For
example (1) the feel of your wristwatch on your wrist – the feeling becomes unnoticeable
after some time. (2) Diving into a cold pool – the water may feel cold at first but
eventually your body becomes used to the temperature.
This shows that adaptation occurs in response to unchanging stimuli.
*Adaptation is also referred to as habituation
The Sensory Systems
is electromagnetic energy which is measured in nanometers (nm). Humans visual
system is sensitive only to wavelengths extending from about 700nm down to 400 nm. Unlike bees, we cannot see ultraviolet light (lower than 400 nm). Also, unlike
snakes we cannot detect infrared energy (above 700 nm)
The Human Eye
Light enters through the cornea (transparent protective structure at front of eye.). behind
is the pupil (dilates or constricts to control amt. of light entering the eye > in dim light,
pupil dilates and in bright light pupil constricts). The pupil’s size is controlled by muscles
in the iris (coloured part of eye). Lens behind pupil becomes thinner to focus on distant
objects, thicker to focus on nearby ones. Retina (multilayered tissue at the rear of the
fluid filled eyeball) focuses the visual image. Lens reverses the image but the brain
reconstructs the image that we perceive.
Myopia > near sightedness (can see close objects but not far)
Hyperopia > far sightedness
The retina contains 2 types of light sensitive receptor cells – rods and cones.
Rods – function best in dim light, primarily black and white brightness receptors. 500
times more sensitive to light than the cones, but do not give rise to colour sensations.
Cones – function best in bright illumination.
Rods and Cones send messages to the brain by
(1) Bipolar cells (synaptic connections with rods and cones)
(2) ganglion cells (axons are collected into a bundle to form the optic nerve)
The optic nerve exits through the back of the eye forming a blind spot.
*Check out the cool picture that proves it on page 178 at the bottom corner.
Transduction – process whereby characteristics of stimulus are converted into nerve
impulses. This is done by rods and cones through the action of protein molecules called
Dark adaptation – progressive improvement in brightness sensitivity. Example when
you walk into a dark cinema after you have just come out of the sunlight… your eyes ‘get
used to’ the dim lighting.
*Human’s difference threshold for light wavelengths are so small that we are able to
distinguish an estimated 7.5 million hue variations! There are 2 different theories of
colour vision attempting to explain how this occurs.
The YoungHelmholtz Trichromatic Theory (three colour theory)
there are 3 types of colour receptors in the retina.
Afterimage – an image in a different colour appears after a colour stimulus has been
viewed steadily and then withdrawn. Example – if you stare at a light for some time and
then look away to a blank/white space, you see green dots after. Hering’s opponentprocess theory – each of the 3 cone types respond to two different
wavelengths. One type responds to red or green and another to blue or yellow, and a 3 to
black or white
Dual process Theory – combines the trichromatic and opponent process theories to
account for the colour transduction process.
Dichromat – person who is col