Vision

Question 1

What do visual illusions such as the motion after effect tell us about our sensory systems?

Answer 1

1. Feature detection = the brain has specific neurons or circuits of neurons specialised for detecting particular features of the sensory world such as colours, movement in particular directions, faces - illusions often trick a specific ‘feature detection’ system
2. Adaptation = the brain is mostly interested in changes in your environment so where a feature remains constant (even if the feature is movement), neural signals are ‘dampened down’

Question 2

Explain the anatomy of the eye

Answer 2

The eye is a fluid filled sphere enclosed by 3 layers of tissue:

Inner most layer = the retina - contains neurons that are sensitive to light and transmit visual signals to central targets

Adjacent layer = uveal tract - contains the retinal photoreceptors and contains light absorbing pigment melanin in the pigment epithelium

Outermost layer = sclera - contains tough white fibrous tissue
Cornea - highly specialised transparent tissue that permits light rays to enter the eye

Cillary body is a ring of tissue that encircles the lens and consists of a muscular component which adjusts the refractive power of the lens and a vascular component that produces fluid that fills the front of the eye

Question 3

What controls the amount of light entering the eye?

Answer 3

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

Pupillary constriction/dilation controlled by amount of light entering eye (pupillary light reflex)

Question 4

How does light affect pupil dilation/constriction?

Answer 4

When its dark the muscles in the iris relax and the pupil dilates - more light enters the eye so sensitivity is improved but acuity is poorer

When its bright the muscle in the iris contracts and the pupil gets smaller - less light enters the eye but image on the retina is sharper so acuity is improved

Question 5

How does the iris affect acuity?

Answer 5

When the pupil is larger when its bright, more light enters the eye but projects to a larger area on the retina and these areas overlap which creates blurring

When the pupil is smaller in the dark, less light enters the eye and each point projects to a smaller and more discrete area on the retina which creates less blurring

Question 6

What is accommodation?

Answer 6

Dynamic changes in the shape of the lens to adjust the focal length of the lens by changing the curvature of the lens

Question 7

What focuses the image?

Answer 7

The cornea covers the front of the eye and along with the lens helps to focus incoming light
The cornea contributes to approx 75% of the eyes focusing power but its focus is fixed

The lens sits behind the pupil and can change shape to change the focal distance of the eye which is called accommodation

Question 8

What holds the lens in place?

Answer 8

Suspensory ligaments called zonule fibres

Question 9

Explain how accommodation works

Answer 9

For close vision =
1. The ciliary muscle contracts
2. The ciliary body with inserted zonula fibres moves closer to the lens
3. The tension is reduced and the lens rounds up

For distant vision =
1. The ciliary muscle relaxes
2. The ciliary body with inserted zonula fibres moves aways from the lens
3. The tension of the zonula fibres increases and the lens flattens

A rounder lens is a thicker/stronger lens - bends the light more and reduces focal distance
A flatter lens is a thinner/weaker lens - increases focal distance

Question 10

How is light transduced into neural code?

Answer 10

The retina contains a layer of photoreceptive cells that convert light into neural signals

Question 11

What is the macula?

Answer 11

Centre of the retina - contains very high concentration of photoreceptor cells

Centre of the macula is the fovea - the site of sharpest vision

Question 12

What is macular degeneration?

Answer 12

Age-related condition resulting in the loss of central vision

Question 13

What sort of photoreceptors are found in the fovea?

Answer 13

Very high density of cones but NO rods

Question 14

What are rods and cones and what are their functions?

Answer 14

2 types of photoreceptor
Rods are best in dim light but very few in the fovea which explains why stars seem to disappear when you look directly at them
Cones require a lot more light and are used to see colour

Question 15

What is the blindspot?

Answer 15

Area at the back of the eye called the optic disc where the optic nerve leaves the eye - there are no cones or rods

Question 16

What is the choroid?

Answer 16

A layer of tissue between the retina and the sclera
The choroid contains many blood vessels and is critical for providing oxygen and glucose to the retinal cells

Question 17

What causes red eye in photos?

Answer 17

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

Question 18

What causes reflective eye?

Answer 18

Many nocturnal animals have a layer of reflective tissue called tapetum lucidum which helps vision in low light conditions

Light bounces of the reflective tapetum lucidum and is given a second chance at hitting the photoreceptors

Question 19

What are the 3 main layers of cells in the retina?

Answer 19

1. Photoreceptors
2. Bipolar cells
3. Retinal ganglion cells

Also a number of interneurons including horizontal cells and amacrine cells

Question 20

How do rods work?

Answer 20

In the dark
- Rhodopsin (G-protein coupled protein receptor) is inactive
- Sodium channels are kept open by cyclic GMP
- Cell is depolarised so rods continually release glutamate, hyperpolarising the bipolar cell

In the light
- Rhodopsin active
- Cyclic GMP broken down and sodium channels close
- Cell is hyperpolarised and glutamate is reduced, depolarising the bipolar cell

Question 21

How do rods and cones detect light of different wavelengths (colours)?

Answer 21

Rods of the retina are sensitive to the intensity of light but cannot distinguish between different wavelengths of light
Cones are the colour-sensing cells on the retina

A single photoreceptor provides only greyscale information
Two or more photoreceptors allow you to differentiate wavelength from intensity
3 different types of cones - blue, green and red each with different wavelengths
If you only had blue cones, a dim light and a bright light could activate the cone to the same amount

Question 22

What are bipolar cells?

Answer 22

One of the main retinal interneurons = Process input from the photoreceptors and output to the retinal ganglion cells
Allow low-level signal processing to occur in the retina aided by the interneurons
Photoreceptors converge via bipolar cells onto retinal ganglion cells

Question 23

What are retinal ganglion cells?

Answer 23

These cells are wired up to bipolar cells and photoreceptors in such a way that they facilitate the detection of edges in images
They detect spots of contrasting illumination

Question 24

Explain on off surround retinal ganglion cells

Answer 24

ON-centre cells are stimulated by illumination of their receptive field centre
OFF-centre cells are inhibited by illumination of their receptive field centre

Question 25

What is the mach band illusion and what causes it?

Answer 25

The Mach band illusion exaggerates the contrast between edges of slightly differing shades of grey when touching each other

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

Activation of central region causes inhibition of cells providing input from surrounding region, thus amplifying the centre-surround difference

Question 26

Explain lateral inhibition in horseshoe crabs

Answer 26

Horseshoe crabs contain large visual receptors called ommatidia
The axons of the ommatidia are interconnected by a lateral inhibitory neural network
When light hits the ommatidia they fire at a rate proportional to the intensity of the light
The more they fire, the more they inhibit their neighbours via lateral inhibition

Question 27

Explain how lateral inhibition works in the human eye

Answer 27

The capacity of excited neurons to reduce the activity of neighbouring neurons
Say there are 6 receptors = ABCDEF
ABC receptors are in the light, DEF receptors are in the dark
Receptor C fires more than receptors A and B because it receives the same light intensity but does not get inhibited by the neighbouring receptor D

Similarly, receptor D fires less than receptors E and F because it receives the same light intensity but is strongly inhibited by receptor C

Question 28

What other illusion is caused by lateral inhibition?

Answer 28

The Hermann Grid effect

Question 29

How does the Hermann grid illusion work?

Answer 29

Grey blobs appear at the intersection of the grid but disappear when you foveate on them
Illusion of blobs are the consequence of lateral inhibition - the intersection is surrounded by more activated receptive fields

When you look at the intersection it falls directly onto your fovea which has very little lateral inhibition due to the small receptive field and high density cones
However, at the peripheral intersections most of the receptive field is flooded with white light causing strong lateral inhibition making the intersection appear grey

Question 30

Why do the blobs disappear when you foveate?

Answer 30

When you foveate on the intersection, you are positioning this onto a part of the retina cells with a much smaller receptive field
Therefore, the surround-inhibition effect no longer produces the grey blobs

Question 31

Is the visual pathway contralateral or ipsilateral?

Answer 31

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

Question 32

How do retinal ganglion cells exit the retina?

Answer 32

Retinal ganglion cells exit the retina through a circular region in its nasal part called the optic disk, where they bundle together to form the optic nerve

Axons in the optic nerve project to the optic chiasm where approx 60% cross and the other 40% continue towards the thalamus on the ipsilateral side

After the optic chiasm, the axons form the optic tract/radiation containing fibres from both eyes

Question 33

What are the main projections of the retinal ganglion cells and what is the function?

Answer 33

1. Retino-geniculate striate pathway = largest pathway, input for complex scene analysis and object identification
   Visual info from retina to the lateral geniculate nucleus to the primary visual cortex
2. Pretectum (between thalamus and the midbrain) = accessory, optic pretectal nuclei responsible for visual reflexes = accommodation, vergence, pupillary
3. Retino-hypothalmic pathway = major target is the suprachiasmatic nucleus the pacemaker for circadian rhythms
4. Retinotectal pathway = coordinates head and eye movements to visual targets - projects to the superior colliculus

Question 34

Explain the pathway responsible for pupillary light reflex

Answer 34

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

Question 35

Explain the retino-tectal pathway

Answer 35

Function = orienting the head and eye movement towards or away unexpected stimuli

Retino-tectal projection to the superior colliculus
Sensorimotor properties of the colliculus initiates the initial gaze-shift towards/away sudden events

Neurons sensitive to luminance change - appear, disappear, move

Question 36

Explain the retino-geniculate striate pathway

Answer 36

1. Neurons in the lateral geniculate nucleus send their axons to the cerebral cortex via the internal capsule
2. These axons pass through a portion of the internal capsule called the optic radiation and terminate in the primary visual cortex or striate cortex which lies largely along and within the calcarine fissure in the occipital lobe

Question 37

What is meant by retinotopic mapping?

Answer 37

Retinotopic mapping is the mapping of visual input from the retina to the neurons
The relationship between the positions of the neurons in the visual centres of the brain and the values of one or more of the receptive field properties of those neurons

The retino-geniculate striate pathway 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

Question 38

How do all signals from the left visual field reach the right primary cortex?

Answer 38

Via the temporal hemiretina of the right eye (ipsilateral)
Via the nasal hemiretina of the left eye (contralateral)

Opposite is true for the right visual field

Question 39

What is binocular overlap?

Answer 39

The overlapping of vision occurs due to the position of the eyes on the head
This overlap allows each eye to view objects with a slightly different viewpoint
As a result of this overlap of vision, binocular vision provides depth
When the brain sees an object with both eyes, the relative angles in which this object is visible gives an estimate of how far this object is located

Question 40

What are the P and M layers?

Answer 40

Layers of the lateral geniculate nucleus of the thalamus
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

Magnocellular layers
- Large cell bodies
- Particularly responsive to luminance change - on, off movement
- Rods provide the majority of the input
- Similar properties to the superior colliculus

Question 41

What is the receptive field?

Answer 41

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

Question 42

What 4 commonalities did Hubel and Wiesel observe in the retino-geniculate-striate system?

Answer 42

1. The receptive fields of neurons representing the foveal area were smaller than those in the periphery
2. They were circular
3. They were monocular - from one eye only
4. Many had receptive fields that comprised an excitatory area and an inhibitory area

Question 43

What are the characteristics of the receptive field of V1?

Answer 43

V1 responds to more complicated visual stimuli than points of light
The receptive fields of most V1 neurons fall into 2 categories –> simple and complex cells

Question 44

What are simple cells?

Answer 44

• Have antagonistic on and off regions
• However, the borders between the on and off regions are straight rather than circular
• This means they respond best to bars of light in a specific orientation at a specific location in the visual field

Question 45

What are complex cells?

Answer 45

• Complex cells are more common than simple cells
• 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
• Some complex cells respond optimally to movement of the bar in a specific direction
• Respond to a variety of stimuli across different locations - receptive field cannot be mapped into fixed excitatory and inhibitory zones - it will respond to patterns of light in a certain orientation within a large receptive field

Question 46

What is ocular dominance?

Answer 46

The tendency to prefer visual input from one eye to the other
Hubel and Wiesal found that some neurons were only responsive to stimuli coming from a single eye

Ocular dominance columns are stripes of neurons in the visual cortex of certain mammals that respond preferentially to input from one eye or the other

Ocular dominance columns are laid out in a striped pattern across the surface of the striate cortex

Question 47

What is the visual hierarchy?

Answer 47

The neural signal flows from neurons with simpler receptive fields to those with more complex receptive fields
From on-centre to off-centre to simple cells and from simple cells to complex cells

Question 48

What is the hierarchy in the extrastriate visual cortex?

Answer 48

V1 = provides the input
V2 = similar to V1 but more complex shape characteristics
V3 = form, motion, depth
V4 = colour, form, stimulus saliency, attention
V5 = motion

Question 49

What is the ventral stream and the dorsal stream?

Answer 49

The ventral stream 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)