THE EYE

Question 1

Fovea

Answer 1

Receives light from the center of the visual field
- Cells other than photoreceptors are displaced laterally at the fovea (forming “pit”): This provides light a clear path to photoreceptors
- Rod-free area in human fovea contains about 35,000 cones (≈0.3mm in diameter)
– No blood vessels at the fovea
Design features improve visual resolution at the fovea

most sensitive part of the retina

Question 2

Optic disk

“blind spot”

Answer 2

Axons from ganglion cells leave the retina here to form the optic nerve (no photoreceptors here)

Question 3

Visual angle

Answer 3

Size is commonly specified as visual angle (θ):
– tan θ = S / D
– where S = size, and D = distance (S and D in the same units, e.g., cm)

Question 4

Anatomy of the retina

Answer 4

Ganglion cells are the only cells in the retina that fire action potentials
Photoreceptors and bipolar cells release neurotransmitter proportional to their level of depolarization (a graded response)
Highest density of cones at fovea: Few cones in retinal periphery
Rods more numerous in retinal periphery: Few, if any, rods in fovea

Question 5

Photoreceptors

Rods and cones

Answer 5

Photoreceptors contain a light-sensitive pigment
– All rods have the same photopigment (rhodopsin)
– Each cone has one of three types of photopigment
Rods and cones differ in light sensitivity
– Rods are sensitive to very dim light
– Cones need much brighter light

Different pigments contain different proteins called opsins giving rise to differences in color sensitivity

Question 6

Rod pathway

Answer 6

A number of rods provide converging input to bipolar cells
– This arrangement amplifies the visual input to ganglion cells
under low-light conditions (but reduces visual acuity)

Question 7

Cone pathway

Answer 7

– Cones in fovea may provide one-to-one input to bipolar cells
– This arrangement preserves detail in the visual image
– Cones also provide color information

Question 8

In dark

Answer 8

– Outer segment membrane permeable to sodium
- Sodium channels open: Sodium channel gated by compound called “cyclic GMP” (cGMP = cyclic Guanine Monophosphate)
– Photoreceptors are depolarized
– Photoreceptors release transmitter (glutamate)

Graded transmitter release:
– Proportional to photoreceptor depolarization
– Transmitter binds to receptors on bipolar cells

Question 9

In light

Answer 9

– Outer segment less permeable to sodium
- Sodium channels closed: Light activates enzyme called “phosphodiesterase”, which breaks down cyclic GMP
– Photoreceptors are hyperpolarized
– Transmitter release reduced

Question 10

Center vs Surround

Bipolar cells have receptive field with center-surround arrangement

Answer 10

– ON-center cell responds well to light in the center of the receptive field, but not in the surround (mGluR6)
– OFF-center cell responds well to light in the surround of the receptive field, but not in the center (AMPA/kainate)

Question 11

ON-center and OFF-center ganglion cells

Ganglion cells have center-surround RFs and are sensitive to contrast

Contrast (i.e., adjacent light and dark areas)
– Ganglion cells respond little with change in brightness over a large area

Answer 11

ON-center ganglion cell responds when:
– Onset of small bright light in center of receptive field
– Offset of annulus of light in surround of receptive field
OFF-center ganglion cell responds when:
– Offset of small bright light in center of receptive field
– Onset of annulus of light in surround of receptive field
Complementary responses: when ON-center cell responds well, OFF-center cell responds poorly

Individual RFs are small, “piecing them together” covers visual field

Center-surround receptive fields (ganglion cells) are useful for detecting edges

Question 12

Trichromacy

Answer 12

– Short wavelength (S) or blue
– Medium wavelength (M) or green
– Long wavelength (L) or red

3 cone types at each point on the retina (ideally)

– We perceive white when 3 cone types equally active
– Colors from different combinations of cone inputs

Question 13

Opponency theory of color vision

Answer 13

– Red and green are opponent colors
– Blue and yellow are opponent colors (Only blue ON)
– We process opponent colors in two channels: red-green channel and blue-yellow channel

Question 14

Computations for color opponency

Answer 14

3 opponent channels carry info from retina to V1
– 2 chromatic channels (green/red and blue/yellow)
– 1 luminance channel (black/white)

Question 15

Retinal ganglion cells that project to the thalamus

Answer 15

– Parasol cell, also called “M-type” (M for magno) to luminance channel (“magnocellular” pathway)
– Midget cell, also called “P-type” (P for parvo) to red-green channel (“parvocellular” pathway)
– Small bistratified cell to blue-yellow channel (“koniocellular” pathway)