Vision 8)

Question 1

What is the motion after effect?

Answer 1

The motion after effect causes a visual stimulus to undergo apparent motion

Also known as the waterfall effect

Question 2

What two key properties of the sensory system does the motion after effect demonstrate?

Answer 2

1. FEATURE DETECTION
   The brain has specific neurons or circuits of neurons specialised for detecting particular features of the sensory world such as edges, colours, lines in particular orientations, movement in different directions, faces
   Illusions often trick theses feature detection systems. Nervous system is designed to detect particualr features
2. ADAPTATION
   Mostly interested in changes in your environment, so where a feature remains constant (even if this feature is movement), neural signals are ‘dampened down’ (they are no longer important). Unless its new information you become undersensitised to it or less aware of it

Question 3

What is the goal of the visual system?

Answer 3

To build a predictive model of the external world based on incident light (the pattern of light that falls on the retina)

Question 4

What matters in vision?

Answer 4

Features that have value for survival e.g. edge detection, threatening nature of stimuli

Question 5

What part of the eye regulates light levels?

Answer 5

The iris- donut-shaped band of contractile tissue that regulates the amount of light that reaches the retina by adjusting the size of the pupil

The pupil- pupillary constriction/dilation is controlled by the amount of the light entering the eye (pupillary light reflex)

Question 6

What are light levels?

Answer 6

Amount of light that enters the eye
-not to bright or too dark

perceptually- what appears to be around twice as bright is actaully around 10x as bright

Question 7

What happens in pupil dilation?

Answer 7

The muscles in the iris relaxes and the pupil dilates

More light enters the eye so sensitivity is improved but acuity is poorer

Question 8

What happens in pupil constriction?

Answer 8

The muscle in the iris contracts and the pupil gets smaller

Less light enters the eye but the image on the retina is sharper so acuity is improved

Question 9

How does the iris affect acuity? (lack of blurring)

Answer 9

There is a trade off between pupillary dilation and image acuity (precision)

In pupil dilation- more light can enter the eye but each point projects to a larger area on the retina/sensor, and these areas overlap, creating blurring

In pupil constriction- less light is able to enter the eyes and each point projects to a smaller and more discrete area on the retina/sensor, with less blurring

Question 10

What parts of the eye ensure a sharp and focussed image?

Answer 10

The transparent cornea- covers the front of the eye and along with the lens, helps to focus incoming light - the cornea contrubutes to 75% of the eyes focussing power, but its focus power is fixed

The lens- sits behind the pupil and works with the cornea to focus the incoming light on the retina

each lens is held in place by suspensory ligaments called zonules

The lens can change shape to change the focal distance of the eye, a process called accommodation

Question 11

How does accommodation help close vision?

Answer 11

During near accommodation, the ciliary muscles contract (i.e., shorten), which relaxes the zonule and rounds the lens (i.e., thickens it). This brings the near object into focus.

Question 12

How does accommodation help far vision?

Answer 12

During far vision, the ciliary muscles relax, the zonule stretch, and the lens flattens.

Question 13

What is the difference between a rounder lens and a flatter lens?

Answer 13

A rounder lens is a thicker/stronger lens
This bends the light more as it enters the eye and reduces the focal distance, as required for near objects to create a sharp ‘image’ on the retina

A flatter lens is a thinner/weaker lens
This increases focal distance, as required for far objects to create a sharp image on the retina

Question 14

What part of the eye converts light into neural signals?

Answer 14

The retina

light=raw material for a neural model of the visual field

The retina is a thin, light sensitive tissue lining the back of the eye. It contains a layer of photoreceptive cells that convert the light into neural signals

Question 15

What is the macula and what is macula degeneration?

Answer 15

At the centre of the retina is an area called the macula
It contains a very high concentration of photoreceptor cells
The very centre of the macula is called the fovea, the site of our sharpest vision
Age-related macular degeneration – loss of central vision, blurring of central vision, visual loss is whereever you look

Question 16

What is the fovea?

Answer 16

Is a small depression within the neurosensory retina where visual acuity is the highest. The fovea itself is the central portion of the macula, which is responsible for central vision

The point of highest visual acuity- when you fixate on something you are positioning that something on the fovea

No rods, only cones, in very high density

Blood vessels and other cells are displaced to the side, minimizing distortion of light hitting the photoreceptors

Around 50% of the nerve fibres in the optic nerve are supplied by the fovea

Question 17

What is the blind spot?

Answer 17

The blind spot – ganglion cell axons leave retina in optic nerve, where there are no photoreceptors
The brain uses completion, whereby it uses information provided by receptors around the blind spot to fill in the gap

Question 18

What are rods?

Answer 18

A photoreceptor

Rods are sensitive in dim/low light
But very few around fovea
This explains why stars seem to disappear when you look directly at them

They are concentrated in the outer areas of the retina and give us peripheral vision
More sensitive to light than cones
High convergence: many rods connected to one bipolar cell so poor acuity but good sensitivity
Only one type so monochromatic (greyscale)

Question 19

What is the sclera?

Answer 19

‘the white of the eye’ - a tough protective layer of connective tissue

Question 20

What is the choroid?

Answer 20

Layer of tissue between the retina and sclera

The choroid contains many blood vessels and is critical for providing oxygen and glucose to the retinal cells

Question 21

What is the difference between red eye and reflective eye?

Answer 21

The flash from a camera is reflected off the blood in the choroid and back through the pupil

Many nocturnal animals have a layer of reflective tissue, called the tapetum lucidum

Question 22

Why is the retina of vertebrates inverted?

Answer 22

Various theories, including that it is actually a space saving solution - eyes would need to be larger to enable light to be focussed on photoreceptors if other way around

Question 23

What are the three main layers of cells in the retina?

Answer 23

Photoreceptors
Bipolar cells
Retinal ganglion cells

There are also a number of interneurons, including horizontal cells and amacrine cells

Question 24

What are photoreceptors?

Answer 24

They convert light into neural signals (visual transduction)

There are 2 main types: rods and cones

Question 25

What are cones?

Answer 25

Cones are a type of photoreceptor cell in the retina. They give us our color vision. Cones are concentrated in the center of our retina in an area called the macula and help us see fine details.

They are photopic (well-lit) vision
Low convergence- each one connects to one bipolar cell so good acuity but poor sensitivity
Three types of cone (boradly referred to as red, green, blue) so responsible for colour
Optimised for central vision

The majority of cones are crammed into the fovea, giving it high acuity, colour vision

Question 26

What is the photoreceptor function of rods?

Answer 26

In the dark, rhodopsin is inactive, sodium channels are kept open by cyclic GMP, cell is depolarised, rods continally release glutamate

So when the rods are signalling dark, what they’re actially doing is continuouslly releasing glutamate to signal dark and that causes a bipolar cell to become hyperpolarised which is where that signal is inverted and you dont get onward propagation of signal in the dark. Signal as though they are an excited cell in the dark.

Question 27

What is the photoreceptor function of cones?

Answer 27

In the light rhodopsin is active, cyclic GMP is broken down and sodium channels close, the cell is hyperpolarised and glutamate release is reduced

This leads the bipolar cell, the next cell in the line to depolarise and will signal on to the retinal ganglion cells. Almost like a double inhibitory circuit, where you’ve got the lights inhibiting the inhibition of the bipolar cell

Question 28

What affects rods/cones sensitivity to light?

Answer 28

Rods/cones are sensitive to light by different amounts depending on the wavelength (colour)

A single photoreceptor provides only greyscale information, cannot differentiate between wavelength and intensity
BUT
Two or more photoreceptors allow you to differentiate wavelength from intensity

Accurate resolving of colour information is done with more than two cones although 2 cones alone is sufficient for a good job

Question 29

What is colour blindness?

Answer 29

Colour blindness rarely means colour blind, rather a range from near normal colour vision to monochromacy

In all cases, it relates to either an altered sensitivity in one of the cones or an absence of one of the cones

Deuteranomoly is the most common, where the sensitivity of the ‘green’ cones is shifted towards the red, making resolution of reds and greens difficult

Question 30

How can rich perception of the world be a product of photopic vision that is restricted to a few degrees of visual angle?

Answer 30

Eye movement data
The eye continually scans the visual field, with the fovea making three fixations per second.
The visual system then integrates this information to produce a wide-angled, high acuity, coloured perception

Question 31

What are bipolar cells?

Answer 31

Process input from the photoreceptors and output to the retinal ganglion cells
Allow some low-level signal processing to occur in the retina, aided by the interneurons (horizontal and amacrine cells)
Photoreceptors converge via bipolar cells onto retinal ganglion cells
Few-to-one convergence for cones (maintain excellent resolution), many-to-one convergence for rods (maintain excellent sensitivity)

Question 32

What are retinal ganglion cells?

Answer 32

These cells are wired up to bipolar cells and photoreceptors =detection of edges in images
They detect ‘spots’ of contrasting illumination

The weighting of the signals of bipolar cells as the feed into retinal ganglion cells is altered so that you’ve got
a negative weighting at the surrounding region and a positive weighting at the mid region, or can be the other way around

When light hits the surrounding parts of the disk, it inhibits the retinal ganglion cell, when you have light hitting the centre of the disk region, you get excitement of the retinal ganglion cell. If light hits both parts, both the centre and the surround, you have a bit of both where exactly what happens depends on the tuning of the retinal ganglion cell

These cells act as a way of optimally detecting a spot of light, we get the most excitment in a retinal ganglion cell by having illumination in the centre but actually darkness around the outside, the retinal ganglion cell really likes that (called ‘on’ centre ‘off’ surround) but we can also have ‘off’ centre ‘on’ surround whereby they would only like the darkness in the middle and the illumination around the outside.

Either way we have something that is detecting spots of contrasting illumination and that is foundational processing for the visual system

Question 33

What are Mach Bands?

Answer 33

The apparent change in lightness between the bands is an illusion caused by lateral inhibition

Mediated by horizontal cells, which when activated inhibit the other cells they contact

This ‘artificially’ enhances the difference between areas of more and less illumination

E.g. for an on-centre/off-surround retinal ganglion cell
-> Activation of central region causes inhibition of cells providing input from surrounding region, thus amplifying the centre-surround difference
->When you look at the boundary between two shades, the difference appears larger than it really is
->Occurs due to the function of retinal ganglion cells to optimise contrast- they are overemphasising the contrast between the two shades

Question 34

Why is sensing contrast important?

Answer 34

Maximising detection of contrast in lightness provides the basis for edge detection – a key component of the information in an image.

Importance of vision-Taking a complex visual scene and breaking it down into simpler elements and then somewhere in the brain it is reconstructed to create a whole perceptual experience

Question 35

What is a key mechanism in retinal processing?

Answer 35

Lateral inhibition

When light hits the ommatidia receptors fire at a rate proportional to the intensity of the light
The more they fire, the more they inhibit their neighbours via lateral inhibition

This lateral inhibition causes the contrast enhancement in the Mach bands

Question 36

What is the Hermann Grid illusion

Answer 36

Grey blobs at intersection (but disappear when intersection foveated)
Illusion of blobs is a consequence of ‘lateral-inhibition’ – the intersection is surrounded by more activated receptive fields

When you foveate on an intersection, you are positioning this onto a part of the retina cells with much small receptive fields
-> the surround-inhibition effect no longer produces the illusory grey blobs

Question 37

What is the Scintillating Grid?

Answer 37

Multiple overlying receptive fields with these three different levels of intensity so we get a dark spot within the white spot basically

Amplifies relative difference.

Horizontal cells responsible – they effectively reverse the sign of their input

Question 38

What is the Koffka Ring Illusion?

Answer 38

When the inner circle is no longer directly attached, the shade of grey changes across the two sides despite the fact that the inner circle remains the same shade of grey the whole time

Question 39

Where do visual pathways project to?

Answer 39

Projections occur contralaterally- so the wiring of the retina is set up so that regardless of which eye is doing the sensing of the visual scene, all the visual information from the left part of the visual world goes to the right side of the brain and vice versa

Retinal ganglion cells sensitive to light in the left and right visual fields have different projections in the optic nerve
They separate at the optic chiasm so that information from the left and right field projects contralaterally to the superior colliculus and visual cortex

Question 40

What are visual pathways carrying information from the eye?

Answer 40

(1) Projections to the brainstem accessory optic and pre-tectal nuclei responsible for visual reflexes – accommodation, vergence, pupillary control
(2) The ancient (pre-cortical expansion - mammals) retino-tectal projection to the superior colliculus orients the head and eyes towards or away from unexpected events
(3) In mammals, especially humans the retino-geniculate-striate pathway is by far the largest and provides the input for complex scene analysis and object identification

Question 41

What is pupillary light control?

Answer 41

1. light is shined on the right eye only
2. action potentials from right eye reach both right and left pretectal nuclei
3. the prectectal nuclei stimulate both sides of the Eddinger-Westphal nucleus even though the light was only perceived in the right eye
4. the right and left sides of the Eddinger-Westphal nuceli generate action potetnials through the right and left occulomotor nerves, causing both pupils to constrict

Its essentially a little feedback circuit to come back and control the muscles of the iris, so as light hits one eye you get a consentual light reflex where it goes back via these two nuclei to cause constriction of theb pupil in both eyes
The presence of that reflex and its normal operation is an important indicator in terms of potential brain damage. Damage may mean that whne you shine light on the eye you won’t get a consentual light reflex in either eye or in one or the other.

Question 42

What is the superior colliculus pathway?

Answer 42

Information from the retina also gets branched off and goes to the superior colliculus
The SC is important for low level visual processing
The ancient (pre-cortical expansion) retino-tectal projection to the superior colliculus orients the head and eyes towards or away from unexpected events (survival value)
The SC can also spot something small and moving in front of you

The retino-tectal projection to the superior colliculus
-> Evolutionary ancient >500 M years – Main visual system in phylogentically lower species – fish, reptiles, amphibia
Layers of the SC contain Retinotopic mapping of the environment which maintains spatial topography
Sensorimotor properties of colliculus initiates initial gaze-shifts towards/away from sudden visual events

Question 43

What is the retino-geniculate striate pathway?

Answer 43

The pathway that we would consider as giving us our conscious visual experience.

This pathway takes visual information from the retina to lower layer IV of the primary visual cortex (V1) via the lateral geniculate nuclei (LGN) of the thalamus

Sensitivity from the visual fields is crossing as necessary at the optic chiasm and then you’ve got this relay of the lateral geniculate nucleus

Spatial mapping is preserved thorughout this pathway whihc means that if damaged it creates dark spots in the vision
The pathways are maintained retinotopically throughout the whole system

Question 44

What is retinotopic mapping?

Answer 44

The retino-geniculate-striate system is retinotopic: each relay within the system is organised according to a spatial map of the retina
Thus two stimuli presented to adjacent regions of the retina excite adjacent neurons at all levels of the system
Note – a large proportion of the visual cortex(~25%) analyses input from a small proportion of retina (fovea)
This principle is in line with the mapping of motor control, mapping of sensation areas, based on fucxntional representation which is guided by functional requirements in evolution

Question 45

What are the projections from right and left visual fields?

Answer 45

All signals from the left visual field (red/blue) reach the right primary cortex via the:
Temporal hemiretina of the right eye (ipsilaterally)
Nasal hemiretina of the left eye (contralaterally)
The opposite is true for the right visual field
Binocular overlap – depth perception

Its a map for left and right with the exception of the fovea for central vision in which there is some overlap

Question 46

What happens when there is damage to the visual pathways?

Answer 46

Reduced field of vision can occur as a result of brain damage

Determining specifics of visual field deficit can be a useful clinical tool to indicate approximate location of damage

Question 47

Lateral geniculate nucleus: thalamic relay. What are the P & M layers?

Answer 47

Parvocellular layers- Small cell bodies,
Responsive to colour, fine detail and to stationary or slow moving objects – scene analysis, object identification
Cones provide majority of input
(overall fine detailed visual information)

Magnocellular layer- Large cell bodies,
Particularly responsive to luminance change – on, off movement
Rods provide the majority of the input
Similar properties to the superior colliulus
(overall luminence change and movement) bit of redunduncy here- we already the the superior colliculus system and the cortical system bolted on top do we need this layer?

These different layers are the start of separation of different types of visual information from a visual scene

Question 48

What are receptive fields?

Answer 48

The receptive field of a visual neuron is the area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron

Visual system neurons tend to be spontaneously active so by influence we mean increase or decrease the firing rate

Question 49

What did Hubel and Wiesel find about Receptive Fields in the Retino-Geniculate-Striate system?

Answer 49

The receptive fields of the neurones representing the foveal area were smaller than those in the periphery
They tend to be circular
They tend to be monocular – from one eye only
Many had receptive fields that comprised an excitatory area and an inhibitory area

However, when investigating responses in visual cortex, neurons here did not seem to respond to small spots in the way that retinal and LGN cells did.
But
Stimulus presentation revealed that these cells were very interested in bars of light (or dark…i.e. contrast) and at specific orientations.

V1 responds to more complicated visual stimuli than points of light
The receptive fields of most V1 neurons fall into two categories:
Simple cells
Complex cells

This lead to the characterisation of simple cells and complex cells

Question 50

What are simple cells?

Answer 50

Simple Cells have antagonistic on and off regions
However, the borders between the on and off regions are straight rather than circular (they detect edges)
This means they respond best to bars of light in a specific orientation at a specific location in the visual field

Question 51

What are complex cells?

Answer 51

Complex cells are more common than simple cells (75% of V1)
Complex cells do not have static on / off regions: they respond to a particular straight-edge stimulus of a particular orientation regardless of its position within the receptive field
E.g. 45 deg bar of light
Some complex cells respond optimally to movement of the bar in a specific direction

Get weaker responses if they are in a non-optimal orientation

Question 52

What is the cortical organisation?

Answer 52

In one dimension we have orientation columns so we can move an electrode in one particular direction and find cells that are responsive to a progressive range of line orientsations

In another dimesnsion we have left eye/right eye sensitivity
->Occular dominance columns seem to be important to 3D vision- if you have sensitivty to something in the visual field with one eye, and sensotivty in the visual field to soemthing in exactly the same spot with the other eye, those images are going to look very slightly different and if you put that data side by side, in theory it should allow us to gain information about the 3D nature of the visual world

V1 neurons are grouped in functional vertical columns (i.e. at right angles to cortical layers)
All cells in a column have a receptive field in the same area of the visual field
All simple and complex cells in a column prefer straight-line stimuli in the same orientation

In contrast, if you move an electrode along a horizontal track the spatial location of the receptive fields of the neurons at the tip shift systematically
Furthermore, the preferred orientation of the neurons at the tip shifts systematically

The neural signal flows from neurons with simpler receptive fields to those with more complex receptive fields
Specific hierarchy: from on-centre and off-centre to simple cells and from simple cells to complex cells
Higher complex receptive field properties required for scene analysis ‘constructed’ from lower simpler ones

Question 53

What is extrastriate visual cortex?

Answer 53

Higher level information is extra to the striate

As information passes forward through the visual hierarchy, the complexity of the neural representation increases
V2: similar to V1 but more complex shape characteristics
V3: form, motion, depth
V4: colour, form, stimulus saliency, attention
V5/MT: motion

Different stimulus qualities are processed in spatially separated cortical areas the perceptual binding problem

Question 54

What are the dorsal and ventral streams?

Answer 54

One unifiying theme is the way thaat visual information leaves the primary visual cortex is two pathways:

The ventral stream (linked to memory) travels to the temporal lobe and is involved in scene analysis and object identification (the “what” stream)

The dorsal stream travels to the parietal lobe and processes spatial locations (the “where” stream).