How do we interpret information
from the eyes to enable us to
see what we see?
Brain growth facts
• 21 days after conception the neural tube is formed. The first
three segments develops into the brain, the rest into the spinal
• Baby is born with > 100 000 neurons
• By 6 months after birth the brain is half the adult size
• By 2 years the brain is 80% of the adult size
• There is no huge increase in the number of neurones; most brain
growth is the result of elongation of axons, myelination,
development of synaptic connections, increase in number of the
brain‟s connective/support tissue- the neuroglial cells.
• Axon growth and making the right synaptic connections is crucial
for the development of brain function.
• We will look at this in the case of visual development.
• The eye forms as an outgrowth of the
• Axons of neurones from the retina grow
back along the optic nerve to the
a relay station that carries sensory information from the sense organs
to the correct part of the cortex and hypothalamus.
• In the thalamus the axons from the retina
form synapses with neurones from the
thalamus itself in a very ordered
• Axons from these thalamus neurones then
grow towards the visual cortex in the
occipital lobe to produce the visual pathway.
• These all occur purely by the genetic encoding
of the cells in the central nervous system and
the genetically driven biochemical processes
that guide it.
The visual pathway
• The cells of the visual cortex are
arranged in columns. Axons from the
thalamus synapse within these columns
Relationship between the cells in the retina of each eye and the cells
in the visual cortex.
Adjacent columns of cells
receive input from the same area
of the retina of both eyes.
This is repeated across the
whole visual cortex to build up a
„map‟ of the retina.
• The formation of the columns of cells is genetically
determined and not influenced by the environment.
• However there are periods of time during brain
development when the brain must obtain sufficient
stimulation from specific external experiences (i.e.
environment) to develop properly.
• These are known as critical windows in development.
• This must happen before a certain age; after this
window no wiring will occur and the brain loses the
ability to develop fully.
The “critical window” – Hubel & Wiesel’s proof
Permanently Hubel & Wiesel investigated the
blind critical window.
They used monkeys and kittens
in their studies
Their work permanently blinded
some animals and can be argued
to be unethical.
Hubel & Wiesel‟s Method:
•Raise monkeys from birth in three groups for 6 months
•Group 1 are the control (no blindfold), Group 2 are blindfolded in
both eyes, Group 3 are blindfolded in one eye (monocular
•Test the monkeys to see whether they can see using each eye
•Test the sensitivity of retinal cells
•Test the activity of nerves in the visual cortex in response to
• Monkeys in Group 2 (both eyes blindfolded)
had impaired vision
• Monkeys in Group 3 (monocular deprivation)
were blind in the deprived eye
• Retinal cells were responsive in all groups
• Cortical activity was reduced in parts of the
brain that process information from the
• Adults undergoing the same tests showed no
difference between groups. All could see.
There is a critical window for visual neural development,
which requires stimulus from the eye.
If this window is missed the monkey is blind, because of
events happening in the brain, not the eye.
In humans there is a burst of new synapse formation in
the visual cortex at 3-4 months and the maximum
density of synapses is reached between 4-12 months
See Activity 8.10
Q 8.22 p217.
• Kittens are born blind, with their eyes shut.
• Hubel and Wiesel tested kittens for the effects of
monocular deprivation at different stages of
development and for different lengths of time. They
• Deprivation at under 3 weeks had no effect
• Deprivation after 3 months had no effect
• Deprivation at four weeks had a catastrophic effect –
even if the eye was closed for merely a few hours.
• How can you explain these results?
• Because kittens are born blind, early
deprivation (under 3 weeks) would have no
• By 3 months connections to the brain have
been made, and deprivation has no effect
since the critical period has ended.
• The critical period is at about 4 weeks so lack
of stimulation from the kitten‟s environment
at this time severely affects visual
What happens during the critical period?
Neurones in the visual cortex
Refinement of columns in the visual
cortex produces the distinct pattern of
column driven by the left and right eyes
Columns that receive input from
light-deprived left eye are much
Dendrites and synapses from the
light-stimulated eye take up more
Mechanism of visual development in the cortex
See Q8.24 p 218. Rewrite the sentences in the correct order to explain
the mechanism for visual development.
What follows happens in the area of the visual cortex where neurones
from both eyes overlap.
1. There is a lack of visual stimulation in one eye.
5. Axons from the visually deprived eye do not pass impulses to cells
in the visual cortex so no neurotransmitter released.
3. Axons from the non-deprived eye pass impulses to cells in the
visual cortex so neurotransmitter released.
4. Synapses made by active axons are strengthened so release more
2. Inactive synapses are eliminated.
So for full development of the visual cortex nerve impulses from
both eyes, and neurotransmitter release from all neurones,
involved must occur
Impulses passing along neurone Fewer impulses passing along
P cause synapses 1 and 2 to neurone Q causes synapse 3
release neurotransmitter. to release less
neurotransmitter. It is
eventually lost if there are
many more impulses passing
along P than Q
Neurone Q no longer
synapses with cell Y so
The initial formation of the columns in the
visual cortex is genetically determined.
Visual stimulation is required for refinement of
the column and so for full development of the
visual cortex so refinement depends on
So full visual development is the result of
genotype + environment (i.e. „nature +
Background reading; see Weblinks
for Activity 8.10
Making sense of what we see
There are many types of neurones in the
visual cortex which respond to, and
interpret, nerve impulses from the
retina in different ways.
• Simple cells
Respond to bars of light
• Complex cells
Respond to edges, slits, angles of edges,
• However in order to make sense of the
image of what we see we need prior
knowledge and experience so the brain
can interpret the image.
Theories of pattern recognition
• How to investigate pattern recognition as
an instance of visual perception in
• Carry out Activity 8.11 and 8.12.
• Record how the investigations were
1. has a „template‟ for key features of patterns stored
in our long term memory, learned from past
• e.g. we learn the shapes of letters so we can
recognise and name them; we know the letter d is
called „dee‟ because that is what we have been told,
so when we see that shape we will call it „dee‟.
• We learn to recognise whole words in the same way.
Aoccdrnig to a rscheearch at an Elingsh
uinervtisy, it deosn’t mttaer in what oredr
the ltteers in a word are, the olny
iprmoetnt tihng is that the frist and lsat
letetr is at the rgihit pclae. The rset can
be a toatl mses and you can still raed it
wouthit a porbelm. This is bcuseae we do
not raed ervey lteter by istlef but the
word as a wlohe.
Wichh wrod avboe is not sepleld croertlcy?
There is actually more than one!
1st line/4th word (should be research)
4th line/1st word (should be important)
5th line/5th word (should be right)
2. makes „assumptions‟ about what it
expects to see, based on
– past experience
• See Qu 8.25 page 219
• Left = strawberry
• Right = pencil
What is this?
Stairs going upwards?
We probably perceive it as stairs going up because we read from left to right
Stairs going downwards?
Arabs perceive it as stairs going down because they read from right to left
Or just a pattern with 3 colours?
We perceive this a picture of stairs because we‟ve seen real stairs before in
three dimensions and our brain has stored that information so that when we
see a pattern of lines such as these the brain retrieves that information such
that we see this as „stairs‟, when in reality it is just a pattern of lines with 3
Which is what somebody who has never seen stairs would perceive it as.
Comment on Activity 8.11
Details of the complex patterns associated with
particular artists are learned by looking at
paintings. When work by an artist is viewed,
key style features are compared with those in
If paintings by the same artist have been seen
previously, when a new work is viewed there
will be the necessary neural pathways already
present for recognition of it‟s style features.
Comment on Activity 8.12
• How is a previously seen pattern recognised?
• Your brain compares images with previously
seen „templates‟. This involves breaking down
any new patterns or pictures into simple
shapes that your brain is able to recognise.
Each separate element is analysed individually.
• Your brain has separate groups of cells
programmed to respond to highly specific
stimuli such as horizontal lines, diagonal lines,
Possible mechanism of picture processing
Light falls on retina the first time the picture is seen
Nerve impulses to visual cortex
Image stimulates groups of cells in visual cortex
Pattern of stimulus created as synapses within a neural pathway are stored
Second time picture is seen light falls on the retina
Nerve impulses to visual cortex
Image stimulates groups of cells in visual cortex
Pattern of stimulus created is compared with previously seen patterns stored
in the memory
Recall of picture
• See Fig 8.43 on page 220
• Faces are recognised on the basis of prior experience. We see a face.
Certain complex cells – called face-recognition units - in the cortex are
stimulated by the lines and contours of the face. These form synapses with
other areas of the brain including the language and association areas. The
image is stored in the memory. When we see the same face again, we
may recognise it, as the necessary neural pathways are already there so
only a few visual clues are necessary. The more familiar the face (i.e. seen
often, so the image is reinforced) the fewer visual clues are required.
• Police Identikit system
• The sad case of Lincoln Holmes
What can you see?
The face or the trees and a woman walking?
Depth and distance perception
Depth perception is the visual ability to perceive the
world in three dimensions
The brain relies on clues to determine how near or far away an
For objects < 30m away
Both eyes look at an object from slightly different angles
Stereoscopic vision involves the use of both eyes. When
you focus on an object each eye has a slightly different
view of it. Your left eye tends to see a little more of the
left side of the object, while your right eye sees a little
more of the right side.
Your brain automatically uses this information, plus the
angle your eyes have to turn to focus on the object,
(feedback from stretch receptors in the muscles which
move your eyes) to supply you with an estimate of the
distance of the object.
Distant objects > 30 m away
For objects > 30 m away the images on our two retinas
are very similar so stereoscopic vision provides no
The brain relies on visual clues and past experience to
interpret how far away something is.
Visual clues e.g.
• Perspective (lines converge in the distance)
• Relative size of objects (although we need to
have seen the object before)
• Overlap of objects
Role of perspective and relative size for determining distance
Relative size clues
size as clues
What is the man pointing at?
Young children have little experience of the world in three-dimensions and
of depth clues. They would say he was pointing at the elephant.
We know better! Depths clues* from experience tell us that he is pointing
at the antelope.
•Familiarity with the relative sizes of the man, elephant and antelope.
•Overlap of one hill by another
Usually we are
unaware of the clues
being used. It is
only when these clues
are not present, or
when the brain is
misinterpret an image
(due to an overload
of conflicting clues)
that we become
aware of them.
Optical illusions rely
Are these two people the same or different
The clues are wrong but they are the same
Interpretation of images using visual clues
It is thought your brain
interprets straight lines
joined by acute and
obtuse angles as being
right angled shapes.
What is this?
You will see it as a cube.
In fact it is just a 2-D
drawing with acute and
obtuse angles joined by
• Interactive tutorial for Activity 8.14
The Muller-Lyer illusion
Which line is the longest?
They are both the same length.
This illusion is well-known - most of us
are aware that the vertical lines here Explanation
are actually the same size but that the
right-hand line appears to be
One hypothesis is that we live in a
„carpentered world‟ and our visual
experience is of straight lines and right
One explanation of why the right-hand
figure appears to be so much larger
involves interpreting the images in
The right-hand figure can be easily
interpreted as representing the inside
corner of a room whilst the arrow like
left-hand figure can be seen as the
outside corner of a building.
As an inside corner the right-hand
figure may appear to be nearer (and
therefore larger) than the outside
corner (because buildings are usually
viewed at a distance and is therefore
perceived as further away.
Are the diagonal lines
the same length?
People who live in a carpentered environment tend to interpret obtuse and
acute angles as right angles because they are used to seeing them in. for
example, the corners of rooms and buildings (the Muller-Lyer illusion). They
perceive lines with more acute angles at the top and bottom (like arrowheads)
as shorter – the argument is that it is the shape you see at the nearest edge of
a building to you, so you interpret the line as being shorter than it actually is.
Lines that have more obtuse angles at the top and bottom (the inside-out
umbrella shapes) are perceived as being longer than they actually are. This
shape corresponds to what you would see in the distant corner of a room. The
mind knows that in a carpentered world rooms are actually the same height in
every part of the room. Accordingly, a line with obtuse angles at its top and
bottom is “seen” as being longer than it really is.
rectangle is the
They are both
the same size!
Is depth perception innate or
• See p224.
• Babies and the “virtual cliff” (Gibson and Walk, 1960)
• Read and answer qu 8.31 – 8.34 (For 8.34 follow
Kenge‟s story through Chapter 8).
• Harry – don‟t cheat!
• 8.31. If the baby does have a perception of
depth, how will she react when invited to
crawl over the „edge‟ of the cliff?
• She will show symptoms of fear and distress,
may refuse, or cry. Young babies are very
reluctant to crawl over the „cliff‟ even when
their mothers encourage them to do so.
• This reaction is assumed to indicate that depth
perception is innate. Explain why.
• The baby has not experienced this before so cannot
have learned it.
• Argue the case for this not illustrating that depth
perception is innate.
• Some perceptual development has certainly taken
place since birth; the experiment requires the baby
to crawl and this isn‟t possible for several months.
• Explain if this supports or does not
support the idea that this type of
perception is not innate.
• The fact that these animals had had
little time for learning suggests that
the behaviour is innate
8.34. Can you suggest why Kenge, a
forest dweller, was fooled into thinking
that the buffalo on the horizon were
• The anthropologist Colin Turnbull described what happened in the
former Congo in the 1950s when a BaMbuti pygmy, used in living in the
dense Ituri forest (which had only small clearings), went with him to the
• And then he saw the buffalo, still grazing lazily several miles away, far
down below. He turned to me and said, 'What insects are those?'
• At first I hardly understood, then I realized that in the forest vision is
so limited that there is no great need to make an automatic allowance
for distance when judging size. Out here in the plains, Kenge was looking
for the first time over apparently unending miles of unfamiliar
grasslands, with not a tree worth the name to give him any basis for
• When I told Kenge that the insects were buffalo, he roared with
laughter and told me not to tell such stupid lies. (Turnbull 1963, 217)
• Because Kenge had no experience of seeing distant objects he saw them
simply as small.
Kenge was not used to vast open spaces and
would have had little opportunity to look far
into the distance; with no experience of
seeing distant objects he would have had
little experience of certain depth cues, such
as size constancy – the tendency to perceive
the object as being the same size no matter
how far away it is; as a result he saw the
buffalo simply as very small animals.
• There is a basic innate perception of
depth and distance but depth clues have
to be learned and refined (more
synapses formed) as a result of
• So depth perception is due to both
nature (i.e. genotype) and nurture (i.e.
• Animal; testing
• Also beep site
• Use activity 8.20