we can see objects even though the background luminance levels change over a range of more than 10...

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.

Time in the Dark (min)

0 10 20 30 40

Log Threshold Luminance

2

3

4

5

6

7

8

9

Cone Branch

Rod Branch

Rod-Cone Break

“Typical” Dark Adaptation Curve

Adapting light goes off at time = 0

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

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.)

Dark Adaptation lab on Thursday

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)


0 10 20 30 40


2

3

4

5

6

7

8

9

Cone Branch

Rod Branch

Rod-Cone Break

Note: If using the Method of Limits, must only use the ascending branch to avoid changing the time-course of the dark adaptation

“sneak up” on threshold from below

The 2009 winning group

Important Stimulus Dimensions

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 **

**

*** = important

parameters in dark

adaptation studies

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

Eccentricity From Fovea (mm)

-20 -15 -10 -5 0 5 10 15 20

Cells/mm2

0

50,000

100,000

150,000

200,000

Eccentricity From Fovea (deg)

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70O

ptic

Ne

rve

Hea

d B

lind

Spo

t

Rods

Cones

NasalTemporal

Distribution of rods and cones along the horizontal meridian in a human retina.Data provided by Dr. Christine Curcio.

Fig. 2.1

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

Retinal Location (2 deg spot)


0 10 20 30 40

Log ThresholdLuminance

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0o

10o

2.5o

5oBroadband 300 millilambert adapting field, 2 min exposure

2° Spot flashed 1 s every 2 s

(μmillilamberts)

Test flash size (centered on fovea)


0 10 20 30


1.0

1.5

2.0

2.5

3.0

3.5

4.0

20o

10o

5o

3o

2o

(μmillilamberts)

Effects of Test Flash Wavelength on the Shape of the Dark Adaptation Curve

400 nm 500 nm 600 nm 700 nm

|

|

Peak rod absorption

Effects of Test Flash Wavelength on the Shape of the Dark Adaptation Curve

-Rods absorb poorly at long wavelengths

400 nm 500 nm 600 nm 700 nm|

Peak rod absorption

Effect of Test flash Wavelength


0 10 20 30 40 50

Threshold Intensity (dB)

20

30

40

50

>680 nm

620-700 nm

550-620 nm485-570 nm400-700 nm

“decibels” (dB) is a log scale

Adapting Light Wavelength

400 nm 500 nm 600 nm 700 nm

|

||

Test flash

Effect of Adapting Light Wavelength

400 nm 500 nm 600 nm 700 nm

|

||

Test flash

|

Adapting light wavelength (blue test flash)


0 5 10 15 20 25


0.0

0.5

1.0

1.5

2.0

2.5

3.0Red at 38.9 mLWhite at 26.3 mL

(μμlamberts)

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

TIme in the Dark (min)

0 10 20 30 40

Log ThresholdIntensity

2

3

4

5

6

7

8

9

263

3,000

19,000

38,000 400,000

Adapting Intensity (trolands)

Illuminance (μTroland)

Adapting light duration


0 10 20 30 40


-3

-2

-1

10 s1 min2 min5 min10 min20 min

333 millilamberts

Luminance (millilamberts)

Luminance needed to detect grating orientation


0 5 10 15 20 25 30 35


-4

-3

-2

-1

0

1

2VA 1.04 VA = 0.62 VA = 0.25 VA= 0.083 VA = 0.042 NO GRATING

If you need cones to do the task, then do not get a rod branch

(millilamberts)

Early Dark Adaptation

1) Rapid decrease in test flash threshold (< 0.4 s)

due to neural (not photopigment) changes

Time of Onset of Stimulus Flash (s)

-0.4 0.0 0.4 0.8 1.2 1.6 2.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1,800 57

57,000

Adapting Field Intensity (Td)

Illuminance (trolands)

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

20 msec test flash

Response to test flash

Adapting light

A

B

D

E

F

C

L

L

Response to adapting light

Response to both

Response to both

Response to both

Response to both

Test flash time

Off

Test flash time

Test flash time

Test flash time

OffOn

On

Time0

Response to threshold test flash alone

Response to adapting light offset alone

Test flash long before adapting light offset

Test flash just before adapting light offset

Test flash same time as adapting light offset

Test flash long after adapting light offset

All of these action potentials are needed to see the test flash

20 msec test flash


Adapting light

A

B

D

E

F

C

L

L


Response to both

Response to both

Response to both

Response to both

Test flash time

Off

Test flash time

Test flash time

Test flash time

OffOn

On

Time0








The response to the test flash is “cut off”; not enough APs to detect

20 msec test flash


Adapting light

A

B

D

E

F

C

L

L


Response to both

Response to both

Response to both

Response to both

Test flash time

Off

Test flash time

Test flash time

Test flash time

OffOn

On

Time0









What happens when the test flash is presented at different times, relative to the adapting light offset?

Remember, we are looking at the response of just ONE neuron, responding to BOTH the test flash and the adapting light offset.


-0.4 0.0 0.4 0.8 1.2 1.6 2.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1,800 57

57,000


20 msec test flash


Adapting light

A

B

D

E

F

C

L

L


Response to both

Response to both

Response to both

Response to both

Test flash time

Off

Test flash time

Test flash time

Test flash time

OffOn

On

Time0









The response to the test flash is supporessed; not enough APs to detect

How do you make the test flash visible again? Raise the intensity to restore the needed number of action potentials


-0.4 0.0 0.4 0.8 1.2 1.6 2.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1,800 57

57,000


20 msec test flash


Adapting light

A

B

D

E

F

C

L

L


Response to both

Response to both

Response to both

Response to both

Test flash time

Off

Test flash time

Test flash time

Test flash time

OffOn

On

Time0









The response to the test flash is supporessed; not enough APs to detect



-0.4 0.0 0.4 0.8 1.2 1.6 2.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1,800 57

57,000


20 msec test flash


Adapting light

A

B

D

E

F

C

L

L


Response to both

Response to both

Response to both

Response to both

Test flash time

Off

Test flash time

Test flash time

Test flash time

OffOn

On

Time0









The response to the test flash is suppressed; not enough APs to detect



-0.4 0.0 0.4 0.8 1.2 1.6 2.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1,800 57

57,000


20 msec test flash


Adapting light

A

B

D

E

F

C

L

L


Response to both

Response to both

Response to both

Response to both

Test flash time

Off

Test flash time

Test flash time

Test flash time

OffOn

On

Time0









The response to the test flash is suppressed; not enough APs to detect


-0.4 0.0 0.4 0.8 1.2 1.6 2.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1,800 57

57,000


Illuminance (trolands)

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


0 10 20 30 40 50


2

3

4

5

6

Oguchi's Disease

Congenital Stationary Night Blindness

Normal

Rod Monochromatism

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


0 10 20 30 40


2

3

4

5

6

7

8

9

Cone Branch

Rod Branch

Rod-Cone Break

Dark Adaptation

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 -


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 muchof visual adaptation

Both for light adaptation and dark adaptation


0 5 10 15 20 25 30 35

Proportion of Pigmentin Bleached State

1.0000

0.5000

0.2500

0.1250

0.0625

0.0000

Retina with only rodsNormal retina

Half-time for

cones = 1.7 min

rods, 5.2 min

Regeneration of rhodopsin follows a exponential decay function

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 muchof 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)?

Rushton derived an equation that approximately relates the

amount of bleached pigment to visual sensitivity is:

(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.

log( / ) I t I o10 HB

The log of the threshold elevation (above absolute threshold) is related to the fraction of bleached rhodopsin

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 -


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.)

TIme in the Dark (min)

0 10 20 30 40

Log Threshold Intensity

2

3

4

5

6

7

8

9

263

3,000

19,000

38,000 400,000

Adapting Intensity (Trolands)


0 5 10 15 20 25 30 35

Proportion of Pigmentin Bleached State

1.0000

0.5000

0.2500

0.1250

0.0625

0.0000

Retina with only rodsNormal retina

Time constant for cones = 1.7 min

rods, 5.2 min


0 5 10 15 20 25 30


0

1

2

3

Percent of PigmentStill Bleached

0.0

2.5

5.0

7.5

13% 24% 42% 99%

Initial amount ofpigment bleached

Symbols = threshold

Lines = bleached pigment

The level of bleached photopigment explains

much of visual adaptation

The Equivalent Background Theory states that: during dark adaptation, the threshold fordetecting a spot will be equivalent to the threshold for detecting the same spot against abackground that bleaches the same fraction of rhodopsin as remains bleached at thatpoint in dark adaptation.

Another way the amount of bleached pigment sets the threshold:

This ties together thresholds during light adaptation (real background light) and during dark adaptation (“equivalent background” set by the fraction of bleached pigment)


0 10 20 30 40 50


0

1

2

3

4

5

6

7

Log Background (Trolands)

-4 -3 -2 -1 0 1 2 3

0

1

2

3

4

5

6

7

5' flash60 flash

deVries-RoseBut plots threshold L not ΔL

Dark Adaptation


0 10 20 30 40 50


0

1

2

3

4

5

6

7


-4 -3 -2 -1 0 1 2 3

0

1

2

3

4

5

6

7

5' flash60 flash

DA-threshold drops as bleached rhodopsin level drops

As background L rises, more rhodopsin is bleached

When the thresholds are the same, the amount of bleached rhodopsin is the same


0 10 20 30 40 50

Log Equivalent Total Background Luminance (Trolands)

-3

-2

-1

0

1

2

35' flash

6o flash

This is the x-axis from the right side of the previous figure

This is the x-axis from the left side of the previous figure

“equivalent background” works for all target sizes

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.

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

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

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

5 s

Vmax

Photoreceptormembrane potential

Test flashintensity

V

V

B High intensity flashes;no adapting light

A Low intensity flashes;no adapting light

C High intensity flashes;low adapting light

D High intensity flashes;high adapting light

V

VVmax

Vmax

Vmax

Photoreceptormembrane potential

Test flash andadapting intensity

Time

Responses to flashes

Flashes

Adaptinglight

Plateau Plateau

0

0 0

0

Top: no adapting light; bottom: with increasing adapting light

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 Test Flash Intensity

-8 -7 -6 -5 -4 -3 -2 -1 0

Log V

-0.5

0.0

0.5

1.0

A

B

C

D

E

F

G H I

A'

B'

C'

E'F'

A''

B''

C''

D''

E'' F''

Dark Adapted-4.2-2.2

D'

ΔV is the Key! Change in membrane potential codes brightness

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


-0.4 0.0 0.4 0.8 1.2 1.6 2.0


0

1

2

3

4

1,800 57

57,000



0 2 4 6 8 10


0.0

0.2

0.4

0.6

0.8

1.0

Ganglion CellIsolated Receptor PotentialHorizontal Cells

Ganglion cells can show dark adaptation when photoreceptors do not

Time in the Dark

0


Log Background Luminance

Low High

-Dark Adaptation - - Light Adaptation -

Receptors

Network

Receptors

Network

This figure is misleading. The network changes really are here


0 10 20 30 40 50


0

1

2

3

4

5

6

7


-4 -3 -2 -1 0 1 2 3

0

1

2

3

4

5

6

7

5' flash60 flash

we can see objects even though the background luminance levels change over a range of more than 10...

Documents

dark adaptation slide

dark adapting

winning group slide

visual system dark

dark adaptation lab

dark adaptation curves

s millilamberts slide

dark adaptation studies