This document discusses light and dark adaptation in the visual system. It describes the roles of rods and cones in scotopic and photopic vision. Dark adaptation allows the visual system to adjust to low light levels and involves both rods and cones, with rods taking longer to recover sensitivity after being bleached. The document outlines the dark adaptation curve and factors that influence it such as pre-adapting light intensity and duration. Light adaptation occurs more quickly and allows the visual system to function across a wide range of background illuminations.

1. LIGHT AND DARK
ADAPTATION

2. Presented by
BIDHUNA S
(2nd year optometry
Govt.medical college
kozhikode)

3. Photoreceptors
RODS
 Scotopic vision.
 Don’t mediate colour
vision
 Low spatial acuity
 Absent in the fovea.
CONES
• Photopic vision
• capable of colour
vision
• High spatial acuity
• Concentrated in the
fovea

4. Visual threshold
• Minimum amount of energy required
for a patient to detect a stimulus
• Low threshold=high sensitivity
• Threshold=1/sensitivity

5. Scotopic threshold
• Threshold of a patient measured in dim light
condition(night)
Photopic threshold
• Threshold of a patient measured in bright light
condition

6. Purkinje shift
Scotopic system
• Stimulus of 507nm are perceived brighter than
other stimuli
Photopic system
• Stimuli of 555nm are perceived brighter than
other stimuli
The difference in the peak sensitivityof the 2
system is the Purkinje shift

7. Visual adaptation
• Temporary change in sensitivity or perception
when exposed to a new or intense stimulus and
the lingering after image may result when the
stimulus is removed.

8. Reflect the neural cording process of visual
system
• Involves 2 types
1) Light adaptation
2) Dark adaptation

9. • Adapt in decrease in illumination
• rodes >cones

10. Temporary Summation:
Bloch’s law of vision:
• Critical period =0.1 sec
• Bt=k
• B=luminous
• T=time
• K=CONSTANT
• Stimulus size background illumination, type of
task.[critical period varies]

11. Broca -Sulzer and Troxler effect
Broca-Sulzer effect-
Critical period in which apparent brightness
undergo temporal summation.
Short flash appear brighter.
Troxler effect-
Spot of light held stationary fades away
because of bleaching.

12. Duplex function
• Rodes are more sensitive than cones
• There are more rods than cones
• Ganglion cells have more post-receptoral
summation than cones

13. DARK ADAPTATION CURVE
Curve plotted with illumination of test object in vertical axis and duration
of dark adaptation along horizontal axis

14. DARK ADAPTATION CURVE
 Cones recover within 5min while rods take up
to 35min to fully recover sensitivity.
 The rod –cone break is the point where the rod
become more sensitive than the cones.
 The two plateaus represent the thresholds of
the rods and cones

15. The first is rapid ,of short
duration and small in extent.
2nd slow, more prolonged and
larger
The inflection of the dark
adaptation curve where the
rod limb begins is called
cone-rod break or alpha point

16. Mechanism of dark adaptation
1. Visual pigmentation mechanism
• Retinal and opsin converted into light
sensitive pigments
• Vit A is reconverted back into retinal
• Sensitivity is proportional to anti
logarithm of rhodopsin conc.
• Bleaching 50%pigments elevates rod
threshold by 100 million and cone by
30

18. Other mechanism
• Change in pupillary size-30 fold
• Neural adaptation-when light intensity
inc.initially signals transmitted inc,
• Followed by rapid dec.
• Occurs in fraction of sec
• Feedback inhibition.

19. FACTORS INFLUNCING DARK ADAPTATION
Factors related to Preadapting light
Factors related to Test stimulus.
Factors Releated to individuals

20. Factors related to
preadapting light
1) Intensity of
preadapting light.
2) Duration of light
used

21. INTENSITY
With increasing level of preadapting
luminance
• The cone branch becomes longer while the rod
branch becomes more delayed
• Absolute threshold takes longer times to reach
At low level of preadapting luminance
• Rod threshold drops quickly to reach absolute
threshold

22. DURATION
• Only after long pre adaptation,a bi-phasic cone
and rod branch are obtained.
• Short duration of the preadapting light, the
more rapid the decrease in dark adaptation
For extremely short preadaptation periods
• A single rod curve is obtained

23. WAVELENGTHS
• 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

24.  Energy of the
light used to
preadapt the
eye.
 Wavelengths
distribution of
adapting light
 Red googles
prevent short
wavelengths
from reaching
the rods.

25. ENERGY
• The total energy depends upon the level of
luminance and duration of exposure.
• An inc cause a dec in the slope of the curve
• Occurs displacement of the curve to the right.
• If it is constant,dark adaptation curve is also
constant.

26. Factors related to
test stimulus
A. Wavelength
B. Duration of exposure of retina to test flash
C. Region of retina where test stimulus applied.

27. Factors related to
the individual
 Vit. A deficiency –depletion of photosensitive
pigments
 anoxia on dark adaptation-inc. threshold
Anasthesia
 Tobacco inhalation
 Opacities in ocular media
 Dark adaptation in retinal degeneration –ARMD
 Myopia between 5-10D
 Glaucoma

29. RHODOPSIN REGENERATION
• Dark adaptation depends upon photo pigment
bleaching
• The time course for dark adaptation and
rhodopsin regeneration was same.
• Bleaching rhodopsin by1% raises threshold by
10 (dec sensitivity by 10)
• Bleaching of cone photo pigment has a smaller
effect on cone thresholds.

30. LIGHT ADAPTATION
• Visual adaptation to increase level of
illumination
• Quick
• Promptly occurring over a period of 5 min.
• The eye has to quickly adapt to background
illumination to be able to distinguish objects
in the background.

33. • As background
illumination increase
,intensity increases.
• If threshold increases
,then absolute
sensitivity decreases
• As illumination we
are less sensitive.

34. Our sensation is determined by the percentage
difference in the luminance of a surface
relative to its background.
This holds over a wide range of background
luminance.

35. Mechanism of light adaptation
Photochemical reaction
• Rhodopsin retinal+opsin
Rod
• Saturates once the light is moderately bright
Cones
• Continues to adapt and respond to brighter
illumination
• Reaches to max.after 5-10 min.

36. • Photoregeneration
• Isomerisation of the immediate products of
bleaching back into the photosensitive
pigments
• Threshold rise when
• Rate of bleaching regeneration of
photopigments