Light and dark adaptation chinnu

Ony a short note on visual adaptation:

1. LIGHT AND DARK ADAPTATION

2. Presented by BIDHUNA S (2 nd 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 Light adaptation Dark adaptation

9. DARK ADAPTATION 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. 2 nd 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 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

17. 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.

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

19. Factors related to preadapting light Intensity of preadapting light. Duration of light used

20. 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

21. 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

22. 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

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

24. 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.

25. Factors related to test stimulus Wavelength Duration of exposure of retina to test flash Region of retina where test stimulus applied.

26. 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

27. 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.

28. 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 .

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

30. 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.

31. 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.

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

33. THANK YOU