Light and dark adaptation

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  • Foveally fixated object blue light= dull
  • With dark adaptation, we noticed that there is progressive decrease in threshold (increase in sensitivity) with time in the dark.
  • Dark adaptation depends upon differing intensities and duration 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.
  • The 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. It is only after long pre-adaptation that a bi-phasic, cone and rod branches are obtained.
  • The retinal location used to register the test spot during dark adaptation will affect the dark adaptation curve due to the distribution of the rod and cones in the retina. When a small test spot is located at the fovea (eccentricity of 0 o ), 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.
  • Light and dark adaptation

    1. 1. Light and Dark AdaptationGauri S. Shrestha, M. Optom, FIACLE Lecturer, BPKLCOS, IOM
    2. 2. 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
    3. 3. Rods Peak density of rods occurs 20o from fovea  150,000 rods/mm2  No rods are present at the fovea:  Total Number: 120 million Gauri S. Shrestha, M.Optom, FIACLE
    4. 4. Cones Cones are most densely packed at the fovea  150,000 cones/mm2  Only 4% of total cones are foveal  Total Number: 6 million Gauri S. Shrestha, M.Optom, FIACLE
    5. 5. 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 Gauri S. Shrestha, M.Optom, FIACLE
    6. 6. 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 Gauri S. Shrestha, M.Optom, FIACLE
    7. 7. Observations Choose particular wave length and increase intensity  Change the wave length from high frequency to low frequency Chromatic interval=0 Gauri S. Shrestha, M.Optom, FIACLE
    8. 8. 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) Gauri S. Shrestha, M.Optom, FIACLE
    9. 9. 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’ Gauri S. Shrestha, M.Optom, FIACLE
    10. 10. 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 Gauri S. Shrestha, M.Optom, FIACLE
    11. 11. 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. Gauri S. Shrestha, M.Optom, FIACLE
    12. 12. 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. Gauri S. Shrestha, M.Optom, FIACLE
    13. 13. 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. Gauri S. Shrestha, M.Optom, FIACLE
    14. 14. Fig 10. Light adaptation using an increment threshold experiment Gauri S. Shrestha, M.Optom, FIACLE
    15. 15. 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. Gauri S. Shrestha, M.Optom, FIACLE
    16. 16. Fig11. Light adaptation curve plotted as increment threshold versusbackground 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). Gauri S. Shrestha, M.Optom, FIACLE
    17. 17. Light Adaptation Experiments Gauri S. Shrestha, M.Optom, FIACLE
    18. 18. 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 Gauri S. Shrestha, M.Optom, FIACLE
    19. 19. Light Adaptation Curve  Section 3:  Slope = 1  Indicates Weber’s Law: threshold contrast remains constant as the illumination changes  Scotopic constant: k = 0.14 Gauri S. Shrestha, M.Optom, FIACLE
    20. 20. 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 Gauri S. Shrestha, M.Optom, FIACLE
    21. 21. Light Adaptation Curve  Section 4:  Slope = infinity  With high levels of illumination, rods cannot detect any stimulus  When does rod saturation occur?  10% of rhodopsin is bleached Gauri S. Shrestha, M.Optom, FIACLE
    22. 22. Light Adaptation Curve 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 Gauri S. Shrestha, M.Optom, FIACLE
    23. 23. 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. Gauri S. Shrestha, M.Optom, FIACLE
    24. 24. 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 Gauri S. Shrestha, M.Optom, FIACLE
    25. 25. 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 Gauri S. Shrestha, M.Optom, FIACLE
    26. 26. The Goldmann-WeekersMachine Gauri S. Shrestha, M.Optom, FIACLE
    27. 27. The Goldmann-WeekersMachine Gauri S. Shrestha, M.Optom, FIACLE
    28. 28. 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 Gauri S. Shrestha, M.Optom, FIACLE
    29. 29. 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. Gauri S. Shrestha, M.Optom, FIACLE
    30. 30. Gauri S. Shrestha, M.Optom, FIACLE
    31. 31. 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 Gauri S. Shrestha, M.Optom, FIACLE
    32. 32. 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. Gauri S. Shrestha, M.Optom, FIACLE
    33. 33. Intensity and duration of pre-adapting light Gauri S. Shrestha, M.Optom, FIACLE
    34. 34. 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. Gauri S. Shrestha, M.Optom, FIACLE
    35. 35. Gauri S. Shrestha, M.Optom, FIACLE
    36. 36. 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. Gauri S. Shrestha, M.Optom, FIACLE
    37. 37. Size and location of the retina used Gauri S. Shrestha, M.Optom, FIACLE
    38. 38. Location of the retina used Gauri S. Shrestha, M.Optom, FIACLE
    39. 39. Size and 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. Gauri S. Shrestha, M.Optom, FIACLE
    40. 40. 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 Gauri S. Shrestha, M.Optom, FIACLE
    41. 41. Wavelength of the threshold light Gauri S. Shrestha, M.Optom, FIACLE
    42. 42. 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?? Gauri S. Shrestha, M.Optom, FIACLE
    43. 43. 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. Gauri S. Shrestha, M.Optom, FIACLE
    44. 44. Rhodopsin regeneration Log relative threshold as a function of the percentage of photopigment bleached Gauri S. Shrestha, M.Optom, FIACLE
    45. 45. Clinical Pearl Gauri S. Shrestha, M.Optom, FIACLE
    46. 46. Clinical Pearl Rod monochromatism  Sensory nystagmus  Blurred vision  Photophobia  Color vision Gauri S. Shrestha, M.Optom, FIACLE
    47. 47. Congenital stationary night blindness Psychophysical and electrophysiological findings in negative electroretinography (ERG) selective loss of the b- wave Gauri S. Shrestha, M.Optom, FIACLE
    48. 48. Retinitis Pigmentosa Depression of the curve occurs in conditions affecting the outer retina and RPE Gauri S. Shrestha, M.Optom, FIACLE
    49. 49. Prolong Dark adaptation Vitamin A deficiency Effect of anoxia Effect of tobacco inhalation Anaesthesia Ocular opacities Retinal degeneration Myopia Glaucoma ? Gauri S. Shrestha, M.Optom, FIACLE
    50. 50.  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. Gauri S. Shrestha, M.Optom, FIACLE

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