Lecture 6 - Rods&Cones

Question 1

Fovea consists solely of

Answer 1

CONES

about 50,000 – 110,000

Question 2

peripheral retina has

Answer 2

both rods and cones.

Question 3

more rods than cones in

Answer 3

periphery

Question 4

Blind Spot

Answer 4

The place where optic nerve leaves the eye: where all the axons leave the eye

one place in the eye with no rods or cones

sometimes called the optic disc

Question 5

Why we don’t notice the blind spot

Answer 5

• One eye covers the blind spot of the other.
• It is located outside of the macula (away from detail vision).
• The brain “fills in” the spot.

Question 6

Age-related macular degeneration

caused by?

Answer 6

– Fovea and surrounding area (macula) are destroyed.

– Caused by interrupted supply of blood to the macula.

– Creates a central “blind spot” on retina.

– Most common in older individuals.


Question 7

Wet AMD (age macular degeneration)

Answer 7

when a lot of blood vessels formed right behind the macula, and growth gets out of control and it starts pushing at the retina: pushing it away from the pigment epithelium and destroying the photosensitivity of that region

Question 8

Dry AMD

Answer 8

protein deposits behind the back of the retina, that pushes the retina away from the epithelium so it kills off the receptivity of those receptors

Question 9

Macular degeneration treatments

Answer 9

rather than have that tightly focused image on one space: you need to spread it out (capture the same number of cones or a similar number of receptors) to duplicate the image

Implantable Miniature Telescopic lens

Telescopic contact lens

Question 10

Implantable Miniature Telescopic lens

Answer 10

replace the lens of the eye with a telescopic lens that takes the image in and steers the image away from the destroyed fovea and opens up the image more broadly

not as high resolution vision

Question 11

telescopic contact lens

Answer 11

lens takes the image coming in and steers everything away from that one foveal point

Question 12

Retinitis pigmentosa

Answer 12

lose the periphery

– Genetic disease affecting receptors in the retina.

– Rods are destroyed first.

– Foveal cones can also be attacked.

– Severe cases result in complete blindness.

– No effective treatment, but onset may be delayed with vitamin A supplements.

Question 13

Glaucoma

caused by?

Answer 13

– Damage to the optic nerve (retinal ganglion cells that carry all the signals from the receptors).

– Most often caused by increased blood pressure in the eye, inhibiting blood flow to the optic nerve.
- cutting off the flow of blood and the operation of these nerves = kills them in patches, but it happens slowly so you adapt

– Onset can be gradual and hard to detect.

– Damage is permanent.

Question 14

How can we do tests to detect the sensitivity of rods and cones? Specifically how little light needs to be present to see things?

Answer 14

Comparing rod and cone sensitivity using dark adaptation

Question 15

dark adaptation

Answer 15

1. Using the method of adjustment, determine the subject’s sensitivity to a test
   light while the room light are still on. This is the light-adapted sensitivity (threshold of sensitivity: when the eyes are adaptive to the light)
   
   • “turn on the light as barely as bright as it needs to be for you to see it (in a well lit condition)”
2. Next, turn off the lights and have the subject continue to adjust the test light until you they no longer improve. This is the dark-adapted sensitivity (final level that they see).
   
   • have the person sit in the dark for awhile and over that period of time tell them to keep turning down the light to the least amount necessary to see it

Question 16

What sensitivity can we attribute to the rods?

experiment

Answer 16

Observer looks at fixation point, but pays attention to a test light to the side (off center, not on the fovea).
– We find a two stage dark adaptation curve:
• Stage one: begins with a rapid increase in sensitivity (3-4 minutes).
- rapid decrease in light increase in sensitivity
• Then sensitivity levels off for seven to ten minutes – the rod- cone break.
- vision gets more and more sensitive
• Stage two shows increased sensitivity for another 20 to 30 minutes.

Question 17

light sensitivity happens in two stages because

Answer 17

two different receptors are regenerating (becoming more sensitive)

at first it’s the cones and then they max out in sensitivity

and then the rods take over because they’re much more sensitive (they need much more light to be present for you to see something there)

Question 18

Cones are about 100 times ____ sensitive than rods

Answer 18

LESS

Question 19

How to test just cones? to know what just cones are responsible for?

Answer 19

put light directly onto the fovea - only stimulates cones.

– Results show that sensitivity increases for three to four minutes and then levels off.

Question 20

You test cone adaptation to the dark by shining a light only in the fovea, which we know only has cones. Can you test rod dark adaption by shining a light only in the periphery? Why or why not?

Answer 20

No, because the periphery contains both rods and cones

Question 21

Experiment for rod adaptation

Answer 21

– Must use a rod monochromat – someone born with no cone receptors.

– Results show that sensitivity increases for about 20-30 minutes and then levels off.

Question 22

Why do rods and cones become more sensitive in the dark?

Answer 22

• After a retinal is isomerized, it separates from the visual pigment.

• This is visual pigment bleaching: the loss of retinal pigment and color.
- pigment is no longer photo-reactive (doesn’t have retinal)

• Darkness provides the opportunity to regenerate.
- takes enzymes from the epithelium

Question 23

Visual pigment regeneration

Answer 23

• Retinal and opsin must recombine to respond to light
• Cone pigment regenerates in about 7-10 minutes.
- not all the cones go on and off at the same time: cycle: losing some but some are coming back on
• Rod pigment takes more than 30 minutes.
• Enzymes necessary for pigment regeneration are found in the pigment epithelium.*
• If your retina separates from the pigment epithelium (detached retina), you lose the ability to regenerate visual pigment.