Visual Perception and Illusions
Lecture One
Seeing is not Believing
Why we cannot see reality
The eye: human eyes are unable to take in all information from the environment, the optic
nerve is thin and could not transfer all environmental cues
Colours: colour illusions can cause confusion e.g. ‘the dress’ picture
Size: surrounding information and different angles can make objects appear larger or smaller
than they are
Movement: some optical illusions can make us believe they are moving if they are not
Shape: different angles can sometimes show a shape that does not exist
Faces: it can be difficult to tell who people are when faces are morphed
Why?
There is too much information in the environment
If your eyes/brain recorded everything you saw it would use far too much energy and
resources
The brain already uses 20% of the bodies energy – it would be impossible to see everything
Solution
Compression means that only important information is transmitted
Don’t transmit things humans do not need to react to (not dangerous)
Changes are more important to attend to rather than things that remain constant
Changes across spaces causes us to see edges
Changes over time indicate new objects and objects that move
Some kinds of information are more important than others
Consequences
Very sensitive to sudden changes and movement
Very poor at detecting slow changes
High resolution for black and white
Low resolution for colour (do not need to see ultraviolet etc)
Good at comparing objects side by side
Poor at making judgements and comparisons from memory
Appearance of things can change over time
Past events change what we perceive presently
Surrounding context can affect what we perceive
Perception changes over time – evidence in the colour after-effect
We have many neurons that encode for primary colours (red, green, blue)
Temporal inhibitors turn off cells if they are active for a long time (e.g. staring at red square)
Temporal inhibitors are slow to work and inhibition takes time to build up and go away
So when red is removed after staring for some time, the white screen appears blueish
White = red + green + blue therefore when staring at red and then a white screen, the white
appears blue/green because the red is inhibited
,Context affects perception – evidence is simultaneous colour contrast
Orange against a red background can make it appear more yellow (e.g.)
This is because G cells detect green at different nearby locations, and spatial inhbitors turn
off their cells if their G neighbours are active
Spatial inhibition acts fast, coming on and disappearing quickly
Only cells on the border between two colours respond strongly, enabling us to see edges
Questions
Q – If the eye only transmits changes why doesn’t everything fade away?
A – If you keep your eye still long enough it can fade away!
Q – If the eye only transmits borders why do I see the thing as whole?
A – Some non-edge info is transmitted and the brain decodes the info from the eye to fill in the
blanks/even when it is not meant to be filled – because neuron excitation spreads among other
nearby neurons to prepare missing info
What happens when balance of inhibition and excitation goes wrong?
Visual stress, looking as though things are moving, migraine and epilepsy
Lecture 2
The Retina, LGN and Visual Pathways
The Retina
A layer of cells that is an outgrowth of the brain during development
Light enters through the pupil, (which size is affected by the cornea) and enters blood
vessels and cell bodies before reaching the photoreceptors, which send signals through the
optic nerve to the brain
Photoreceptors are cones or rods – cones are most concentrated in the fovea and deal with
colour (separate cones for blue, green and red wavelengths) and rods are responsible for
seeing in dim light
They have their own receptive fields – an area of light from a small part of space, the brain
must piece these together to create a whole image
Photoreceptors will respond when light falls on their receptive field
The blind spot is where blood vessels are highly concentrated
The retina only has a small area where there is high resolution (fovea) because if the whole
thing was high res, the blind spot would end up taking up the whole retina – even if every
photoreceptor was connected directly to the brain the blind spot would be huge
If all the cells in the retina were active all of the time the amount of energy would be huge –
which would require more blood vessels in the eye – which would result in a large blind spot
too
This means that the eye must only transmit important information e.g. movement and edges
, Orange = bipolar
Grey = retinal
ganglion cell
Ganglion cells receptive fields
Ganglion cells have cells that are called ‘on centre’ and ‘off centre’ cells
On centre cells give an excitatory response when light falls on the centre of the receptive
field, and an inhibitory response when light falls on the surrounding receptive field
Off centre cells are opposite – excitatory response for surrounding receptive field and
inhibitory response for the centre receptive field
When light covers the whole receptive field, there is little to no response as excitation and
inhibition cancel each other out
These cells allow us the clearly perceive edges, but can lead to some interesting effects
(illusions)
E.g. simultaneous contrast illusion – when two grey squares are placed on a background of
white fading to dark grey, the square above looks darker than what it is
This is because a ganglion cell with an on centre looking at the light side will cause the off-
surround to turn the cell off (inhibition)
Meanwhile the on-centre cells looking at the dark side will not be so inhibited, so this end
appears lighter
This also means in all grey images, edges where there is a lighter colour are enhanced and
appear different shades of grey, whereas areas of no change appear the same grey
Can be seen in the Craik-O’Brien-Cornsweet illusion
The Troxler fading effect – inhibition over time causes neurons to reduce activity, and the
lack of crisp edges means that spatial location is not signalled well either
Reduced signal due to inhibition and lack of edges allows filling in of grey regions by the
cortex, thus If a stimulus stays on for a long time it will appear to fade away
And when the stimulus is removed the time lag on inhibition produces negative after image
The Lateral Geniculate Nucleus (LGN) to the cortex
Signals are sent from the retina to the optic nerve, before some information is transferred in
the optic chiasm
After this, it travels down the optic tract to the LGN (and other areas)
The LGN contains different cells that have different roles
Magnocellular (large ganglion cells) are mostly responsible for motion and depth perception,
and are dominant in peripheral vision and coarse detail
Lecture One
Seeing is not Believing
Why we cannot see reality
The eye: human eyes are unable to take in all information from the environment, the optic
nerve is thin and could not transfer all environmental cues
Colours: colour illusions can cause confusion e.g. ‘the dress’ picture
Size: surrounding information and different angles can make objects appear larger or smaller
than they are
Movement: some optical illusions can make us believe they are moving if they are not
Shape: different angles can sometimes show a shape that does not exist
Faces: it can be difficult to tell who people are when faces are morphed
Why?
There is too much information in the environment
If your eyes/brain recorded everything you saw it would use far too much energy and
resources
The brain already uses 20% of the bodies energy – it would be impossible to see everything
Solution
Compression means that only important information is transmitted
Don’t transmit things humans do not need to react to (not dangerous)
Changes are more important to attend to rather than things that remain constant
Changes across spaces causes us to see edges
Changes over time indicate new objects and objects that move
Some kinds of information are more important than others
Consequences
Very sensitive to sudden changes and movement
Very poor at detecting slow changes
High resolution for black and white
Low resolution for colour (do not need to see ultraviolet etc)
Good at comparing objects side by side
Poor at making judgements and comparisons from memory
Appearance of things can change over time
Past events change what we perceive presently
Surrounding context can affect what we perceive
Perception changes over time – evidence in the colour after-effect
We have many neurons that encode for primary colours (red, green, blue)
Temporal inhibitors turn off cells if they are active for a long time (e.g. staring at red square)
Temporal inhibitors are slow to work and inhibition takes time to build up and go away
So when red is removed after staring for some time, the white screen appears blueish
White = red + green + blue therefore when staring at red and then a white screen, the white
appears blue/green because the red is inhibited
,Context affects perception – evidence is simultaneous colour contrast
Orange against a red background can make it appear more yellow (e.g.)
This is because G cells detect green at different nearby locations, and spatial inhbitors turn
off their cells if their G neighbours are active
Spatial inhibition acts fast, coming on and disappearing quickly
Only cells on the border between two colours respond strongly, enabling us to see edges
Questions
Q – If the eye only transmits changes why doesn’t everything fade away?
A – If you keep your eye still long enough it can fade away!
Q – If the eye only transmits borders why do I see the thing as whole?
A – Some non-edge info is transmitted and the brain decodes the info from the eye to fill in the
blanks/even when it is not meant to be filled – because neuron excitation spreads among other
nearby neurons to prepare missing info
What happens when balance of inhibition and excitation goes wrong?
Visual stress, looking as though things are moving, migraine and epilepsy
Lecture 2
The Retina, LGN and Visual Pathways
The Retina
A layer of cells that is an outgrowth of the brain during development
Light enters through the pupil, (which size is affected by the cornea) and enters blood
vessels and cell bodies before reaching the photoreceptors, which send signals through the
optic nerve to the brain
Photoreceptors are cones or rods – cones are most concentrated in the fovea and deal with
colour (separate cones for blue, green and red wavelengths) and rods are responsible for
seeing in dim light
They have their own receptive fields – an area of light from a small part of space, the brain
must piece these together to create a whole image
Photoreceptors will respond when light falls on their receptive field
The blind spot is where blood vessels are highly concentrated
The retina only has a small area where there is high resolution (fovea) because if the whole
thing was high res, the blind spot would end up taking up the whole retina – even if every
photoreceptor was connected directly to the brain the blind spot would be huge
If all the cells in the retina were active all of the time the amount of energy would be huge –
which would require more blood vessels in the eye – which would result in a large blind spot
too
This means that the eye must only transmit important information e.g. movement and edges
, Orange = bipolar
Grey = retinal
ganglion cell
Ganglion cells receptive fields
Ganglion cells have cells that are called ‘on centre’ and ‘off centre’ cells
On centre cells give an excitatory response when light falls on the centre of the receptive
field, and an inhibitory response when light falls on the surrounding receptive field
Off centre cells are opposite – excitatory response for surrounding receptive field and
inhibitory response for the centre receptive field
When light covers the whole receptive field, there is little to no response as excitation and
inhibition cancel each other out
These cells allow us the clearly perceive edges, but can lead to some interesting effects
(illusions)
E.g. simultaneous contrast illusion – when two grey squares are placed on a background of
white fading to dark grey, the square above looks darker than what it is
This is because a ganglion cell with an on centre looking at the light side will cause the off-
surround to turn the cell off (inhibition)
Meanwhile the on-centre cells looking at the dark side will not be so inhibited, so this end
appears lighter
This also means in all grey images, edges where there is a lighter colour are enhanced and
appear different shades of grey, whereas areas of no change appear the same grey
Can be seen in the Craik-O’Brien-Cornsweet illusion
The Troxler fading effect – inhibition over time causes neurons to reduce activity, and the
lack of crisp edges means that spatial location is not signalled well either
Reduced signal due to inhibition and lack of edges allows filling in of grey regions by the
cortex, thus If a stimulus stays on for a long time it will appear to fade away
And when the stimulus is removed the time lag on inhibition produces negative after image
The Lateral Geniculate Nucleus (LGN) to the cortex
Signals are sent from the retina to the optic nerve, before some information is transferred in
the optic chiasm
After this, it travels down the optic tract to the LGN (and other areas)
The LGN contains different cells that have different roles
Magnocellular (large ganglion cells) are mostly responsible for motion and depth perception,
and are dominant in peripheral vision and coarse detail
