Our senses allow us to experience the world
Sensation – the detection of the elementary properties of a stimulus (brightness, colour,
Humans are capable of sensing more than the principle 5 senses (visual, auditory, gustatory,
olfactory, and somatosensory) humans can also detect temperature, vibration, pain and
Perception is the detection of more complex properties of a stimulus (location,
Transduction is the conversion of physical stimuli into changes in the activity of receptor
cells of sensory organs (environmental events translated into neural activity by the sense
organs). Different sense organs are sensitive to different kinds of energy.
Transduction is the first step to sensation.
Sense organs detect stimuli (light, temp … ) info about stimuli is transmitted to the brain
by nerve impulses action potential in sensory nerves the brain analyzes
For most senses neurons known as receptor cells release chemical transmitter substances
that stimulate other neurons (changes rate of firing the action potential).
Action potentials are fixed (size and duration) and cannot be changed.
We have the ability to detect many different stimuli with each sense organs.
The brain tells colours apart by the information from the sense organs becoming coded
while the axons are carrying info from sensory organs to the brain.
Code – system of symbols or signals that gives information. It takes two general forms:
anatomical coding and temporal coding.
Anatomical Coding – a means by which the nervous system represents information;
different features are coded by the activity of different neurons.
Anatomical coding is used because the brain doesn’t have direct info about the
physical energy impinging on a sense organ. Anatomical coding helps the brain
interpret location and type of sensory stimulus.
Sensory organs located in different parts of the body send their signals to different
locations in the brain.
Example: when you rub your eyes you see stars because your visual senses are
activated and the brain has believes that the change is caused by light.
Temporal coding – coding of info in terms of time. The simplest temporal coding is
rate. The intensity of a stimulus indicates the rate of firing (Action potential). Nerve
cells can fire at different rates this is known as the rate of neural firing.
Psychophysics – branch of psychology that measures the quantitative relation between
physical stimuli and perceptual experience.
The principle of the just-noticeable difference (Jnd)
Ernest Weber conducted the experiment. He investigated the ability of humans to
discriminate between various stimuli. The just-noticeable difference is the smallest
change in magnitude that a person can detect. He found that the weights could not be distinguished unless they differed by a factor of 1 in 40. This ratio is known as
Gustav Fechner also conducted an experiment based on the principle of the just-
noticeable difference. He found that the jnds for brightness was 2.
S.S Stevens suggested a power function that relates to physical intensity to the
magnitude of sensation. S=kI ( S = sensation, k = constant, I= intensity, b= fraction).
Signal Detection Theory – a mathematical theory of the detection of stimuli, which
involves discriminating signal from the noise in which it is embedded and which
takes into account participants’ willingness to report detecting the signal.
Threshold is the line between not perceiving and perceiving. Difference threshold is
the minimum detectable difference between two stimuli (just-noticeable difference).
Absolute threshold is the minimum value of a stimulus that can be detected.
Threshold is the point at which 50% of the time participants can detect stimuli
Jastrow (1897) believed there were two sensitivities to senses
They claim they found the absolute threshold (being aware to unaware)
Separating bias from sensitivity – 1974 Green and Swets found a way to measure
sensitivity free of response bias this is called signal detection theory
- Hit – saying a stimulus is present when it is
- Miss – saying a stimulus is not present when it is
- False alarm – saying a stimulus is present when it is not
- Correct Rejection – saying a stimulus is not present when it isn’t
Null sensitivity in a graph refers to a straight line
Light consists of radiant energy. The wavelength for visible light is short ranging
from 380 (violet) to 760 nm (red). The different wavelengths show different
colours. There are other radiant energy that is invisible to us this includes uv
radiation, x-ray, gamma ray, infrared radiation, radar, and radio waves (first three
have shorter than visible wavelength and the rest have longer wavelengths). The
entire range of wavelengths is known as electromagnetic spectrum. The
wavelengths that we can see are known as visible spectrum. Some animals such as
bees and birds can see uv radiation therefore the visible spectrum for animals are
The eye and its functions
The eye is well protected by the bony socket and is covered by the eyelid. The
eyelashes prevent foreign matter from falling onto the eye. Eyebrows prevent sweat
from dripping into the eye. Reflex mechanisms also protect the eye.
The cornea admits light. The sclera coats the eye.
The iris consists of two bands of muscles that control
the amount of light admitted into the eye. The pupil
constricts in bright light and dilates in dim light.
The fluid found behind the cornea is filled with an
aqueous humour which filters fluid from blood.
The lens contains no blood vessels and is therefore dead tissue The lens is flexible and the change in the shape of the lens when adjusting is called
accommodation. When someone’s eye is too long they are nearsighted (concave lens
is needed) and when it is too short they are farsighted (convex lens is needed).
The retina performs the sensory function. The retina has more than 130 million
photoreceptors. Information from photoreceptors neurons axons back of
the eye (optic disc) optical nerve brain
Johannes Kepler suggested that the retina contains the receptive tissue and not the
lens. This was later proven by Christoph Scheine who demonstrated that the lens
was just a focusing device (Ox’s eye). The retina has three layers. Light passes
through the ganglion cell layer, the bipolar cell layer (middle), and then the
photoreceptor layer (back). Photoreceptors respond to light by passing info to the
bipolar cells and the bipolar cells transmit the info to the ganglion cells and then the
information reaches the brain. The retina contains 2 types of photoreceptors 125
million rods and 6 million cones. Rods function in dim light (sensitive to light but
not to differences in colour). Cones function in bright light (responsible for colour
vision). Fovea (pit at the back of the retina) contains cones and is responsible fro
our detailed vision. The father away from the fovea the number of cones decrease
and the number of rods increase.
Transduction of light by photoreceptors
A molecule derived from vitamin A is the central ingredient in the transduction of
the energy of light into neural activity. When there’s no light present this molecule is
attached to another molecule and together they form a photopigment. When a
photon strikes the photopigment the photopigment splits into two molecules. The
splitting causes a series of chemical reactions to take place that stimulates the
photoreceptors and causes it to send a message to the bipolar cell, which forms a
synapse. The message is sent from the bipolar cell to the ganglion cell and then to
the brain. Rhodopsin is a pink photopigment. Once photopigments split they loose
colour. This phenomenon was discovered by Franz Boll (1876) when he looked at
the retina in dim light (image was seen) and in bright light the retina turned pink.
The recombination of the two molecules is caused by the metabolism of the
Adaptation to Light and Dark
Dark adaptation is when you adapt to the dark (slowly things in the dark will seem
more clear). Detection of light requires photons to split the molecule of
photopigments. Increased levels of illumination = rate of regeneration of rhodopsin
falls behind the rate of the bleaching process. With few rhodopsin intact the rods
aren’t sensitive to light. Once the regeneration of rhodopsin overcomes the
bleaching effects the eye will eventually become dark adapted.
When we gaze at a specific point it’s called fixation point. Jerky