From Retina to the Brain

Question 1

Where does the pupil reflex originate in the brain

Answer 1

Pretectum

Question 2

How does your eye differ when viewing long and short distances?

Answer 2

The pretectum adjusts the lens to ‘curve’ it more for longer distances

Question 3

Where is the automatic head movement towards a stimulus caused in the brain?

Answer 3

Superior colliculus

Question 4

What causes migraines?

Answer 4

Constriction of arteries in the visual cortex

Question 5

If someone is completely blind, what does this indicate?

Answer 5

They are not receiving any input to the hypothalamus anymore

Question 6

What function of the hypothalamus relates to eyesight?

Answer 6

Regulation of circadian rhythms (eyesight)

Question 7

Why in older ages do people often need glasses for certain distances?

Answer 7

The lens becomes less flexible with age

Question 8

What are the movements caused by superior colliculus called, and how often do they occur approximately?

Answer 8

saccadic eye movements approximately three times a second (more or less automatically, steered by stimuli in the environment)

Question 9

Which layers in the LGN correlate to which cells and input?

Answer 9

Layer 1-2: Magnocellular; Y-type (parasol) input

Layer 3-6: Parvocellular; X-type (midget) input

Question 10

Which layers in the LGN correlate to which eyes?

Answer 10

Layers 1, 4, 6: contralateral eye
Layers 2, 3, 5: ipsilateral eye

(based on visual field; left hemisphere - right visual field in both eyes)

Question 11

If you have blindness in one eye, what could this indicate?

Answer 11

damage to the optic nerves of that eye before it gets to the optic chiasm

Question 12

If you have blindness in the left half of your visual field in both eyes, what could this indicate?

Answer 12

Damage to the optic tract in the right hemisphere beyond the optic chiasm (likely before the LGN)

Question 13

Neurologically, what is happening during a migraine attack?

Answer 13

Constriction and dilation of blood vessels in the visual cortex

Question 14

If you have blindness in the left upper quadrant of your visual field in both eyes, what could this indicate?

Answer 14

damage to an area in the visual cortex (or area of optic tract after LGN)

Question 15

If you have blindness in a quadrant or half of your visual field in both eyes, sparing the foveal area, what could this indicate? Explain why this is the case

Answer 15

There is some controversy regarding whether there is a difference between this and not sparing the foveal area. This could either be damage to the optic tract or damage to areas of the visual cortex itself

It is likely that it is in the cortex because it is thought that the foveal area is represented in both hemispheres.

Question 16

If you have blindness in the left half of your visual field in your left eye and right half in your right eye, what could this indicate?

Answer 16

Damage to the optic chiasm

Question 17

How can damage to the optic chiasm occur

Answer 17

The pituitary gland sits directly behind the optic chiasm. It can grow larger (hypotrophy) or it can contract cancer and invade the surrounding structures. This can damage or even destroy the optic chiasm. This is fairly rare

Question 18

Give three alternative names for the primary visual cortex

Answer 18

V1
Striate cortex (The Myelinated stria refer to the folds in this area)
area 17 (Broadman)

Question 19

Describe the areas on the visual field in terms of the primary visual cortex

Answer 19

V1 takes up a large area of the visual cortex (to about where the calcarine sulcus meets the Parietal-occipital sulcus). The foveal area of the visual field (macula) takes up a large caudal area to V1. The para-foveal areas (Binocular portion) take up a smaller area directly rostral to that, and the peripheral visual area (monocular portion) take up an area rostral to that of an even size.

Question 20

what is this phenomenon of the areas closest to the fovea being bigger in the brain known as?

Answer 20

Retico-cortical expansion ( much more cortex for the central parts of the retina than the more peripheral parts of the retina)

Question 21

What unfortunate circumstances led to the discovery of this cortical organisation?

Answer 21

Before the Hague convention, people shot at each other with bullets which exploded inside them. It was later decided that this was inhuman and FMJ bullets were used (which stay whole and go out the other side) which produced clean brain lesions and started the study of retinotopy. Some researcher took advantage of this and studied these patients.

Question 22

What is area V1’s role in relation to the rest of the visual cortex?

Answer 22

V1 is the starting point of projection to the other visual areas and the rest of the brain (gateway to visual cortex)

Question 23

Define what is meant by a visual area of the brain?

Answer 23

Each area contains a separate, retinotopic, map of the visual field. Need not be a complete map, sometimes only central visual field, or peripheral, or only lower or upper. (The visual world is represented over and over again!)

Question 24

How is this similar to our other senses?

Answer 24

In the proimary and secondary auditory cortex there is a mapping for different frequencies (tonotopy). and in the sensory and motor cortex there is a mapping for different areas of the body (somatopy) where there is something similar to reticular expansion where the face and hands act as the foveal area.

Question 25

How was this retinotopic mapping made possible in humans?

Answer 25

fMRI displayed retinotopic mapping in the human visual cortex and displayed the similarities and differences between human and monkey visual areas

Question 26

How did reticular mapping occur in monkeys previously?

Answer 26

Through single unit recordings (Therefore a serious achievement to make such a map and scheme)

Question 27

How is the fMRI used to carry out this retinotopic mapping?

Answer 27

Wedges (angular position) or circles (eccentricity) of the visual field are displayed on a screen above the fMRI while a participant lays in it and recordings are made. angular position or eccentricity maps can therefore be made.

Question 28

Why do these maps discovered not individually represent different divisions in the retinotopic map?

Answer 28

Because the map was constructed as a merged version of these maps

Question 29

People often confuse these maps with another map. What is this map?

Answer 29

Brodmann’s anatomical map which does not always correspond to the functional map

Question 30

What exception is there, where there is correspondence and why is this?

Answer 30

Area V1 with Brodmann area 17 because of it’s distinctive stria

Question 31

where specifically does the information from each LGN layer go in the striate cortex? What is significant about this

Answer 31

Magnocellular (Layers 1 & 2) > Layer 4C alpha, 4B

Parvocellular (LGN layers 4 &6, 3&5) > Layers 4C beta, 2 & 3

P and M cells from the LGNproject to different layers of V1 independent of each other

Question 32

Describe where the information from the magnocellular layer of the LGN goes after the striate cortex

Answer 32

V1 (4Ca > 4b)
V2(Thick), V3
MT
Parietal Cortex, superior colliculus, Pv

Question 33

Describe where the information from the parvocellular layer of the LGN goes after the striate cortex

Answer 33

V1 (4Cb, 4A (blobs, inter blobs))
V2 (Thin, Inter)
V4
Temporal Cortex

Question 34

What are occular dominance columns?

Answer 34

Columns which demonstrate that information from each eye is kept separate

Question 35

Is there a difference in the layers of the occular dominance columns?

Answer 35

strong seperation in layer 4

weaker in layers 2/3 & 5/6

Question 36

how are these ocular dominance columns observed?

Answer 36

De-oxy glucose labelling of the columns by injecting a tracer in one eye then killing the animal. Then a horizontal section is made through V1.

Question 37

How do you check which eye is your dominant eye?

Answer 37

Doing illuminati triangle shit with your hands then closing one eye at a time to see which eye can see what is in the centre of your visual field

Question 38

What can affect which eye is dominant?

Answer 38

Time of day, diurnal rhythm. Stable for most people, changes more for others.

Question 39

What medical condition is associated with this ocular dominance?

Answer 39

Pathological dominance: Amblyopia

One eye is so dominant that information from the other eye is completely ignored

Question 40

What can cause amblyopia?

Answer 40

• Squinting results in suppression of the input of one eye
• If this happens during the critical period in development (2-6 years), input from that eye occupies less space in V1, the OD column of that eye becomes smaller
• After the critical period, this reduced representation remains, and amblyopia (reduced acuity, ‘lazy eye’) remains

Eye works fine, its more of a lazy visual cortex

Question 41

What treatments can be done for amblyopia?

Answer 41

in the critical period, a patch is worn over the good eye to force the other cortex to form. If not this becomes ‘permanent’ although there are some recent breakthroughs in treatments

Question 42

Name another way to organise the visual cortex

Answer 42

Cytochrome oxidase activity in V1, V2

Thin stripes, thick stripes, interstripes and blobs can be observed

Question 43

What is the use of cytochrome oxidase in the body?

Answer 43

An enzyme primarily used for metabolism, this means these are areas in which the brain requires the most oxygen.

Question 44

What did Hubel and Wiesel discovered regarding the visual cortex that earned them a nobel prize

Answer 44

1) that wiesel is an unfortunate name
2) That there were maps in the visual cortex recording various angles of orientation. There were columns that all responded to one angle, then another column would all respond to another, each column would shift by about 10 degrees

Question 45

How did they learn this information?

Answer 45

by strapping down an anaesthised cat, strapping it with single cell cell recorders and presenting line stimuli with varying degrees and moving in different directions

Question 46

What added implication did this have for the neurons in the visual cortex? Name the rule associated with this

Answer 46

Receptive field tuning: A neuron will only respond to a stimulus within its receptive field if that stimulus has certain characteristics (feature selectivity)

Question 47

What two features is area V1 selective to? (that we know)

Answer 47

Orientation and direction of motion

Question 48

How can this feature selectivity be made more apparent in animals in an experimental setting? Describe this in detail

Answer 48

Optical Imaging

An opening is made in the animals skull called the optical chamber where the brain can be observed
A video camera with a macro lens is directed at this brain area
Images of the various features are displayed on a monitor in front of the animal
A light is shone on this optical chamber. The light is reflected depending on the activity of different neurons. Different dyes can also be applied to increase the signal. The brain gives of a small amount of light when it is active

Question 49

How can this be observed in humans?

Answer 49

Using a 7 tesla function fMRI

Question 50

What orthogonal organisational components are there in the primary visual cortex?

Answer 50

Orientation columns
Ocular dominance columns
CO blobs

Question 51

How is this organisational problem solved?

Answer 51

The hypercolumn (Hubel & Wiesel): basic processing unit in V1

Each unit is comprised of one full rotation of the orientation columns and two ocular dominance columns (one left and one right). There is a CO blob in the centre of each set of columns.

refer to google doc for diagram

Question 52

How does this hypercolumn contribute to your visual experience?

Answer 52

Each hypercolumn is responsible for one receptive field

Question 53

what questions did this hypercolumn spark regarding the information that came from the retina?

Answer 53

How did the information go from circular receptive fields in the retina/ LGN to orientated receptive fields in the cortex

Question 54

what answer was given to this problem?

Answer 54

The computational power of a neuron:

Adds up all inputs in a weighted manner to generate an action potential or not. Only when enough inputs combine at the same time, an action potential is generated: the neuron is a coincidence detector

Elongated (orientated) RF’s in V1 are constructed from LGN inputs that have their (circular ) RF along a line in visual space.

Question 55

What name was given to this type of elongated cell?

Answer 55

Simple cell: linear

Question 56

In terms of polarisation what does this look like?

Answer 56

depolarisation down the line, hyperpolarisation either side

Question 57

What is a 3D representation of this polarisation called?

Answer 57

Gabor filters (sinusoid combined with gaussian envelope)

Question 58

What difference does a moving stimulus of the same orientation have on the signal?

Answer 58

it makes the signal stronger

Question 59

What can activate the complex cells shown in the 70’s experiment?

Answer 59

When a bar of a certain orientation is moving in a certain direction. Will not be activated when the bar is just flashing or when it is moving in the other direction

Question 60

What can activate the ‘hypercomplex’ cells shown in the 70’s experiment?

Answer 60

A signal is only produced when a line, moving in a certain direction at a certain orientation with a certain width and length

Question 61

What is the defining characteristic of a hypercomplex cell?

Answer 61

inhibitory “zones” which form almost 90 degree angles marking the length required for it to be activated

Question 62

How are the theoretical models of V1 simple cells related to models of complex cells?

Answer 62

Simple cells are ‘Gabor’ filters tuned to orientation and spatial frequency with on and off zones. These can have low or high spatial frequency but remain simple cells.

‘Quadrature pairs’ of simple cells combine to form complex cells

Orientation (and spatial frequency) selective but without clear ON and OFF zones

Question 63

What illusions are associated with these orientation cells?

Answer 63

Cafe wall illusion- when one straight line of squares is in the middle of other lines of squares at angles, it appears that that line is also at an angle

This is due to some (largely unknown) interaction between orientation selective neurons