Light Adaptation (M1) Flashcards Preview

II. Neurophysiology and Perception > Light Adaptation (M1) > Flashcards

Flashcards in Light Adaptation (M1) Deck (21)

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



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

1. m=0
2. background light no different from neutral noise


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

1. m=1/2
2. at low light levels, threshold detection is limited by quantal fluctuations in light source (Devires-Rose law)


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

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


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

by about 1 standard deviation (square root of mean)


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

1. m= about 1
2. threshold change in I is larger than predicted by quantal fluctuations (Weber's Law)


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?

Weber's law


What is Weber's maximum fraction in rods?



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

sensitivity regulation


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

1. m=infinity
2. rods have become saturated


What generally occurs before the rods become completely saturated?

switch to cone-mediated vision


What is Weber's maximum fraction in cones?



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

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


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

blue stimulus light on yellow background


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?

increase proportionately


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

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


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

1. M cones
2. S cones


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

2. more heavily influenced by ocular media opacities like cataracts


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

1. gain control
2. Ca2+


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

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


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

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