Cognitive Psychology – Visual Perception
The eye
A large area of our brains is devoted to visual processing.
When light reaches the retina there are three consequences:
o Reception: absorption of physical energy by the receptors in
the eye.
o Transduction: physical energy is converted into an
electrochemical signal in the retina.
o Coding: direct one-to-one mapping between the physical
stimulus (the light) and the resultant pattern of neural activity.
Light enters the eye through the lens which can be adjusted by a
process called accommodation to focus the light on the retina.
The retina consists of two types of receptor cells:
o Cones: colour vision and sharpness of vision
o Rods: vision in dim light and movement
The cells that absorb the light are behind blood vessels meaning the
light has to travel through the blood vessels in order to reach the
photo receptors.
In the fovia there are lots of cones found where you are looking
whereas the rods tend to be on the outside of where you are
looking.
The fact that there are a lot of rods found in the space outside
where you are looking means that we have very poor colour
perception in our periphery.
Colour Vision
Why do we have colour vision?
o Colour helps us segregate objects from their background and
helps perception
o Colour helps us to recognise and categorise objects
Trichromatic theory
o First, Thomas Young found that all colours of the spectrum can
be generated by combining three primary colours (red, green,
blue)
o Hermann von Helmholtz then proposed that there must be
three different types of colour sensors in the retina/brain
o The theory assumes that there are three types of cone
receptors differing in the light wavelengths to which they most
strongly.
Cone receptors that are more sensitive to short-
wavelength = blue
Cone receptors that are more sensitive to medium-
wavelength = yellow/green
, Cone receptors that are more sensitive to long-
wavelength = orange/red
o Many forms of colour deficiency are consistent with
trichromatic theory
Most individuals with colour deficiency have dichromacy,
which means that they are lacking one of the cone
classes.
o This system is efficient as it is using just three receptors to
identify many colours.
o Problems of the trichromatic theory: negative afterimage
Opponent process theory
o One major problem of the trichromatic theory is that it cannot
account for what happens after the receptor level.
o Ewald Hering’s opponent process theory:
There are three types of opponent processes:
Red – green
Blue – yellow
Black – white
According to this theory we cannot perceive opposing
colours at the same time.
Opponent process theory also explains negative
afterimages.
It says that it combines the information from different
cones and sends this to the brain so it knows the
colours.
Dual Process Theory integrates the trichromatic theory and the
opponent process theory. According to this theory, signals from
three different cone types identified by trichromacy theory are sent
to the opponent cells.
According to this theory, there are three colour channels:
o One achromatic channel that combines activity of medium
and long-wavelength cones to simply measure the intensity of
the incoming light
o One blue-yellow channel that measures the sum of the
medium and long-wavelength cone signals, and subtracts the
short-wavelength cone signal from this sum
o One red-green channel that represents the difference between
medium and long-wavelength cone signals
Colour constancy
o The tendency for a surface to appear to have the same colour
despite a change in wavelengths contained in the light source.
o For example, you will still a banana as yellow even if there is a
red illuminant
o What we perceive is not entirely driven by the wavelengths of
light that hit our retina…
The eye
A large area of our brains is devoted to visual processing.
When light reaches the retina there are three consequences:
o Reception: absorption of physical energy by the receptors in
the eye.
o Transduction: physical energy is converted into an
electrochemical signal in the retina.
o Coding: direct one-to-one mapping between the physical
stimulus (the light) and the resultant pattern of neural activity.
Light enters the eye through the lens which can be adjusted by a
process called accommodation to focus the light on the retina.
The retina consists of two types of receptor cells:
o Cones: colour vision and sharpness of vision
o Rods: vision in dim light and movement
The cells that absorb the light are behind blood vessels meaning the
light has to travel through the blood vessels in order to reach the
photo receptors.
In the fovia there are lots of cones found where you are looking
whereas the rods tend to be on the outside of where you are
looking.
The fact that there are a lot of rods found in the space outside
where you are looking means that we have very poor colour
perception in our periphery.
Colour Vision
Why do we have colour vision?
o Colour helps us segregate objects from their background and
helps perception
o Colour helps us to recognise and categorise objects
Trichromatic theory
o First, Thomas Young found that all colours of the spectrum can
be generated by combining three primary colours (red, green,
blue)
o Hermann von Helmholtz then proposed that there must be
three different types of colour sensors in the retina/brain
o The theory assumes that there are three types of cone
receptors differing in the light wavelengths to which they most
strongly.
Cone receptors that are more sensitive to short-
wavelength = blue
Cone receptors that are more sensitive to medium-
wavelength = yellow/green
, Cone receptors that are more sensitive to long-
wavelength = orange/red
o Many forms of colour deficiency are consistent with
trichromatic theory
Most individuals with colour deficiency have dichromacy,
which means that they are lacking one of the cone
classes.
o This system is efficient as it is using just three receptors to
identify many colours.
o Problems of the trichromatic theory: negative afterimage
Opponent process theory
o One major problem of the trichromatic theory is that it cannot
account for what happens after the receptor level.
o Ewald Hering’s opponent process theory:
There are three types of opponent processes:
Red – green
Blue – yellow
Black – white
According to this theory we cannot perceive opposing
colours at the same time.
Opponent process theory also explains negative
afterimages.
It says that it combines the information from different
cones and sends this to the brain so it knows the
colours.
Dual Process Theory integrates the trichromatic theory and the
opponent process theory. According to this theory, signals from
three different cone types identified by trichromacy theory are sent
to the opponent cells.
According to this theory, there are three colour channels:
o One achromatic channel that combines activity of medium
and long-wavelength cones to simply measure the intensity of
the incoming light
o One blue-yellow channel that measures the sum of the
medium and long-wavelength cone signals, and subtracts the
short-wavelength cone signal from this sum
o One red-green channel that represents the difference between
medium and long-wavelength cone signals
Colour constancy
o The tendency for a surface to appear to have the same colour
despite a change in wavelengths contained in the light source.
o For example, you will still a banana as yellow even if there is a
red illuminant
o What we perceive is not entirely driven by the wavelengths of
light that hit our retina…
