CHAPTER 4. Adaptation to Light and Dark
We can see objects even though the
background luminance levels change
over a range of more than 10 orders of
magnitude (1010 ).
How do we do it?
Reminder about why we are doing all this:

As a clinician, you need to understand the
scientific basis on which measurements of
vision are made and how they can be made in
the future as new tests of visual function are
developed and put into clinical practice.

For instance, dark adaptation rate may turn out
to be a way to diagnose Age-related Macular
Degeneration (AMD) very early – trials
underway
Three main purposes of course
1) Learn how vision is measured
2) Basic facts about monocular visual function (What is
normal?)
3) Neural basis of visual function (Why does the visual
system respond as it does?)
Three main purposes of course - Adaptation
1) Learn how vision is measured
•
Will measure a group dark adaptation curve in lab
2) Basic facts about monocular visual function (What is
normal?)
•
Different curves from different test flash & adapting
light conditions
3) Neural basis of visual function (Why does the visual
system respond as it does?)
•
mechanisms
The visual system uses four mechanisms to adapt to a wide
range of light levels
1) two different photoreceptor sub-systems (duplicity theory)
Rods - low luminance (scotopic) conditions
Very sensitive at low background luminance
Saturate at high luminance (~102 cd/m2)
Poor color discrimination
Low spatial resolution (e.g., low spatial acuity) because large ganglion cell
receptive fields
Low temporal resolution (e.g., low temporal acuity) because slower recovery from
quantal absorption
Cones - high luminance (photopic) conditions
Insensitive to low luminance (high threshold)
Active in high luminance
color selective (3 cone pigments)
high spatial resolution (especially in the fovea)
high temporal resolution
2) change the pupil size
alters the retinal illuminance by about 1.2 log units
3) changes in the concentration of photopigment
4) changes in neural responsiveness (also called “network” responsiveness.)
Visual adaptation is the process whereby the visual
system adjusts its operating level to the prevailing light
level.
Light adaptation is the process that decreases sensitivity
(increases threshold luminance) in response to an
adapting light.
Dark adaptation is defined as the increase in sensitivity
(decrease in threshold luminance) as a function of time in
darkness.
“Typical” Dark Adaptation Curve
Log Threshold Luminance
9
8
Rod-Cone Break
7
Cone Branch
6
5
4
Rod Branch
3
2
0
10
20
Time in the Dark (min)
Adapting light goes off at time = 0
30
40
Dark Adaptation
The task:
Measure the threshold intensity as the visual system dark
adapts
This is a “moving target” because the threshold decreases
over time.
Dark Adaptation lab on Thursday
The task: measure a “group dark adaptation curve”
Everyone in the group will light adapt. Then everyone will
take a turn as a subject (have your threshold measured)
and as an examiner (measure the threshold intensity of
your classmate) as the visual system dark adapts
This is a “moving target” because the threshold decreases
over time.
The winning group will be awarded two six-packs*
The winning group gets to decide the content of each six-pack (water, beer,
Coke, Pepsi, etc.)
1) Rods and cones both start dark adapting at time 0
2) the more sensitive system at that time determines the
threshold
3) cones dark adapt faster than rods
4) the lowest thresholds obtained using cones are much
higher than the lowest thresholds obtained with rods (rods,
potentially, are more sensitive than cones)
Log Threshold Luminance
9
8
Rod-Cone Break
7
Cone Branch
6
5
4
Rod Branch
3
“sneak up” on threshold from below
2
0
10
20
Time in the Dark (min)
30
40
Note: If using the
Method of Limits,
must only use the
ascending branch to
avoid changing the
time-course of the
dark adaptation
The 2009 winning group
Important Stimulus Dimensions
* = important
*
parameters in dark
adaptation studies
Intensity (of adapting light)
*
wavelength
*
*
size
exposure duration (to adapting light)
frequency
shape
relative locations of elements of the stimulus
cognitive meaning
In addition,(not a stimulus dimension)
*
*
location on the subject’s retina
light adaptation of the subject’s visual system
Variations in the dark adaptation curves help to
illustrate the importance of knowing what you are
doing when making psychophysical measurements.
What you get depends on how you make the measures
Different situations give very different results
Variations in the Shape of the Dark Adaptation Curve
Depend on:
1) the part of the retina that is stimulated by the test flash
a) fovea; no rods, only see the cone branch
b) periphery; both rod and cone branches possible
2) the size of the test flash
3) the wavelength of light used for the adapting light
and/or
4) the wavelength of the test flash
5) the intensity of the adapting light
6) the duration of the adapting light
7) the task that the subject is asked to perform.
The subject always will see first with the more sensitive system
In order to see both the rod and cone branches during dark
adaptation, the adapting light and test spot must stimulate both
rods and cones
Cells/mm
Fig. 2.1
200,000
2
Cones
Optic Nerve Head Blind Spot
150,000
100,000
50,000
0
Rods
Nasal
Temporal
-20
-15
-10
-5
0
5
10
15
20
Eccentricity From Fovea (mm)
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
Eccentricity From Fovea (deg)
Distribution of rods and cones along the horizontal meridian in a human retina.
Data provided by Dr. Christine Curcio.
70
Retinal Location (2 deg spot)
Log Threshold
Luminance (μmillilamberts)
4.5
4.0
3.5
0o
3.0
2.5
2.0
1.5
2.5o
Broadband 300 millilambert adapting field,
2 min exposure
5o
10o
2° Spot flashed 1 s every 2 s
1.0
0
10
20
Time in the Dark (min)
30
40
Retinal Location (2 deg spot)
Log Threshold
Luminance (μmillilamberts)
4.5
4.0
3.5
0o
3.0
2.5
2.0
1.5
2.5o
Broadband 300 millilambert adapting field,
2 min exposure
5o
10o
2° Spot flashed 1 s every 2 s
1.0
0
10
20
Time in the Dark (min)
30
40
Test flash size (centered on fovea)
Log Threshold
Luminance (μmillilamberts)
4.0
2o
3.5
3o
3.0
2.5
5o
2.0
10o
1.5
20o
1.0
0
10
20
Time in the Dark (min)
30
Effects of Test Flash Wavelength on the
Shape of the Dark Adaptation Curve
Peak rod absorption
|
400 nm
|
500 nm
600 nm
700 nm
Effects of Test Flash Wavelength on the
Shape of the Dark Adaptation Curve
-Rods absorb poorly at long wavelengths
Peak rod absorption
400 nm
500 nm
|
600 nm
700 nm
Effect of Test flash Wavelength
Threshold Intensity (dB)
“decibels” (dB) is a log scale
50
40
>680 nm
620-700 nm
30
550-620 nm
485-570 nm
400-700 nm
20
0
10
20
30
Time in the Dark (min)
40
50
Adapting Light Wavelength
|
400 nm
|
|
500 nm
Test flash
600 nm
700 nm
Effect of Adapting Light Wavelength
|
|
400 nm
500 nm
600 nm
|
Test flash
|
700 nm
Adapting light wavelength (blue test flash)
Log Threshold
Luminance (μμlamberts)
3.0
Red at 38.9 mL
White at 26.3 mL
2.5
2.0
1.5
1.0
0.5
0.0
0
5
10
15
Time in the Dark (min)
20
25
Variations in the Shape of the Dark Adaptation Curve
Depend on:
1) the part of the retina that is stimulated by the test flash
a) fovea; no rods, only see the cone branch
b) periphery; both rod and cone branches possible
2) the size of the test flash
3) the wavelength of light used for the adapting light
and/or
4) the wavelength of the test flash
5) the intensity of the adapting light
6) the duration of the adapting light
7) the task that the subject is asked to perform.
The subject always will see first with the more sensitive system
Adapting light intensity
Log Threshold
Intensity
Illuminance (μTroland)
9
Adapting Intensity (trolands)
400,000
38,000
19,000
3,000
263
8
7
6
5
4
3
2
0
10
20
TIme in the Dark (min)
30
40
Adapting light duration
Log Threshold
Intensity
Luminance (millilamberts)
20 min
10 min
5 min
2 min
1 min
10 s
-1
333 millilamberts
-2
-3
0
10
20
Time in the Dark (min)
30
40
Luminance needed to detect grating orientation
Log Threshold
Luminance (millilamberts)
VA 1.04
VA = 0.62
VA = 0.25
VA= 0.083
VA = 0.042
NO GRATING
2
1
0
If you need cones
to do the task,
then do not get a
rod branch
-1
-2
-3
-4
0
5
10
15
20
Time in the Dark (min)
25
30
35
Early Dark Adaptation
1) Rapid decrease in test flash threshold (< 0.4 s)
due to neural (not photopigment) changes
Log Threshold
Intensity
Illuminance (trolands)
4.0
Adapting Field Intensity (Td)
57,000
1,800
57
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.4
0.0
0.4
0.8
1.2
1.6
Time of Onset of Stimulus Flash (s)
2.0
Three Points about Early Dark Adaptation
1) Rapid decrease in test flash threshold (< 0.4 s)
due to neural (not photopigment) changes
2) Increase in threshold to detect test flash if it is presented exactly at time zero
signal to noise issue
3) Threshold for detecting test flash starts to rise just before time zero
threshold response to test flash “cut off” by response to adapting light
A
On
20 msec test flash
Off
L
Response to threshold test flash alone
Response to test flash
All of these action potentials are
needed to see the test flash
B
Adapting light
On
Off
Response to adapting light offset alone
L
Response to adapting light
C
Test flash long before adapting light offset
D
Test flash just before adapting light offset
Test flash time
Response to both
Test flash time
Response to both
E
Test flash time
F
Test flash time
Test flash same time as adapting light offset
Response to both
Test flash long after adapting light offset
Response to both
0
Time
A
On
20 msec test flash
Off
L
Response to threshold test flash alone
Response to test flash
All of these action potentials are
needed to see the test flash
B
Adapting light
On
Off
Response to adapting light offset alone
L
Response to adapting light
C
Test flash long before adapting light offset
D
Test flash just before adapting light offset
Test flash time
Response to both
Test flash time
Response to both
The response to the test flash is
“cut off”; not enough APs to detect
E
Test flash time
F
Test flash time
Test flash same time as adapting light offset
Response to both
Test flash long after adapting light offset
Response to both
0
Time
A
On
20 msec test flash
Off
L
Response to threshold test flash alone
Response to test flash
All of these action potentials are
needed to see the test flash
B
Adapting light
On
Off
Response to adapting light offset alone
L
Response to adapting light
C
Test flash long before adapting light offset
Test flash time
Response to both
What happens when the test flash is
D
presented at different
times, relative to the
Test flash just before adapting light offset
adapting light offset?
The response to the test flash is
Test flash time
Response to both
“cut off”; not enough APs to detect
E
Remember,
we are
looking
at
Test flash
same time as adapting
light offset
Response to both
the
response of just ONE neuron, responding
F
to BOTH the test flash
and the adapting
Test flash long after adapting light offset
light offset.
0
Test flash time
Test flash time
Response to both
Time
Log Threshold
Intensity
4.0
Adapting Field Intensity (Td)
57,000
1,800
57
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.4
0.0
0.4
0.8
1.2
1.6
Time of Onset of Stimulus Flash (s)
2.0
A
On
20 msec test flash
Off
L
Response to threshold test flash alone
Response to test flash
All of these action potentials are
needed to see the test flash
B
Adapting light
On
Off
Response to adapting light offset alone
L
Response to adapting light
C
Test flash long before adapting light offset
D
Test flash just before adapting light offset
Test flash time
Response to both
Test flash time
Response to both
The response to the test flash is
How do you make the test flash visible again? Raise
the
“cut off”; not enough APs to detect
E
intensity to restore the needed
of action
Testnumber
flash time
Test flash same time as adapting light offset
Response to both
potentials
F
Test flash long after adapting light offset
Test flash time
The response to the test flash is
supporessed; not enough APs to
detect
Response to both
0
Time
Log Threshold
Intensity
4.0
Adapting Field Intensity (Td)
57,000
1,800
57
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.4
0.0
0.4
0.8
1.2
1.6
Time of Onset of Stimulus Flash (s)
2.0
A
On
20 msec test flash
Off
L
Response to threshold test flash alone
Response to test flash
All of these action potentials are
needed to see the test flash
B
Adapting light
On
Off
Response to adapting light offset alone
L
Response to adapting light
C
Test flash long before adapting light offset
D
Test flash just before adapting light offset
Test flash time
Response to both
Test flash time
Response to both
The response to the test flash is
“cut off”; not enough APs to detect
E
Test flash time
Test flash same time as adapting light offset
Response to both
The response to the test flash is
supporessed; not enough APs to
detect
How do you make the test flash visible again? Raise the
F
Test flash time
to restore
needed
number of action
Test flash intensity
long after adapting
lightthe
offset
Response to both
potentials
0
Time
Log Threshold
Intensity
4.0
Adapting Field Intensity (Td)
57,000
1,800
57
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.4
0.0
0.4
0.8
1.2
1.6
Time of Onset of Stimulus Flash (s)
2.0
A
On
20 msec test flash
Off
L
Response to threshold test flash alone
Response to test flash
All of these action potentials are
needed to see the test flash
B
Adapting light
On
Off
Response to adapting light offset alone
L
Response to adapting light
C
Test flash long before adapting light offset
D
Test flash just before adapting light offset
Test flash time
Response to both
Test flash time
Response to both
The response to the test flash is
“cut off”; not enough APs to detect
E
Test flash time
Test flash same time as adapting light offset
Response to both
F
Test flash long after adapting light offset
The response to the test flash is
suppressed; not enough APs to
detect
Test flash time
Response
to both again? Raise the
How do you make the test flash
visible
0
intensity to restore the needed number of action
Time
potentials
Log Threshold
Intensity
4.0
Adapting Field Intensity (Td)
57,000
1,800
57
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.4
0.0
0.4
0.8
1.2
1.6
Time of Onset of Stimulus Flash (s)
2.0
A
On
20 msec test flash
Off
L
Response to threshold test flash alone
Response to test flash
All of these action potentials are
needed to see the test flash
B
Adapting light
On
Off
Response to adapting light offset alone
L
Response to adapting light
C
Test flash long before adapting light offset
D
Test flash just before adapting light offset
Test flash time
Response to both
Test flash time
Response to both
The response to the test flash is
“cut off”; not enough APs to detect
E
Test flash time
Test flash same time as adapting light offset
Response to both
F
Test flash long after adapting light offset
The response to the test flash is
suppressed; not enough APs to
detect
Test flash time
Response to both
0
Time
Log Threshold
Intensity
Illuminance (trolands)
4.0
Adapting Field Intensity (Td)
57,000
1,800
57
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.4
0.0
0.4
0.8
1.2
1.6
Time of Onset of Stimulus Flash (s)
2.0
Three Points about Early Dark Adaptation
1) Rapid decrease in test flash threshold (< 0.4 s)
due to neural (not photopigment) changes
2) Increase in threshold to detect test flash if it is presented exactly at time zero
signal to noise issue
3) Threshold for detecting test flash starts to rise just before time zero
threshold response to test flash “cut off” by response to adapting light
Log Threshold Luminance
6
Oguchi's Disease
Congenital Stationary Night Blindness
5
4
Normal
3
Rod Monochromatism
2
0
10
20
30
Time in the Dark (min)
40
50
New research (Greg Jackson, just
moved from CEFH, UAB) suggests that
dark adaptation is slower in people who
are developing age-related macular
degeneration
Clinical trial ongoing on HPB 4th floor
Dark Adaptation
Log Threshold Luminance
9
8
Rod-Cone Break
7
Cone Branch
6
5
4
Rod Branch
3
2
0
10
20
Time in the Dark (min)
30
40
The visual system uses four mechanisms to adapt to a wide
range of light levels
1) two different photoreceptor sub-systems (duplicity theory)
Rods Cones 2) change the pupil size
alters the retinal illuminance by about 1.2 log units
3) changes in the concentration of photopigment.
4) changes in neural responsiveness (also called “network” responsiveness.)
The level of bleached photopigment explains much
of visual adaptation
Both for light adaptation and dark adaptation
Proportion of Pigment
in Bleached State
Regeneration of rhodopsin follows
a exponential decay function
1.0000
Retina with only rods
Normal retina
Half-time for
cones = 1.7 min
0.5000
rods, 5.2 min
0.2500
0.1250
0.0625
0.0000
0
5
10
15
20
Time in the Dark (min)
25
30
35
How much rhodopsin is still bleached after a given time in the dark? The general
equation is:
B = B0 x (0.5) (t/)
(4.1)
where B is the fraction of pigment remaining bleached, B0 is the initial fraction of pigment
bleached, t is the time after the bleaching light has been turned off, and  is the half-life
for the process.
At a practical level, the amount of bleached
photopigment is cut in half every 1.7 min for
cones and every 5.2 min for rods
The level of bleached photopigment explains much
of visual adaptation
Both for light adaptation and dark adaptation
If you bleach half of the photopigment, how
much does the threshold rise? If you bleach
¼ of the photopigment, is the threshold
elevated half as much (linear increase)?
The log of the threshold elevation (above
absolute threshold) is related to the fraction
of bleached rhodopsin
Rushton derived an equation that approximately relates the
amount of bleached pigment to visual sensitivity is:
log( I t / I o)  10 HB
(4.2)
where It is the threshold for detecting the test stimulus, I0 is
the absolute threshold, H is a constant, specific for the test
conditions, with a value of about 2, and B is the fraction of
pigment that is still bleached.
This gives how much the threshold is raised
above absolute threshold
The visual system uses four mechanisms to adapt to a wide
range of light levels
1) two different photoreceptor sub-systems (duplicity theory)
Rods Cones 2) change the pupil size
alters the retinal illuminance by about 1.2 log units
3) changes in the concentration of photopigment.
4) changes in neural responsiveness (also called “network” responsiveness.)
Log Threshold Intensity
9
Adapting Intensity (Trolands)
400,000
38,000
19,000
3,000
263
8
7
6
5
4
3
2
0
10
20
TIme in the Dark (min)
30
40
Proportion of Pigment
in Bleached State
1.0000
Retina with only rods
Normal retina
Time constant for
cones = 1.7 min
0.5000
rods, 5.2 min
0.2500
0.1250
0.0625
0.0000
0
5
10
15
20
Time in the Dark (min)
25
30
35
Percent of Pigment
Still Bleached
Log Threshold Intensity
Symbols = threshold
3
7.5
Lines = bleached pigment
2
5.0
1
Initial amount of
pigment bleached
13%
24%
42%
99%
2.5
0
0.0
0
5
10
15
20
Time in the Dark (min)
25
30
The level of bleached photopigment explains
much of visual adaptation
Another way the amount of bleached
pigment sets the threshold:
The Equivalent Background Theory states that: during dark adaptation, the threshold for
detecting a spot will be equivalent to the threshold for detecting the same spot against a
background that bleaches the same fraction of rhodopsin as remains bleached at that
point in dark adaptation.
This ties together thresholds during light adaptation
(real background light) and during dark adaptation
(“equivalent background” set by the fraction of
bleached pigment)
Log Threshold Intensity
7
deVries-Rose
Dark Adaptation
But plots threshold L
not ΔL
5' flash
60 flash
6
7
6
5
5
4
4
3
3
2
2
1
1
0
0
0
10
20
30
40
Time in the Dark (min)
50 -4
-3
-2
-1
0
1
2
Log Background (Trolands)
3
Log Threshold Intensity
DA-threshold drops as
bleached rhodopsin level
5' flash
drops
0
7
6
As background L rises, more
rhodopsin is bleached
7
6
6 flash
When the thresholds are
the same, the amount of
bleached rhodopsin is
the same
5
4
5
4
3
3
2
2
1
1
0
0
0
10
20
30
40
Time in the Dark (min)
50 -4
-3
-2
-1
0
1
2
Log Background (Trolands)
3
Log Equivalent Total Background
Luminance (Trolands)
3
5' flash
6o flash
2
“equivalent background”
works for all target sizes
1
This is the xaxis from the
right side of
the previous
figure
0
-1
-2
-3
0
10
20
30
Time in the Dark (min)
This is the x-axis from the left side of
the previous figure
40
50
Light adaptation alters the responses
of the photoreceptors
(looking at the neural changes that
occur during light adaptation)
What happens to the
response of rods as the
background L is raised?
We know that the threshold ΔL rises as
the background L is increased (Ch. 3)
We also know that the amount of
bleached photopigment increases as L is
increased.
Look now at what effect increasing L has
on photoreceptor responses. This
should explain the increase in threshold
ΔL.
These are the responses (hyperpolarization) of a rod to different flash intensities
Low intensity, brief flash
of light produces a small
hyperpolarization with
longer latency
As the flash intensity
rises, the amount of
hyperpolarization rises,
an overshoot develops,
and the latency is
shorter. The membrane
is slow to return to
baseline
Low intensity, brief flash
of light produces a small
hyperpolarization with
longer latency
If you slow down time on
the x-axis, this just looks
like a line of differing
lengths
For simplicity, represent the responses
just with vertical lines
A Low intensity flashes;
no adapting light
Test flash
intensity
Flashes
0
Photoreceptor
membrane potential
Vmax
B High intensity flashes;
no adapting light
0
V
V
Vmax
Responses to flashes
Top: no adapting light; bottom: with increasing adapting light
C High intensity flashes;
low adapting light
Test flash and
adapting intensity
0
Photoreceptor
membrane potential
Adapting
light
Plateau
D High intensity flashes;
high adapting light
0
Plateau
V
Vmax
Vmax
5s
Time
V
 V is the Key!
Three important points about the responses of photoreceptors.
1) the same flash intensity produces a smaller response (V) when
the amount of light adaptation increases.
2) at each adaptation level, there is a “linear region” of intensities,
where a given increase in flash intensity will produce a given
increase in V. (important for coding “brightness”)
3) at each adaptation level, there is a maximum response (V) the
photoreceptor can produce and this maximum response
decreases as the adapting light becomes more intense.
Log V
ΔV is the Key!
Dark Adapted
-4.2
-2.2
1.0
F
F'
E
E'
E''
D'
D
Change in
membrane
potential
codes
brightness
I
G H
F''
D''
C'
0.5
C''
C
B
0.0
B''
B'
A
A''
A'
-0.5
-8
-7
-6
-5
-4
-3
-2
Log Test Flash Intensity
-1
0
There are neural (non-photopigment) changes
that also produce light and dark adaptation
Neural (“network”) (non-photopigment)
Early dark adaptation
“early” Light adaptation – non-photopigment
based photoreceptor changes; Ganglion
cell sensitivity changes even though
photoreceptors are dark adapted
“Loss” (disconnection) of receptive-field
surround in full dark adaptation
Circadian changes – dark adaptation is
more complete at night
Log Threshold Intensity
4
Adapting Field Intensity (Td)
57,000
1,800
57
3
2
1
0
-0.4
0.0
0.4
0.8
1.2
1.6
Time of Onset of Stimulus Flash (s)
2.0
Log Threshold
Luminance
1.0
Ganglion cells can show dark adaptation
when photoreceptors do not
Ganglion Cell
Isolated Receptor Potential
Horizontal Cells
0.8
0.6
0.4
0.2
0.0
0
2
4
6
Time in the Dark (min)
8
10
Log Threshold
Luminance
-Dark Adaptation -
- Light Adaptation -
Network
Receptors
Receptors
0
This figure is
misleading. The
network changes
really are here
Network
Low
Time in the Dark
High
Log Background Luminance
Log Threshold Intensity
7
7
5' flash
60 flash
6
6
5
5
4
4
3
3
2
2
1
1
0
0
0
10
20
30
40
Time in the Dark (min)
50 -4
-3
-2
-1
0
1
2
Log Background (Trolands)
3