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Light and dark adaptation
1. Light and Dark Adaptation
Gauri S. Shrestha, M. Optom, FIACLE
Lecturer, BPKLCOS, IOM
2. Rods
Rhodopsin
most readily absorbs wavelengths of 507 nm.
a molecule of rhodopsin absorbs one quanta of
light, it is ‘bleached’
Bleached: the molecule is not capable of
capturing another quantum
Spontaneously become ‘unbleached’
50% recover within 5 minutes
Gauri S. Shrestha, M.Optom, FIACLE
3. Rods
Peak density of rods occurs
20o from fovea
150,000 rods/mm2
No rods are present at the
fovea:
Total Number: 120 million
Gauri S. Shrestha, M.Optom, FIACLE
4. Cones
Cones are most
densely packed at the
fovea
150,000 cones/mm2
Only 4% of total
cones are foveal
Total Number: 6
million
Gauri S. Shrestha, M.Optom, FIACLE
5. Cones
3 types of cone photopigments:
Erythrolabe: maximum absorption at 565 nm
‘Long wavelength cones’ (L-cones), red cones
Chlorolabe: maximum absorption at 535 nm
‘Middle wavelength cones’ (M-cones), green cones
Cyanolabe: maximum absorption at 430 nm
‘Short wavelength cones’ (S-cones), blue cones
Recover from bleaching more rapidly than rhodopsin
50% of cones will recover within 1.5 minutes
Gauri S. Shrestha, M.Optom, FIACLE
6. Specific Cone Distribution
Ratio of L-cones to M-cones to S-cones = 16/8/1
S-cones are less numerous than either L-cones or
M-cones.
No S-cones at the fovea
Peak distribution occurs just outside the fovea
Small field Tritanopia
unable to see small blue objects when centrally
fixated
Gauri S. Shrestha, M.Optom, FIACLE
7. Observations
Choose particular wave
length and increase intensity
Change the wave length
from high frequency to low
frequency
Chromatic interval=0
Gauri S. Shrestha, M.Optom, FIACLE
8. Visual Thresholds
Minimum amount of energy required for a patient to detect a
stimulus
low threshold = high sensitivity
Threshold = 1/Sensitivity
Scotopic Threshold:
threshold of a patient measured in dim light conditions
(night)
Photopic Threshold:
threshold of a patient measured in bright light conditions
(sunny)
Gauri S. Shrestha, M.Optom, FIACLE
9. Purkinje Shift
Scotopic System:
Stimuli of 507 nm are perceived brighter than other stimuli
Photopic System:
Stimuli of 555 nm are perceived brighter than other stimuli
The difference in the peak sensitivity of the 2 systems is
the ‘Purkinje Shift’
Gauri S. Shrestha, M.Optom, FIACLE
10. Visual Adaptation
Human visual system
Sensitive to a variety of range of illumination
Phenomenon of our visual system
to be capable of functioning in various
illumination visual adaptation
Types:
Light Adaptation
Dark Adaptation
Gauri S. Shrestha, M.Optom, FIACLE
11. Light adaptation
visual adaptation to increased levels of
illumination.
Promptly occurring over a period of 5
minutes
With light adaptation, the eye has to quickly adapt
to the background illumination to be able to
distinguish objects in this background.
Gauri S. Shrestha, M.Optom, FIACLE
12. Light adaptation
Light adaptation can be explored by determining
increment thresholds
In an increment threshold experiment, a test stimulus is
presented on a background of a certain luminance
The stimulus is increased in luminance until detection
threshold is reached against the background
Therefore, the independent variable is the luminance of the
background and the dependent variable is the threshold
intensity or luminance of the incremental test required for
detection.
Such an approach is used when visual fields are measured
in clinical practice.
Gauri S. Shrestha, M.Optom, FIACLE
13. Light adaptation
The experimental conditions shown in figure can
be repeated by changing the background field
luminance.
Depending upon the choice of test, background
wavelength, the test size and retinal eccentricity
a monophasic or biphasic threshold versus
intensity curve is obtained.
Gauri S. Shrestha, M.Optom, FIACLE
14. Fig 10. Light adaptation using an increment threshold experiment
Gauri S. Shrestha, M.Optom, FIACLE
15. Light adaptation
Fig11 illustrates such a curve for parafoveal
presentation of a yellow test field on a green
background.
This stimulus choice leads to two branches.
A lower branch belonging to the rod system.
As the background light level increases, visual
function shifts from the rod system to the cone
system.
A dual-branched curve reflects the duplex nature
of vision, similar to the bi-phasic response in the
dark adaptation curve.
Gauri S. Shrestha, M.Optom, FIACLE
16. Fig11. Light adaptation curve plotted as increment threshold versus
background luminance (or a threshold-versus-intensity: tvi curve). The
above plot shows increment threshold (Nl ) and background
luminance (Mm ). Light of two different wavelengths are used in this
case (580 nm for the test and 500 nm for the background).
Gauri S. Shrestha, M.Optom, FIACLE
18. What does the graph mean?
Sections 1-4 are the ‘scotopic
portion’
Section 1: (Slope = 0)
Detection is limited by neural
noise in very low light levels
(practically black)
Section 2: (Slope = ½)
Background is very dim.
Fluctuations inherent in the
light source play primary role
in determining threshold
Gauri S. Shrestha, M.Optom, FIACLE
19. Light Adaptation Curve
Section 3:
Slope = 1
Indicates Weber’s Law:
threshold contrast
remains constant as the
illumination changes
Scotopic constant: k =
0.14
Gauri S. Shrestha, M.Optom, FIACLE
20. Weber’s Law
What do we know from Weber’s Law?
∆I = kIb
As background illumination increases, the
increment intensity (JND, increment
threshold) increases
If threshold increases, what happens to
absolute sensitivity?
Absolute sensitivity decreases
As illumination increases, we are less
sensitive
Gauri S. Shrestha, M.Optom, FIACLE
21. Light Adaptation Curve
Section 4:
Slope = infinity
With high levels of illumination, rods cannot
detect any stimulus
When does rod saturation occur?
10% of rhodopsin is bleached
Gauri S. Shrestha, M.Optom, FIACLE
22. Light Adaptation Curve
Section 5:
Slope = 1
makes up the photopic portion of the curve
Photopic system is following Weber’s Law (k =
0.015)
Photopic system is much more sensitive to
contrast than the scotopic system
Gauri S. Shrestha, M.Optom, FIACLE
23. Mechanism of light adaptation
Photochemical reaction
Rhodopsin Retinal + Opsin
Rod:
Saturates once the light is moderately bright.
Cones:
Continue to adapt and respond to brighter
illumination
Reaches to maximum after 5-10 mins.
Gauri S. Shrestha, M.Optom, FIACLE
24. Mechanism of light adaptation
Photoregeneration:
Isomerisation of the immediate products of bleaching-
back into the photosensitive pigments.
Threshold rise when,
Rate of bleaching regeneration of
photopigment
Gauri S. Shrestha, M.Optom, FIACLE
25. Dark Adaptation
Subject is exposed to a bright adapting light so that
most of the photo pigments are bleached
Light is then turned off and detection threshold is
measured repeatedly over a period of time
The background is totally dark
The stimulus is large, centrally fixated
Wavelength is 420 nm
Gauri S. Shrestha, M.Optom, FIACLE
28. Cones and Rods…
First Section
Rapid threshold reduction (5 min) followed by
plateau
This represents the cone thresholds
Cone-Rod Break
Break in curve that occurs after 10 min. of
adaptation
Second Section
Reduction in threshold that extends to 35 minutes
This represents the rod thresholds
Gauri S. Shrestha, M.Optom, FIACLE
29. Dark Adaptation Curve
Cones recover within 5 min while rods take up to
35 min to fully recover sensitivity
The Rod-Cone break is the point where the rods
become more sensitive than the cones
The two plateaus represent the thresholds of the
rods and cones.
Gauri S. Shrestha, M.Optom, FIACLE
31. Factors Affecting Dark Adaptation.
Intensity and duration of the pre-adapting light
Size and position of the retina used in
measuring dark adaptation
Wavelength distribution of the light used
Rhodopsin regeneration
Gauri S. Shrestha, M.Optom, FIACLE
32. Intensity of pre-adapting light
With increasing levels of pre-adapting luminance
the cone branch becomes longer while the rod branch
becomes more delayed.
Absolute threshold takes longer time to reach.
At low levels of pre-adapting luminance
rod threshold drops quickly to reach absolute
threshold.
Gauri S. Shrestha, M.Optom, FIACLE
34. Duration of pre-adapting light
Shorter the duration of the pre-adapting light, the more
rapid the decrease in dark adaptation.
For extremely short pre-adaptation periods
a single rod curve is obtained.
Only after long pre-adaptation, a bi-phasic, cone and
rod branches are obtained.
Gauri S. Shrestha, M.Optom, FIACLE
36. Size and location of the retina used
The retinal location used to register the test spot
during dark adaptation will affect the dark adaptation
curve
When a small test spot is located at the fovea
(eccentricity of 0o), only one branch is seen with a
higher threshold compared to the rod branch.
When the same size test spot is used in the peripheral
retina during dark adaptation
the typical break appears in the curve representing
the cone branch and the rod branch.
Gauri S. Shrestha, M.Optom, FIACLE
37. Size and location of the retina used
Gauri S. Shrestha, M.Optom, FIACLE
38. Location of the retina used
Gauri S. Shrestha, M.Optom, FIACLE
39. Size and location of the retina used
A single branch obtained when a small test spot is
used
only cones present at the fovea are stimulated.
A rod-cone break is seen when a larger test spot is
used
the test spot stimulates both cones and rods.
As the test spot becomes even larger, incorporating
more rods, the sensitivity of the eye in the dark is
even greater
reflecting the larger spatial summation
characteristics of the rod pathway.
Gauri S. Shrestha, M.Optom, FIACLE
40. Wavelength of the threshold light
From figure, a rod-cone
break is not seen when
using light of long
wavelengths such as
extreme red.
This occurs due to rods
and cones having
similar sensitivities to
light of long
wavelengths
Gauri S. Shrestha, M.Optom, FIACLE
41. Wavelength of the threshold light
Gauri S. Shrestha, M.Optom, FIACLE
42. Wavelength of threshold light
depicts the photopic and scotopic spectral
sensitivity functions
to illustrate the point that the rod and cone
sensitivity difference is dependent upon test
wavelength.
when light of short wavelength is used,
the rod-cone break is most prominent??
Gauri S. Shrestha, M.Optom, FIACLE
43. Rhodopsin regeneration
Dark adaptation also depends upon photopigment
bleaching
The time course for dark adaptation and rhodopsin
regeneration was same
Using retinal densitometry.
Bleaching rhodopsin by 1% raises threshold by 10
(decreases sensitivity by 10)
Bleaching of cone photopigment has a smaller
effect on cone thresholds.
Gauri S. Shrestha, M.Optom, FIACLE
44. Rhodopsin regeneration
Log relative threshold as a function of the
percentage of photopigment bleached
Gauri S. Shrestha, M.Optom, FIACLE
46. Clinical Pearl
Rod monochromatism
Sensory nystagmus
Blurred vision
Photophobia
Color vision
Gauri S. Shrestha, M.Optom, FIACLE
47. Congenital stationary night blindness
Psychophysical and
electrophysiological
findings in negative
electroretinography (ERG)
selective loss of the b-
wave
Gauri S. Shrestha, M.Optom, FIACLE
48. Retinitis Pigmentosa
Depression of the curve occurs in conditions affecting
the outer retina and RPE
Gauri S. Shrestha, M.Optom, FIACLE
49. Prolong Dark adaptation
Vitamin A deficiency
Effect of anoxia
Effect of tobacco inhalation
Anaesthesia
Ocular opacities
Retinal degeneration
Myopia
Glaucoma ?
Gauri S. Shrestha, M.Optom, FIACLE
50. Which type of tint is better to preserve bleaching
of rods?
Does photochromic lens help preserve dark
adaptation?
Progressive conditions with impaired dark
adaptations are __________ & ________
Name the tints that help in optimizing dark
adaptation.
Gauri S. Shrestha, M.Optom, FIACLE
Hinweis der Redaktion
Foveally fixated object blue light= dull
With dark adaptation, we noticed that there is progressive decrease in threshold (increase in sensitivity) with time in the dark.
Dark adaptation depends upon differing intensities and duration of pre-adapting light. With increasing levels of pre-adapting luminance, the cone branch becomes longer while the rod branch becomes more delayed. Absolute threshold takes longer time to reach. At low levels of pre-adapting luminance, rod threshold drops quickly to reach absolute threshold.
The shorter the duration of the pre-adapting light, the more rapid the decrease in dark adaptation. For extremely short pre-adaptation periods, a single rod curve is obtained. It is only after long pre-adaptation that a bi-phasic, cone and rod branches are obtained.
The retinal location used to register the test spot during dark adaptation will affect the dark adaptation curve due to the distribution of the rod and cones in the retina. When a small test spot is located at the fovea (eccentricity of 0 o ), only one branch is seen with a higher threshold compared to the rod branch. When the same size test spot is used in the peripheral retina during dark adaptation, the typical break appears in the curve representing the cone branch and the rod branch.