B&C: Vision: Retina and V1

Question 1

What is meant by remote sensing or exteroceptive perception?

Answer 1

Perceiving information at a distance

Question 2

How is the physical world translated onto the retina?

Answer 2

As light passes through the lens of the eye, the image is in- verted and focused to project on the back surface of the eye (Figure 5.21), the retina.

Question 3

Describe the structure of the retina

Answer 3

The retina is only about 0.5 mm thick, but it is made up of 10 densely packed layers of neurons. The deepest layers are composed of millions of photoreceptors, the rods and cones.

Question 4

What is contained in the rods and cones?

Answer 4

Photopigments; protein molecules that are sensitive to light

Question 5

What happens when these proteins are exposed to light?

Answer 5

When exposed to light, the photopigments become unstable and split apart.

Question 6

What happens when rods and cones fire action potentials?

Answer 6

Unlike most neurons, rods and cones do not fire action potentials. The decomposition of the photopigments alters the membrane potential of the photoreceptors and triggers action potentials in downstream neurons. Thus, photoreceptors provide for translation of the external stimulus of light into an internal neural signal that the brain can interpret.

Question 7

What pigment do cones (1) and rods (2) contain?

Answer 7

Rods- Rhodopsin

Cones- Photopsin

Question 8

What characteristic of rods makes them useful for what element of sight

Answer 8

rhodopsin is destabilized by low levels of light. Rods are most useful at night when light energy is reduced

Question 9

How do cones make up for the shortcomings of rods?

Answer 9

Rods also respond to bright light, but the pigment quickly becomes depleted and the rods cease to function until it is replenished. Because this takes several minutes, they are of little use during the day. photopsin. Cones require more intense levels of light but can replenish their photopigments rapidly. Thus, cones are most active during daytime vision.

Question 10

Describe the three types of cones

Answer 10

(a) a cone that responds to short wavelengths, the blue part of the spectrum; (Smurfs) (b) one that responds to medium wavelengths, the greenish region; and (c) one that responds to the long “reddish” wavelengths (Rockets)

Question 11

What maximal wavelength is correspondences to each type of cone? Which is activated by sunlight?

Answer 11

Blue- 430 (book) 419 (slides)
Rods- 495/ 496
Green- 530/ 531
Red- 560/ 559

Question 12

Where are cones most densely packed?

Answer 12

Cones are densely packed near the center of the retina, in a region called the fovea.

Question 13

Where are rods distributed most?

Answer 13

throughout the retina

Question 14

Where does information travel from the rods and cones?

Answer 14

The rods and cones are connected to bipolar neurons that then synapse with the ganglion cells, the output layer of the retina. The axons of these cells form a bundle, the optic nerve, that transmits in formation to the central nervous system.

Question 15

How do rods and cones differ in their output to ganglion cells?

Answer 15

though humans have an estimated 260 million photoreceptors, we have only 2 million ganglion cells to telegraph information from the retina. Many rods feed into a single ganglion cell. By summing their outputs, the rods can activate a ganglion cell even in low light situations. For cones, however, the story is different: Each ganglion cell is innervated by only a few cones. Thus, they carry much more specific information from only a few receptors, ultimately providing a sharper image.

Question 16

Describe how information in the optic nerve in the eye gets to the central nervous system

Answer 16

before entering the brain, each optic nerve splits into two parts. The temporal (lateral) branch continues to traverse along the ipsilateral side. The nasal (medial) branch crosses over to project to the contralateral side. Each optic nerve divides into several pathways that differ with respect to where they terminate in the subcortex. Almost all of the fibers terminate in the primary visual cortex

Question 17

What is the benefit of the two nasal branches crossing over each other and what is this point called?

Answer 17

the optic chiasm ( crossover of nasal fibers ) ensures that visual information from each side of external space will be projected to contralateral brain structures.

Question 18

What is the name of the pathway that contains 90% of the axons in the optic nerve

Answer 18

The retinogeniculate pathway, the projection from the retina to the lateral geniculate nucleus (LGN) of the thalamus.

Question 19

How do ganglion cells correlate with layers of the geniculate nucleus?

Answer 19

One type of ganglion cell, the M cell, sends output to the bottom two layers (Magno cellular). Another type of ganglion cell the, P cell, projects to the top four layers (Parvo Cellular)

Question 20

Where do the other 10% of optic nerves go? (2)

Answer 20

The remaining 10 % of the optic nerve fibers in- nervate other subcortical structures, including the pulvinar nucleus of the thalamus and the superior colliculus of the midbrain.

Question 21

What do the pulvinar and superior collicus play a role in?

Answer 21

The superior colliculus and pulvinar nucleus play a large role in visual attention. The superior colicus is important in orientating movement of head and eyes. (quickly bringing sudden and salient objects to attention)

Question 22

What pathway is utilised in the final projection to the visual cortex?

Answer 22

the geniculocortical pathway. This bundle of axons exits the LGN and ascends to the cortex, and almost all of the fibers terminate in the primary visual cortex

Question 23

What four distinct neurons has visual information been processed by by the time it reaches the cortex?

Answer 23

photoreceptors, bipolar cells, ganglion cells, and LGN cells.

Question 24

Where are the receptive field neurons located?

Answer 24

external space is represented continuously within neural regions such as the LGN or V1 (Visual cortex.) A full retinotopic map contains a representation of the entire contralateral hemifield

Question 25

How do receptive field in the LGN and the visual cortex differ?

Answer 25

Receptive fields range in size, becoming larger across the visual system. LGN cells have receptive fields responding only if the stimulus falls within a very limited region of space, about one degree of visual angle. Cells in V1 have slightly larger receptive fields, and this magnification process continues through the visual sys- tem: Cells in the temporal lobe have receptive fields that may encompass an entire hemifield.

Question 26

How do chips replacing photoreceptors work?

Answer 26

Light enters the eye through the lens, passes through
the transparent retina, and hits the chip. The image is simultaneously captured several times per minute by all
of the photodiodes, each of which controls a tiny ampli-
fier connected to an electrode, together known as an element (pixel). Each element generates a voltage at its
electrode, the strength of which depends on the intensity of light hitting the photodiode. The voltage then passes to the adjacent bipolar neurons in the retina, and the signal proceeds through the rest of the visual pathway.

Question 27

What is the quality of image captured on this and why?

Answer 27

When you consider that the eye contains millions of photo- receptors, 1,500 seems like a drop in the bucket. Indeed, this number produces only crude images.

Question 28

How is the chip powered?

Answer 28

an implanted cable that runs from the eye under the temporalis muscle and out from behind the ear, where it is attached to a wirelessly operated power control unit that the patient wears around his neck.

Question 29

What brain areas contain retinotopic maps? (discovered in monkey- humans indicated to have more)

Answer 29

Occipital cortex
Parietal cortex
Inferotemporal cortex

Question 30

What is the the name of the initial projection region of geniculate axons?

Answer 30

Striate cortex (V1)

Question 31

In the diagram displaying the different visual areas in the brain what do the lines represent?

Answer 31

The lines connecting these extrastriate visual areas demonstrate extensive convergence and divergence across visual areas. In addition, connections between many areas are reciprocal; areas frequently receive input from an area to which they project.

Question 32

Give a possible reason on why it would be advantageous for the primate brain to have evolved so many visual areas

Answer 32

One possibility is that visual processing is hierarchical. Each area, representing the stimulus in a unique way, successively elaborates on the representation derived by processing in earlier areas.

Question 33

How does the book theorise this elaboration of cells would work?

Answer 33

The simple cells of the primary visual cortex calculate edges. Complex cells in secondary visual areas use the information from many simple cells to represent corners and edge terminations. In turn, higher order visual neurons integrate information from complex cells to
represent shapes. Successive elaboration culminates in formatting the representation of the stimulus so that it matches (or doesn’t match) information in memory

Question 34

What is a problem with this theory?

Answer 34

there is no simple hierarchy; rather, extensive patterns of convergence and divergence result in multiple pathways.

Question 35

What effect does rhodopsin have as it is released in the rods? What does this mean regarding light

Answer 35

Causes the sodium channel to close which leads to hyperpolarisation, therefore the more light there is the more hyperpolarised they become.

Question 36

What signals are transmitted within the retina and eye?

Answer 36

Graded potentials

Question 37

Why are different cones and rods sensitive to different wavelengths?

Answer 37

Different versions of rhodopsin are sensitive to different wavelengths

Question 38

What happens when someone is lacking one of these versions of rhodopsin?

Answer 38

Retinal colour blindness: absence of a particular cone type (or colour)

Question 39

Where are the bipolar and ganglion cells located in the retina? Why is this?

Answer 39

In front of the cones and rods, almost ‘In the way’ of the light. This allows rods and cones to lie in a very pigmented layer where light is very readily absorbed. This helps prevent light reflecting or ‘light scattering’ and makes the vision less hazy and more sharp due to other cones picking up the scattered light (less noise)

Question 40

Cats don’t have this pigmented layer, instead they have a reflected one. What is the consequences of this

Answer 40

They cannot see something very well unless it moves however they have much better night vision (much more sensitive to light)

Question 41

What shape is the fovea and what in the eye is known as the blind spot?

Answer 41

Cup shaped and where ganglion cell fibres leave the retina

Question 42

Why does visual information go to the thalamus?

Answer 42

to regulate day and night cycles

Question 43

What is the pretectum?

Answer 43

bunch of nuclei which function is reflex control of pupil and lens (making you focus, adapting when you go up close, constricting and expanding in response to light etc.)

Question 44

What type of input do the different LGN layers have?

Answer 44

Layer 1-2: Y-type (parasol) input
Layer 3-6: Parvocellular X-type (midget) input
I.E P goes to M and M goes to P

Question 45

Why is the primary visual cortex known as the striate cortex? (as well as V1/area 17)

Answer 45

The visual cortex has a very distinct feature called the (myelinated) stria which is visible from the outside at the back of the brain

Question 46

In respect to the visual cortex why can it be so dangerous to text while driving/cycling?

Answer 46

The centre of your visual field gets a much larger part of the visual field devoted to it

Question 47

How is a visual area in the brain defined?

Answer 47

Each area must contain a separate, retinotopic map of the visual field. This needn’t 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 48

Do the anatomical areas identified by brodmann correspond to the functional areas?

Answer 48

No except for area 17 (visual cortex)

Question 49

What is involved in the development of amblyopia?

Answer 49

(Lazy eye); squinting results in the suppression of one eye, if this happens during the critical period in development (2-6 years), input from that eye occupies less space in V1 and the OD column of that eye gets smaller. After the critical period, this reduced representation remains, and amblyopia remains.

Question 50

This development of amblyopia shows an exaggerated form of what feature about us?

Answer 50

Occular dominance: We all have a dominant eye (can change due to diurnal rhythm, over the day etc)

Question 51

What is the primary way to treat amblyopia?

Answer 51

Putting the patch on it over the critical period

Question 52

Name another anatomical/ functional parcellation in the visual cortex

Answer 52

Cytochome oxidase activity in V1 and V2

Question 53

What features can you see on the enzyme Cytochome oxidase and what does this represent?

Answer 53

Thick stripes, thin stripes, interstripes (V2) and blobs. (V1) These blobs can represent higher enzyme activity

Question 54

What is notable about magnocellular and Parvocellular inputs from the LGN when projecting to other visual areas?

Answer 54

They remain segregated

Question 55

What is meant by receptive field tuning?

Answer 55

A neuron will only respond to a stimulus within its receptive field if that stimulus has certain characteristics (different columns representing different angles.)

Question 56

What orthogonal organisational components in the primary visual cortex were highlighted during the presentation? What diagram attempts to describe this?

Answer 56

• Orientation columns
• Occukar dominance columns
• CO blobs

Question 57

What diagram/concept attempts to summarise these organisational components?

Answer 57

The hypercolumns: You go through all the orientations in one hypercolumn, you have both occur dominance columns (influence from two eyes) and you also have the CO blobs, typically lying in the centre. This is the basic functional unit of the visual cortex, each representing an area in the visual field

Question 58

as well as orientation selective these cells are often _______ selective

Answer 58

direction ( motion )

Question 59

How do receptive fields differ in the retina and the cortex?

Answer 59

Circular in the retina, orientated in the cortex

Question 60

How do they change from circular to orientation

Answer 60

Elongated (oriented) RF’s in V1 are constructed from LGN inputs that have their (circular) RF along a line in visual space. All the action potentials of the cells in the lateral ganglion nucleus representing these receptive fields in a line feed into the one cortical cell which represents that angle in that visual field