Light Adaptation (M1)

Question 1

What happens to the threshold luminance with an increase in background light levels?

Answer 1

increases

Question 2

What is the slope of the first section of the incremental threshold curve? 1. What does this section represent? 2

Answer 2

1. m=0

2. background light no different from neutral noise

Question 3

What is the slope of the second section of the incremental threshold curve? 1. What does this section represent? 2

Answer 3

1. m=1/2

2. at low light levels, threshold detection is limited by quantal fluctuations in light source (Devires-Rose law)

Question 4

What is the law that states that the eye essentially behaves as an ideal detector? 1. Does this hold in low background light levels? 2. Does this hold in high background light levels? 3

Answer 4

1. Devries-Rose law
2. yes
3. no

Question 5

In order for a signal to be detected above the background how much must the luminance of the two differ?

Answer 5

by about 1 standard deviation (square root of mean)

Question 6

What is the slope of the third section of the incremental threshold curve? 1. What does this section represent? 2

Answer 6

1. m= about 1

2. threshold change in I is larger than predicted by quantal fluctuations (Weber’s Law)

Question 7

What law states that as the background light changes, the threshold increment (ΔI) also increases so that the ratio of ΔI to the background light intensity remains constant?

Answer 7

Weber’s law

Question 8

What is Weber’s maximum fraction in rods?

Answer 8

0.14

Question 9

What is the term for the absolute sensitivity changing while the relative sensitivity remains the same?

Answer 9

sensitivity regulation

Question 10

What is the slope of the forth section of the incremental threshold curve? 1. What does this section represent? 2

Answer 10

1. m=infinity

2. rods have become saturated

Question 11

What generally occurs before the rods become completely saturated?

Answer 11

switch to cone-mediated vision

Question 12

What is Weber’s maximum fraction in cones?

Answer 12

0.02

Question 13

What are the advantages of visual field testing with some background luminance?

Answer 13

1. close to ambient lighting of waiting room
2. comfortable for patient
3. pt’s response variability is low
4. in a lighting range where Weber’s law holds

Question 14

How would you isolate S-cones using an increment threshold?

Answer 14

blue stimulus light on yellow background

Question 15

Based on Weber’s law, if the intensity of he background increases what must the absolute intensity of the target light at detection threshold do?

Answer 15

increase proportionately

Question 16

What does the yellow background light do in order to isolate S-cones?

Answer 16

adapts M and L cones to increase their threshold (thereby decreasing sensitivity)

Question 17

Using dim conditions, which cones are more sensitive to blue light with the yellow background? 1. Under brighter conditions which cones? 2

Answer 17

1. M cones

2. S cones

Question 18

What tool can be better at detecting early VF defects in glaucoma than standard white-on-white perimetry? 1. What are the disadvantages of this tool? 2

Answer 18

1. SWAP

2. more heavily influenced by ocular media opacities like cataracts

Question 19

What is the alteration of the ratio of the signal output to the signal input within a given system? 1. What is the essential molecule involved in this within photoreceptors? 2

Answer 19

1. gain control

2. Ca2+

Question 20

If a constant light is shown on photoreceptors, what is the adaptation within the PR? 1. What type of PR is it more prominent in? 2

Answer 20

1. rollback in response by depolarizing cell to be able to be hyperpolarized with even brighter light
2. cones>rods

Question 21

What is the physiological explanation for photoreceptor adaptation to a background light?

Answer 21

1. Calcium continues to be pumped out of cell but no longer flowing into cell
2. GC is not inhibited by Ca2+ so converts GTP into cGMP
3. some cGMP-gated channels open so cell depolarizes
4. balance between PDE and GC on cGMP achieved as background light is on
5. membrane potential in between dark state and immediately after light