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Visual Science > Lecture 14: Dark and Light Adaptation (Lecture 9) > Flashcards

Lecture 14: Dark and Light Adaptation (Lecture 9) Flashcards

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1
Q
  1. Dark Adaptation forms the basis of what theory?
  2. What is Radiometry concerned with?
  3. What does Photometry deal with?
A
  1. Duplicity Theory
  2. The power produced by a source of Electromagnetic Radiation, irrespective of its effect on vision.
  3. Deals w/the effect that this radiation has on the Visual System
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2
Q
  1. The basis for Photometric measurements is what?
A
  1. The Photopic Luminosity Curve (V(lambda)) which shows that certain wavelengths are more efficient at stimulating the visual system than others.
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3
Q
  1. When is the cone mechanism involved in mediating vision? (Above what Luminance Level)
A
  1. About 0.03 cd/m^2 (Photopic Vision)
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4
Q

Retinal Densitometry

  1. What does Dark Adaptation also depend upon?
  2. What is Retinal Densitometry based on measuring?
    a. What can it be used to determine?
A
  1. Photopigment Bleaching
  2. on measuring the Light Reflected from the Fundus of the Eye
    a. The amt of photopigment bleached
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5
Q
  1. How does light adaptation work? (3 things)

2. Absorption curve is the reciprocal of what curve?

A
  1. Turn off the source; use a Red Fixation Light, and Test spot at 20 degrees from the fovea
  2. The Transmission curve
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6
Q
  1. How much light is needed to bleach a Molecule of Rhodopsin?
  2. When Rhodopsin is in the Bleached state, what can it not do?
  3. Bleached rhodopsin will spontaneously revert back to the unbleached state. What is the chance of that happening?
  4. What quanta have the highest probability of absorption?
A
  1. 1 Quantum of light
  2. It’s not capable of capturing another Quantum, so it will transmit a quantum of light
    incident on it.
  3. 0.50 over a 5 minute period.
  4. 507 nm quanta.
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7
Q

Factors Affecting Dark Adaptation

  1. What 2 things have to be considered about Pre-Adapting Light?
  2. What other 3 things?
A
  1. Intensity and Duration of the pre-adapting light
  2. a. Size and position of the Retinal area used in Measuring dark adaptation
    b. Wavelength distribution of the light used
    c. Rhodopsin Regeneration
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8
Q

Intensity and Duration of Pre-Adapting Light

  1. With increasing levels of pre-adapting luminances, the CONE BRANCH becomes what?
  2. At low levels of pre-adapting luminances, what happens?
A
  1. becomes LONGER and the ROD branch becomes more delayed

2. the Rod threshold drops quickly to REACH ABSOLUTE THRESHOLD

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9
Q

Effects of Stimulus Wavelength

  1. If the stimulus is 650 nm, what is seen?
  2. Is the Scotopic system more sensitive than the photopic system?
A
  1. ONLY the CONE PORTION. Rod aspect is missing

2. Scotopic system is NOT more sensitive than the photopic system. There’s not an obvious rod-cone break.

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10
Q

Dark Adaptation Curves for Stimuli of 465 and 610 nm

  1. 465 nm stimulus: What happens?
    a. At 40 minutes, what happens?
  2. The Photochromatic interval: What is it?
A
  1. 15 minutes of dark adaptation: we see a Cone plateau
    a. Rod portion of the dark adaptation curve for 465 nm levels off
  2. It’s the SEPARATION of the Cone and Rod Plateaus!
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11
Q

Stimulus Size and Location

  1. What do we see if we make the stimulus VERY SMALL (0.5 degrees) and Confine it to the FOVEA?
  2. What if a larger test spot is used?
A
  1. Only see a CONE FUNCTION

2. a rod-cone break is present. As the test spot gets larger, the sensitivity of the EYE in the DARK is GREATER!!

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12
Q

How long does it take for 50% of cone photopigments to recover after bleaching?

A
  1. 1.5 minutes. So it’s faster than the Rod Photopigments (Rhodopsin)
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13
Q

Looking at the Photopic Spectral sensitivity as a function of wavelength, where do we see a SINGLE BROAD PEAK?

Purkinje SHift: Defint

A
  1. In the region of about 555 nm. So this represents the addition of M and L cone absorption spectra. It’s thought that S-cones make little, if any, contribution to spectral sensitivity.
  2. Lighting conditions change from scotopic to photopic, wavelength to which we are most sensitive increases from 507 to 555 nm. (Relative increase in brightness of longer wavelength stimuli as lighting changes)
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14
Q

Physiological Basis of Dark Adaptation

  1. What happens during dark adaptation?
  2. Photochemical Explanation of dark adaptation: What is it?
A
  1. Photopigment regeneration increases the probability of quantal absorptions, thus increasing sensitivity.
  2. Photopigment regeneration fully explains the recovery of sensitivity that occurs during dark adaptation
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15
Q

Light Adaptation

  1. What does the eye have to quickly adapt to?
  2. How is threshold determined?
    a. What happens as Intensity is increased?
  3. Procedure is performed for background light levels ranging from what?
    a. What does this result in?
A
  1. The Background illumination in order to be able to distinguish objects in this background
  2. It’s determined for a Flash of light background
    a. Threshold measurements are increased
  3. From darkness to Extreme brightness.
    a. in a light adapation curve that show increment threshold (I) as a function of the background adapting intensity (IB)
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16
Q

Scotopic Light Adaptation (3-16 C: Pg 45)

  1. Slope (m = 0): What does this represent?
  2. m = 1/2 represents what?
  3. In order to detect a stimulus, the stimulus must EXCEED what?
  4. What is DeVries-Rose Law?
A
  1. range of light levels where detection is LIMITED by neural noise inherent w/in the visual system.
    a. Thermal Isomerisations of photopigment, spontaneous opening of photoreceptor membrane channels and spontaneous neurotransmitter release
  2. Reflects Quantal Fluctuations in the background (DeVries-Rose Law: predicts that Change of I is equal to IB^1/2)
  3. the fluctuations of the background (noise)
  4. predicts that I is Equal to (IB)^(1/2)
17
Q

Scotopic Light Adaptation

  1. As the Background Brightness is increased, what must be increased and to what?
  2. What is Weber’s fraction for Scotopic Vision?
  3. A Slope of infinity means what?
    a. What does this mean?
  4. When about 10% of rhodopsin molecules are bleached, what does this mean?
A
  1. Increment Intensity has to be increased such that the ratio of the Increment Intensity (I) to the background Intensity (IB) remains CONSTANT: I/IB = Weber’s fraction or Weber’s constant
  2. 0.14
  3. Rods are saturated
    a. It’s not possible for rods to signal the Increment stimulus, no matter how bright it is, cuz they’re overwhelmed by the brightness of the background
  4. All Na+ Channels are basically closed, and further bleaching of Rhodopsin will not produce anymore Hyperpolarization (so the rod is saturated)
18
Q

Photopic Light Adaptation

  1. What is the Weber Fraction here?
  2. What does this indicate?
  3. Even though this may be the case, What about ABSOLUTE SENSITIVITY?
A
  1. 0.015
  2. Photopic system is MORE SENSITIVE to CONTRAST than the Scotopic System
  3. It is less in the photopic system than in scotopic system
19
Q

Spatial Resolution and Spatial Summation

  1. Photopic VA?
  2. Scotopic VA? Difference?
  3. Contrast Sensitivity higher where?
  4. While visual resolution and contrast sensitivity are SUPERIOR under photopic conditions, ABSOLUTE SENSITIVITY is what?
A
  1. 20/20
  2. 20/200; about 1 log-unit difference
  3. under photopic conditions. Weber fraction is 0.015 (compared to 0.14 for scotopic vision)
  4. is greater under scotopic conditions
20
Q

Spatial Resolution and Spatial Summation

  1. Rods are connected in such a manner that they do what with info?
    a. What does this produce?
  2. What do cones do?
  3. 500-nm stimulus to be detected under photopic conditions has to be how much more intense?
A
  1. They sum up info over space
    a. Great sensitivity, but poor resolution
  2. Have connection that MAXIMIZE visual Resolution at the expense of sensitivity
  3. about 3 log units (1000 times) more intense
21
Q

Spatial Resolution and Spatial Summation

  1. 2 spots of light produce a total of 10 quantal absorptions. What does the ganglion do?
  2. What does the Scotopic System show?
A
  1. It sums this information to produce a signal that indicates the presence of a SINGLE LIGHT (this is just an example)
  2. Excellent Spatial Summation and Poor Spatial Resolution
22
Q

Spatial Resolution and Spatial Summation

  1. No stimulus under the same conditions is seen in Photopic System: Why?
  2. What if we double the number of quanta?
A
  1. Due to LIMITED SPATIAL SUMMATION of the photopic system. Prevents it from adding up the info contained in both spots of light.
  2. 2 stimuli would be seen. (info gathered from 2 separate ganglion)
23
Q

Ricco’s Law

  1. Equation?
  2. Threshold number of quanta can be delivered in how much time?
  3. What does this mean?
  4. Difference b/w scotopic and photopic systems?
A
  1. IA = K
    a. I = stimulus intensity (Quanta/Area)
    b. A = Stimulus Area
    c. K = Constant
  2. in a 1-minute Arc test spot, or spread out over a larger area, up to 10-minutes arc, which is the so-called CRITICAL DIAMETER

3 That the Scotopic system manifest total spatial summation for stimuli that fall w/in the Critical Diameter

  1. A Difference in Critical Diameters for the 2 systems. Critical Diameter of the Photopic System is SMALLER than that for the Scotopic System. This results in REDUCED SPATIAL SUMMATION capability of the photopic system.
24
Q

Temporal Resolution and Temporal Summation

  1. Photopic System is better able to distinguish what?
  2. Scotopic system’s HIGH degree of Temporal summation limits its ability to do what?
A
  1. 2 flashes of light separated by a brief interval in time

2. to resolve distinct temporal events

25
Q
  1. Although the photopic system demonstrates poor temporal summation, it does manifest what?
  2. High degree of temporal summation of the scotopic system is consistent with what?
A
  1. superior temporal resolution

2. with its greater absolute sensitivity

26
Q

Bloch’s Law

  1. Eqn?
  2. Multiple Flashes presented w/in this critical duration are what?
  3. Scotopic vision shows a critical duration of what?
    a. Photopic?
A
  1. It = K
    a. I = stimulus intensity (quanta/time)
    b. t = stimulus duration
    c. K = Constant
  2. are not resolved, and only 1 flash is seen
  3. of about 100 milliseconds
    a. About 10-50 milliseconds

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