Chapter 9 - Vision

Question 1

What is in the optical system of the eye?

Answer 1

Cornea and lens

Question 2

What does the optical system do?

Answer 2

Projects physical image of the world onto the retina

Question 3

What does the retina do with the optical image?

Answer 3

It converts it into a neural image

Question 4

What constitutes the retina?

Answer 4

3 interconnected layers and 5 classes of neurons

Question 5

What are the 3 stages of vertical information flow?

Answer 5

1) Transduction of the image by photoreceptors (physical energy (photons) to electrochemical energy)
2) photoreceptors synapse on bipolar cells
3) bipolar cells synapse on ganglion cells

Question 6

What is the lateral information flow?

Answer 6

At each stage of the vertical transmission there are specialized laterally connecting neurons called horizontal cells and amacrine cells.

These cells modify forward transmission across the synaptic layers ( largely by inhibition)

Question 7

When are rods and cones depolarized?

Answer 7

When they are in darkness

Question 8

In the rod, what maintains sodium and potassium concentrations?

Answer 8

Na/K pump

Question 9

What happens to channels in outer segment when in the light?

Answer 9

Cell hyperpolarizes

Question 10

What is the role of the visual pigment molecules?

Answer 10

Absorb electromagnetic energy and divert the energy into a biological process

Question 11

What could you compare the rhodopsin to?

Answer 11

Is like a G-protein receptor

Question 12

What could you compare retinal?

Answer 12

To a ligand (it’s a chromophore)

Question 13

What are the steps of phototransduction?

Answer 13

1) Light stimulation of rhodopsin leads to activation of a G-protein, transducin
2) Activated G-protein activates cGMP phosphodiesterase (PDE)
3) cGMP binds Na+ channels opening them
4) PDE hydrolyzes cGMP, reducing its concentration
5) this leads to closure of sodium channels

Question 14

Explain dark reaction

Answer 14

1) cGMP is high in cytoplasm
2) inward current of Na+ through cGMP-Gated channels which depolarized cell. This is the dark current which is usually ~ -50pA (in current) keeping cell’s Vm at ~ -40mV
3) depolarization causes transmitter release (glutamate) at terminal region

Question 15

Explain light reaction

Answer 15

1) Light activates rhodopsin
2) Transducin, the g-protein is active
3) phosphodiesterase (PDE), the effector enzyme, is activated
4) PDE activity reduces the cGMP level
5) cGMP-gated N’a+ channels close- no Na+ current (dark current)
6) Cell hyperpolarizes to ~ -70mV, transmitter release stops

Question 16

1) OFF bipolars are … by …/… by light

2) ON bipolars are … by…/… by light

Answer 16

1) depolarized — glutamate—hyperpolarized

2) hyperpolarized—glutamate—-depolarized

Question 17

What are the two ways of thinking about the receptive fields’ visual system?

Answer 17

1- the set of photoreceptors to which the cell is connected

2- the set of the visual field in which various visual stimuli can affect the discharge rate of the cell (slide 39)

Question 18

What are the two pathways of the receptive field? What are the components of each?

Answer 18

A) vertical pathway: - photoreceptors

• bipolar cell
• ganglion

Forms the center of receptor field (blue)

B) horizontal pathway: - photoreceptors

• horizontal cells
• bipolar cells
• ganglion cell

Forms the inhibitory surround of receptive field

Question 19

What are the different types of ganglion cells?

Answer 19

M: large receptive fields, not wavelength selective, respond well to large objects and movement, terminate in M (magnocellular) layers of LGN

P: small receptive fields, selective to particular wavelengths of light (color), concerned with analysis of fine detail and color, terminate in P (parvocellular) layers of LGN

Question 20

What is scotopic vision?

Answer 20

Vision under dim-light (dark)

Question 21

What is photopic vision?

Answer 21

Vision under well lit conditions

Question 22

Name the parallel pathways

Answer 22

Magnocellular M(motion)

Parvocellular - P(shape, detail)

Koniocellular - non-M-non-P (color)

Question 23

For every point of the visual field we have at least 10 ganglion cells. What are they?

Answer 23

• On-M
• Off-M
• On-P (2)
• Off-P (2)
• non-M-non-P (4)

Question 24

What does LGN stand for?

Answer 24

Lateral geniculate nucleus

Question 25

NonM-nonP ganglion cells project to which pet of the LGN

Answer 25

Koniocellular

Question 26

What is disparity tuning?

Answer 26

Many binocular cells code for retinal disparity - differences in images from the left and right eye that the brain uses as a binocular cue (stereopsis) to determine depth or distance of an object.

Question 27

What are the components of the horizontal organization of V1?

Answer 27

1) Occular dominance columns (cells in 4C are monocular)
2) orientation columns
3) cytochrome oxidase blobs

Question 28

What are receptive field properties emerging in V1?

Answer 28

• Disparity tuning (for stereopsis, depth perception)
• orientation tuning (for form perception)
• direction selectivity (for lotion perception)
• wavelength (color) selectivity (from retinal P pathway)
• blobs - koniocellular pathway

Question 29

Which pathways continue to be segregated in parallel pathways?

Answer 29

M and P pathways are processed in segregated in parallel pathways by the visuals system

Question 30

What are the two streams of processing?

Answer 30

Dorsal and ventral

Question 31

Magnocellular Pathway

Answer 31

M-type ganglion
Magnocellular LGN (Layer 1,2)
V1 Layer IV C alpha (monocular)
V1 Layer IV B (binocular)
dorsal pathway

Question 32

Where is motion represented?

Answer 32

It is represented in middle temporal area MT

Question 33

Are MT receptive fields larger than V1 r fields?

Answer 33

Yes, much larger

Question 34

Which neurons respond to optic flow stimuli?

Answer 34

MST neurons represent the motion of the world as you move through the world

Question 35

What is color constancy?

Answer 35

Our ability to determine the color of an object despite changing illumination

Question 36

Where are the cells responsible for color constancy located?

Answer 36

In V4

Question 37

Dorsal Pathway Explanation

Answer 37

“Where”

V1
V2
V3
MT - Motion
MST - navigation
other dorsal areas

Question 38

Rods vs Cones

Answer 38

Rods: High sensitivity to light
One type
Active in low light (scotopic)
Concentrated in peripheral retina

Cones: Low sensitivity to light
Three types (red, green blue)
Active in bright light (photopic)
Concentrated in central retina (fovea)

Question 39

P cell traits

Answer 39

95% of ganglion cells
color sensitive
smaller receptive fields
slower adaptation
detect shapes + details

Question 40

M Cell Traits

Answer 40

~5% of ganglion cells
color insensitive
larger receptive fields
rapid adaptation
detect motion

Question 41

Name 6 layers in LGN and what they do

Answer 41

1, 2 - Magnocellular

3, 4, 5, 6 - Parvocellular, some koniocellular

Question 42

Ventral Pathway

Answer 42

"What"
V1
V2
V3
V4 (color vision)
Inferior Temporal (IT - face recognition))
other ventral areas

Question 43

Topographic Mapping in Visual System - Name and explanation

Answer 43

Retinotopic: Retinal cells in are close together, and the same in each layer, at the LGN level and cortex level

Question 44

Parvocellular Pathway

Answer 44

Perception of shape & detail

P-type ganglion
LGN Layer 3-6 Parvocellular
V1 Layer IV C Beta (monocular)
V1 Layer II & III (binocular)
Interblob