Absolute Threshold of Vision (M1)

Question 1

How many photons does it take to activate one rhodopsin molecule? 1. How many transducins are activated by a active rhodopsin? 2. How many PDE are activated by a transducin? 3. How many cGMP can be hydrolyzed per second by an active PDE? 4. Therefore, how many cGMP can be hydrolyzed by a single photon? 5

Answer 1

1. one
2. 1000
3. one
4. 200
5. 8000

Question 2

What percentage of photons get back to the retina? 1. What percentage pass through the retina or are absorbed by cones? 2. What percentage of photons of the original light flash are absorbed by cones? 3

Answer 2

1. 50%
2. 40%
3. 10%

Question 3

What happens as a result of spontaneous isomerization of 11-cis retinal to all-trans retinal? 1. What causes an increase in this? 2

Answer 3

1. noise

2. higher temperatures

Question 4

How often does spontaneous photoisomerization occur for a given molecule?

Answer 4

every 500 to 1000 years

Question 5

What are the reasons rods are more sensitive than cones?

Answer 5

1. Stiles-Crawford effect reduces sensitivity to off-axis rays
2. cones are noisier
3. less signal amplification in cones
4. voltage response to 1 photon in cone only 1/10 or rod
5. rods more spatial and temporal summation

Question 6

What is the adding together of responses that occur in different photoreceptors called?

Answer 6

spatial summation

Question 7

Do rods or cones sum up more information over space? 1. What is the reason for this difference? 2

Answer 7

1. rods

2. organization of synaptic connections between rods, bipolar cells, and ganglion cells

Question 8

What is the law that states up to the critical spot diameter, threshold is independent of spot size if quanta in spot remains constant?

Answer 8

Ricco’s law

Question 9

What is the adding together of events that occur at slightly different times?

Answer 9

temporal summation

Question 10

Do rods or cones sum up more info over time? 1. What is the reason for this difference? 2

Answer 10

1. rods

2. cones inactivate faster after photon absorption

Question 11

What is the temporal summation period for the scotopic system?

Answer 11

100 msec

Question 12

What is the law that states up to the critical time duration, as long as the threshold number of quanta are delivered during this period, it does not matter how they are delivered?

Answer 12

Bloch’s Law

Question 13

How many rods in close spatial proximity must be stimulated within a short time period for perception to occur?

Answer 13

10

Question 14

What does each number in the 100:10:1 rule stand for?

Answer 14

100: photons on the cornea
10: photons absorbed at retina
1: photon needed to get detectable response of rod

Question 15

What is the absolute threshold of number of photons in a small, brief flash for a response in scotopic conditions? 1. Photopic conditions? 2

Answer 15

1. 100

2. 600

Question 16

What is the absolute threshold of number of photons in a large, long flash for a response in scotopic conditions? 1. Photopic conditions? 2

Answer 16

1. 10^-4 td

2. 10^-1 td