Light and dark adaptation

Light and Dark Adaptation

Gauri S. Shrestha, M. Optom, FIACLE
Lecturer, BPKLCOS, IOM

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

Rods
 Peak density of rods occurs
20o from fovea
 150,000 rods/mm2
 No rods are present at the
fovea:
 Total Number: 120 million


Cones
 Cones are most
densely packed at the
fovea
 150,000 cones/mm2
 Only 4% of total
cones are foveal
 Total Number: 6
million


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


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


Observations
 Choose particular wave
length and increase intensity

 Change the wave length
from high frequency to low
frequency

Chromatic interval=0

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)


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’


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


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.


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.

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.


Fig 10. Light adaptation using an increment threshold experiment


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.

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).

Light Adaptation Experiments


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


Light Adaptation Curve
 Section 3:
 Slope = 1
 Indicates Weber’s Law:
threshold contrast
remains constant as the
illumination changes
 Scotopic constant: k =
0.14


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


 Section 4:
 Slope = infinity
 With high levels of illumination, rods cannot
detect any stimulus
 When does rod saturation occur?
 10% of rhodopsin is bleached



 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


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.


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


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


The Goldmann-Weekers
Machine


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


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.


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


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.


Intensity and duration of pre-adapting light


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.


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.


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.

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


Wavelength of the threshold light


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??


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.


Rhodopsin regeneration

Log relative threshold as a function of the
percentage of photopigment bleached

Clinical Pearl


Clinical Pearl
 Rod monochromatism
 Sensory nystagmus

 Blurred vision

 Photophobia

 Color vision


Congenital stationary night blindness
 Psychophysical and
electrophysiological
findings in negative
electroretinography (ERG)
selective loss of the b-
wave


Retinitis Pigmentosa
 Depression of the curve occurs in conditions affecting
the outer retina and RPE


Prolong Dark adaptation
 Vitamin A deficiency
 Effect of anoxia
 Effect of tobacco inhalation
 Anaesthesia
 Ocular opacities
 Retinal degeneration
 Myopia
 Glaucoma ?


 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.


Light and dark adaptation

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