Visual Perception and Illusions
LECTURE 1: SEEING IS DECEIVING (CHAPTERS 0, 1 – TO 37, 3 - 84-85)
Vision is active not passive
Why study vision?
• 1/3 of the cortex is devoted to vision
• Other senses work in a similar way to vision. Between them occupying
more than half the brain
• Everything we do starts with sensation
• Even imagination, dreams and hallucinations use sensory areas of the brain
• We do not perceive physical reality
• Vision as we experience it is ‘impossible’ – vision tells us how we see the world not how it actually is
• We cannot be sure:
o what colour something is
o how big something is
o what direction something is moving in, its speed or even if it is moving at all
o what shape something is
o who someone is, whether they are happy or their gender
• This is because we are faced with too much information
o the energy required to process all the information would be huge (if our eyes recorded everything
we saw like a camera)
▪ the whole brain consumes only 20 watts of power. that’s one low power light bulb and
about 10% of a desktop PC
▪ even so, this is 20% of the power consumed by the whole body (with sophisticated energy
saving measures
• Compression - solves the problem of too much information
o Only transmit important information Similar methods enable tools
o Don’t transmit things humans don’t need to react to like iPlayer to send tv
o Changes are more important than things that stay the same programmes over the telephone
o Changes across space = edges network. Update only changes,
o Changes over time = new objects, things that move send some information in low
o Some kinds of information are more important than others resolution
• Consequences of compression are:
o Very sensitive to sudden changes and movement
o Very poor at detecting slow changes
o High resolution for black and white
o Low resolution for colour
o Good at comparing things side by side
o Poor at making absolute judgements or comparisons at different times
o Appearance of things can change over time
o Past events change what we perceive
o Surrounding context affects what we perceive
Encoding changes over time =AFTER EFFECT
, • R cells detect red
• G cells detect green (most mammals cannot detect red-green)
• B cells detect blue
T T T
• Temporal inhibitors turn off cells if they are active for a long time
R G B
o temporal inhibitors are slow
o temporal inhibition takes time to build-up
o temporal inhibition takes time to go away
• White=red+green+blue (perceived colour)
o if we look at red for a long time, our ‘R’ cells will become inhibited
o if we then look at something white, the green and blue cells respond but the red cells do not.
o So, the white now looks blue/green
NB colours are an invention of our brain (aided by light) –necessary information to help us behave appropriately
Encoding changes over space = COLOUR CONTRAST
• G cells detect green at different (nearby) locations
• Spatial inhibitors turn off cells if their like-minded neighbours are
active
• Every G cell has a spatial inhibitor cell linked to it
• Spatial inhibition is fast
o spatial inhibition develops and disappears quickly
• As a consequence the cells on either side of the stimulus are
reduced and central is increased
Illusions
• Most illusions can be explained by the brains willingness to produce three dimensions from the poorest of
information – in order to try and make sense of the world through scaling and distortion
• The brain makes assumptions about the world and uses these to give us the information we need (which
leads to incorrect perception)
• For example, the two shapes are the same but appear different.
They are the same on the back of the eye, but the brain overrides
this and produces our perception. We do not care what is on the
back of the eye and therefore do not need conscious access to it
• Illusions provide us with a means of examining the way the visual
system works
o if we can understand how things occur, we may be able to
do something to change them
▪ e.g with visual agnosia
The eye
The eye is sometimes likened to
a camera, both have:
-an aperture
-a lens
-something to absorb the light at
the back
,• The cornea is the first thing we notice – is the transparent window through which light enters the eyes. It is
curved and acts as a lens (the main lens – ¾ of the eyes focusing power comes from the cornea)
o they focus light on the retina
o they are curved and made of a surface which light travels through more slowly than through air
▪ this means light gets bent when it hits the surface
• how much bending that takes place depends on the nature of the materials on
either side of the boundary
o underwater vision is blurry because light travels at the similar speed through
water and the cornea, so little deflection of light takes place and you are
unable to focus the world onto your retina (wearing goggles re-establishes
and air-cornea boundary)
• Behind the cornea is the anterior chamber filled with aqueous humour, a watery liquid that flows from the
ciliary body, through the zonules of Zinn, through the pupil and into the anterior chamber.
• It then passes through the trabecular meshwork and leaves the eye down the canal of Schlemm.
• The pressure of the aqueous humour is very important, too much pressure = glaucoma/blindness
o this is checked through a puff of air to see if the surface of the cornea distorts enough
• The iris provides an adjustable aperture and is the coloured part of the eye.
o When light levels are high, the iris constricts the pupil to make it smaller, limiting the amount of light
reaching the retina
▪ when our pupils constrict, our depth of focus is increased
o When the light is dim, the iris relaxes the pupil to make it bigger and allows more light through to
the retina
• Beyond the iris, we reach the lens. Although the cornea is more powerful than
the lens, the lens is adjustable (accommodation occurs here)
o the lens is held between the zonules of Zinn
▪ Contraction of the ciliary muscles relaxes the zonules of zinn,
making the lens fatter, giving us more refractive power
• required for focusing on close objects
▪ Relaxation of the ciliary muscles tightens the zonules of zinn,
making the lens skinner
giving us less refractive power
• required for focusing on things further away
o Light rays from distant objects that reach the eye are near
parallel and need little bending to bring them into focus
on the retina
o Light from close objects that reach the eye are diverging,
which need to be bent more to bring them into focus
• If the lens is fully functioning vision is emmetropic
• If you are short sighted, you are myopic (lens = too strong, eye =
long)
o distant things are blurred because the parallel light rays get bent too much and focus comes before
the retina
o To fix: less lens power - through a diverging lens
, • If you are long sighted, you have hypermetropia (lens = weak, eye = short)
o close things are blurred because the divergent rays are not bent enough towards each other, and
focus comes behind the retina
o To fix: more lens power – through a converging lens
• As we age, the natural elasticity of the lens reduces, meaning that when we relax our ciliary muscles, the
lens does not become as skinny as it should, which causes our near-point to become further away from us
(this is called presbyopia)
• By age 30 – ½ focusing power is lost
• By age 50 – most focusing power is lost
• The main cavity of the eye, behind the lens is filled with a gelatinous substance called the vitreous humour
o This keeps the eyeball in shape and the retina pinned to the back of the eye
o If the vitreous humour shrinks (like it does in old people) the retina may become detached and may
needed to be spot-welded back with a laser
• The retina is the light-sensitive layer at the back of the eye. Here is the start of real visual processing
o light-sensitive photo pigments are found in the outer segments of the photoreceptors
o light must travel a lot before reaching the receptors
▪ one reason for this is that transduction (turning of light energy into electrochemical energy
within the nervous system) requires a lot of energy and that must be supplied by the blood
supply
• blood can be delivered to the back of the eye more easily than the front of the eye
o Once the light reaches the outer segments, neural processing can begin
o The receptors are connected to bipolar cells, which synapse with retinal ganglion cells
▪ the axons of the retinal ganglion cells carry information from the eye to the visual cortex
o Connecting across the retina are two further cell types
▪ horizontal cells (where the receptors synapse with bipolar cells)
▪ amacrine cells (where bipolar cells synapse with retinal ganglion cells)
o The blind spot is caused by the neural bundle passing through the retina
• Photoreceptors:
o There are two types of photoreceptors in the human eye: cones and rods
o All of our rods are basically the same – they all contain the same photopigment in their outer-
segments (what absorbs the light).
▪ this is called rhodopsin
• reflects red and blue light and absorbs green
o Cones, cause colour vision, come in three different types
▪ red – most sensitive to long wavelengths of light
▪ green – most sensitive to medium wavelengths of light
▪ blue – most sensitive to shorter wavelengths of light
o Rods respond very well to dim light and are therefore very useful in dim conditions.
▪ as light level increases, so does the activity of the rods
▪ because rods are so sensitive, they respond as much as they can in dim light
▪ they are useless/bleached in full daylight (rods take time to recover their sensitivity when
going from daylight to a dark room, which results in poorer vision temporarily)
o Cones are much less sensitive and responsible for most of our daytime vision (not in dim)
LECTURE 1: SEEING IS DECEIVING (CHAPTERS 0, 1 – TO 37, 3 - 84-85)
Vision is active not passive
Why study vision?
• 1/3 of the cortex is devoted to vision
• Other senses work in a similar way to vision. Between them occupying
more than half the brain
• Everything we do starts with sensation
• Even imagination, dreams and hallucinations use sensory areas of the brain
• We do not perceive physical reality
• Vision as we experience it is ‘impossible’ – vision tells us how we see the world not how it actually is
• We cannot be sure:
o what colour something is
o how big something is
o what direction something is moving in, its speed or even if it is moving at all
o what shape something is
o who someone is, whether they are happy or their gender
• This is because we are faced with too much information
o the energy required to process all the information would be huge (if our eyes recorded everything
we saw like a camera)
▪ the whole brain consumes only 20 watts of power. that’s one low power light bulb and
about 10% of a desktop PC
▪ even so, this is 20% of the power consumed by the whole body (with sophisticated energy
saving measures
• Compression - solves the problem of too much information
o Only transmit important information Similar methods enable tools
o Don’t transmit things humans don’t need to react to like iPlayer to send tv
o Changes are more important than things that stay the same programmes over the telephone
o Changes across space = edges network. Update only changes,
o Changes over time = new objects, things that move send some information in low
o Some kinds of information are more important than others resolution
• Consequences of compression are:
o Very sensitive to sudden changes and movement
o Very poor at detecting slow changes
o High resolution for black and white
o Low resolution for colour
o Good at comparing things side by side
o Poor at making absolute judgements or comparisons at different times
o Appearance of things can change over time
o Past events change what we perceive
o Surrounding context affects what we perceive
Encoding changes over time =AFTER EFFECT
, • R cells detect red
• G cells detect green (most mammals cannot detect red-green)
• B cells detect blue
T T T
• Temporal inhibitors turn off cells if they are active for a long time
R G B
o temporal inhibitors are slow
o temporal inhibition takes time to build-up
o temporal inhibition takes time to go away
• White=red+green+blue (perceived colour)
o if we look at red for a long time, our ‘R’ cells will become inhibited
o if we then look at something white, the green and blue cells respond but the red cells do not.
o So, the white now looks blue/green
NB colours are an invention of our brain (aided by light) –necessary information to help us behave appropriately
Encoding changes over space = COLOUR CONTRAST
• G cells detect green at different (nearby) locations
• Spatial inhibitors turn off cells if their like-minded neighbours are
active
• Every G cell has a spatial inhibitor cell linked to it
• Spatial inhibition is fast
o spatial inhibition develops and disappears quickly
• As a consequence the cells on either side of the stimulus are
reduced and central is increased
Illusions
• Most illusions can be explained by the brains willingness to produce three dimensions from the poorest of
information – in order to try and make sense of the world through scaling and distortion
• The brain makes assumptions about the world and uses these to give us the information we need (which
leads to incorrect perception)
• For example, the two shapes are the same but appear different.
They are the same on the back of the eye, but the brain overrides
this and produces our perception. We do not care what is on the
back of the eye and therefore do not need conscious access to it
• Illusions provide us with a means of examining the way the visual
system works
o if we can understand how things occur, we may be able to
do something to change them
▪ e.g with visual agnosia
The eye
The eye is sometimes likened to
a camera, both have:
-an aperture
-a lens
-something to absorb the light at
the back
,• The cornea is the first thing we notice – is the transparent window through which light enters the eyes. It is
curved and acts as a lens (the main lens – ¾ of the eyes focusing power comes from the cornea)
o they focus light on the retina
o they are curved and made of a surface which light travels through more slowly than through air
▪ this means light gets bent when it hits the surface
• how much bending that takes place depends on the nature of the materials on
either side of the boundary
o underwater vision is blurry because light travels at the similar speed through
water and the cornea, so little deflection of light takes place and you are
unable to focus the world onto your retina (wearing goggles re-establishes
and air-cornea boundary)
• Behind the cornea is the anterior chamber filled with aqueous humour, a watery liquid that flows from the
ciliary body, through the zonules of Zinn, through the pupil and into the anterior chamber.
• It then passes through the trabecular meshwork and leaves the eye down the canal of Schlemm.
• The pressure of the aqueous humour is very important, too much pressure = glaucoma/blindness
o this is checked through a puff of air to see if the surface of the cornea distorts enough
• The iris provides an adjustable aperture and is the coloured part of the eye.
o When light levels are high, the iris constricts the pupil to make it smaller, limiting the amount of light
reaching the retina
▪ when our pupils constrict, our depth of focus is increased
o When the light is dim, the iris relaxes the pupil to make it bigger and allows more light through to
the retina
• Beyond the iris, we reach the lens. Although the cornea is more powerful than
the lens, the lens is adjustable (accommodation occurs here)
o the lens is held between the zonules of Zinn
▪ Contraction of the ciliary muscles relaxes the zonules of zinn,
making the lens fatter, giving us more refractive power
• required for focusing on close objects
▪ Relaxation of the ciliary muscles tightens the zonules of zinn,
making the lens skinner
giving us less refractive power
• required for focusing on things further away
o Light rays from distant objects that reach the eye are near
parallel and need little bending to bring them into focus
on the retina
o Light from close objects that reach the eye are diverging,
which need to be bent more to bring them into focus
• If the lens is fully functioning vision is emmetropic
• If you are short sighted, you are myopic (lens = too strong, eye =
long)
o distant things are blurred because the parallel light rays get bent too much and focus comes before
the retina
o To fix: less lens power - through a diverging lens
, • If you are long sighted, you have hypermetropia (lens = weak, eye = short)
o close things are blurred because the divergent rays are not bent enough towards each other, and
focus comes behind the retina
o To fix: more lens power – through a converging lens
• As we age, the natural elasticity of the lens reduces, meaning that when we relax our ciliary muscles, the
lens does not become as skinny as it should, which causes our near-point to become further away from us
(this is called presbyopia)
• By age 30 – ½ focusing power is lost
• By age 50 – most focusing power is lost
• The main cavity of the eye, behind the lens is filled with a gelatinous substance called the vitreous humour
o This keeps the eyeball in shape and the retina pinned to the back of the eye
o If the vitreous humour shrinks (like it does in old people) the retina may become detached and may
needed to be spot-welded back with a laser
• The retina is the light-sensitive layer at the back of the eye. Here is the start of real visual processing
o light-sensitive photo pigments are found in the outer segments of the photoreceptors
o light must travel a lot before reaching the receptors
▪ one reason for this is that transduction (turning of light energy into electrochemical energy
within the nervous system) requires a lot of energy and that must be supplied by the blood
supply
• blood can be delivered to the back of the eye more easily than the front of the eye
o Once the light reaches the outer segments, neural processing can begin
o The receptors are connected to bipolar cells, which synapse with retinal ganglion cells
▪ the axons of the retinal ganglion cells carry information from the eye to the visual cortex
o Connecting across the retina are two further cell types
▪ horizontal cells (where the receptors synapse with bipolar cells)
▪ amacrine cells (where bipolar cells synapse with retinal ganglion cells)
o The blind spot is caused by the neural bundle passing through the retina
• Photoreceptors:
o There are two types of photoreceptors in the human eye: cones and rods
o All of our rods are basically the same – they all contain the same photopigment in their outer-
segments (what absorbs the light).
▪ this is called rhodopsin
• reflects red and blue light and absorbs green
o Cones, cause colour vision, come in three different types
▪ red – most sensitive to long wavelengths of light
▪ green – most sensitive to medium wavelengths of light
▪ blue – most sensitive to shorter wavelengths of light
o Rods respond very well to dim light and are therefore very useful in dim conditions.
▪ as light level increases, so does the activity of the rods
▪ because rods are so sensitive, they respond as much as they can in dim light
▪ they are useless/bleached in full daylight (rods take time to recover their sensitivity when
going from daylight to a dark room, which results in poorer vision temporarily)
o Cones are much less sensitive and responsible for most of our daytime vision (not in dim)
