Dark adaptation 

            The physiological description of dark adaptation is the eye's adjustment to low illumination by increasing the sensitivity of the photoreceptors to light.  Both cones and rods participate in dark adaptation, slowly increasing their sensitivity to light in a dim environment. Cones adapt faster, so the first few minutes of adaptation reflect cone-mediated vision. The bulk of the process takes 30 minutes and involves expansion of pupils and retinal alterations, specifically the regeneration of rhodopsin and iodopsin.1 In the vision science literature, dark adaptation means the recovery response following bright light stimulation (bleaching) of the photoreceptors.  There are two types of dark adaptation, foveal and extrafoveal.  Foveal dark adaptation measures cone function and extrafoveal dark adaptation measures both rod and cone function. The eye's response to and recovery from light involves the conversion of 11-cis-retinal isomer to the all-trans isomer by the eye.  Eventually, all-trans-retinal is converted back to 11-cis-retinal through a series of enzymatic steps, termed the retinoid cycle. 

            The kink (cone-rod break) in the extrafoveal dark adaptation curve was first described by Aubert (1865)2.   Cobb3 in 1919 while studying aviators found the rate of recovery was variable between individuals. The times of recovery were less diverse after a short blinding than after a longer one. Mote and Riopelle5 in1950 found for two subjects, the longer the preadaptation light exposure (blinding) the more diverse the recovery time and the higher the intensity and the longer the duration of preadaptation light exposure (blinding), the longer the time required to recover.

            Extrafoveal dark adaptation endpoint is limited to the light perception threshold, whereas foveal dark adaptation has a broad range of endpoints, including the light perception threshold, the ability to read a specified size symbol and after-image resolution of brightness or color change.  Both foveal and extrafoveal dark adaptation are affected by age, AMD, and other diseases.  Testing times are shorter for foveal than extrafoveal dark adaptation, making foveal dark adaptation potentially favorable for clinical testing.14  In addition, central foveal cones are well preserved in older normal subjects whereas the number of rods declines significantly with age even in normal subjects without AMD, confounding intergroup comparisons.14    For both foveal and extrafoveal dark adaptation, recovery time is prolonged by increasing the intensity and duration of light exposure during light adaptation (bleaching),3,5,12  with aging,14,15 in AMD7,11,14, and other diseases11. 

            Extrafoveal dark adaptation is related to the general health of the retina but does not directly measure macula function, whereas foveal dark adaptation instantaneously measures macular function.11,14

 

 

Extrafoveal Dark Adaptation

            Loss of foveal cones and parafoveal rods and cones have been reported in eyes with AMD and clinical studies have shown a substantial impairment of rod dark adaptation is eyes with AMD. In older adults’ substantial delays in dark adaptation occur.15 For older adults in their 70s, the transition point in the dark at which the rod system takes over is delayed almost 2 1/2 minutes, as compared to those in their 20s. The time taken for 70-year-olds to reach pre-bleach light sensitivity is over 10 min longer than for those in their 20s.15 
           Hecht et. al6 were the first to study dark adaptation of retinal fields of different size and location where 2-degree fields were tested 0 to 15 degrees from the fovea. They found that the retina gradually increases in sensitivity from center to periphery.6  
                 In recent studies, dark adaptation measurements following light adaptation (bleaching) of the peripheral retina in a 1.77-to-4.08-degree field located 5-degrees on the inferior visual meridian found that peripheral retinal testing is a sensitive and specific indicator of age-related macular degeneration.  Delayed rod-mediated dark adaptation in older adults in normal macular health is associated with early AMD three years later, and thus is a functional biomarker for early disease.13 The speed of dark adaptation was characterized by the rod intercept value.13The rod intercept is defined as the duration required for sensitivity to recover to a criterion sensitivity value of 5.0 × 10−3 scotopic cd/m2 (3.0 log units of attenuation of the stimulus).8 Abnormal dark adaptation was defined as a rod-intercept of ≥ 12.3 minutes.8
                 

Foveal Dark Adaptation

            One term for bleaching of the fovea is photostress. Foveal dark adaptation was first reported by Hecht4 (1921), he used the term "light-adaptation" for what is today is synonymous with photostress9,10 or bleaching.12 Light adaptation was achieved with an incandescent bulb providing a brightness field of 90 millilamberts for 5 minutes4.  A red cross having adjustable brightness was used to measure the foveal dark adaptation threshold.4 The dark adaption of the fovea was complete in 4 to 6 minutes. 4

                  The baseline cone threshold can assess visual dysfunction in early AMD which consistently detected deficits in AMD subjects and differentiated them from age-matched controls with high test–retest repeatability.14  Newsome and Negreiro11 using a novel light flash device found return of retinal sensitivity, or dark adaptation based upon the ability to read numbers can provide a useful indicator of central retinal health, disease, and worsening of disease.

            Increasing the bleaching intensity, duration or both, the foveal dark adaptation light- perception threshold (endpoint) rises, the slope of the curve decreased, and the time to reach a final steady threshold value increased".5  

 References

1. American Psychological Association, https://dictionary.apa.org. 

2. . Klin Monbl Augenheilkd. 1983 Jan;182(1):107-8. Summary: After exposure to strong light, the visual threshold of extrafoveal parts of the human retina in darkness is lowered in two steps, an early cone and a later rod portion, which are separated by a kink (Kohlrausch, 1922). A reappraisal of the pioneering experiments of H. Aubert (1826-1892) has furnished evidence that the first description of the kink in the dark adaptation curve on the basis of sensitivity measurements after strong light adaptation, was given as early as 1865. Klin Monbl Augenheilkd. 1983 Jan;182(1):107-8.
3.  Cobb, P. W. (1919). Dark adaptation with especial reference to the problems of night flying. Psychological Review, 26(6), 428–453.  https://doi.org/10.1037/h0071312
4. Hecht, S., The nature of foveal dark adaptation, J. Gen.  Physiol., 1921,4, 113.

5. Mote, F.A., Riopelle, A.J. The effect of varying the intensity and the duration of pre-exposure upon subsequent dark adaptation in the human eye.

Journal of Comparative and Physiological Psychology, 46(1), 49–55. 
6. S Hecht, C Haig, G Wald. Affiliation. The dark adaptation of retinal fields of different size and location. J Gen Physiology. 1935 Nov 20;19(2):321-37. doi: 10.1085/jgp.19.2.321.
7. Jackson GR & Edwards JG. A short-duration dark adaptation protocol for assessment of age-related maculopathy. j ocul biol dis inform (2008) 1:7–11 DOI 10.1007/s12177-008-9002-6.
8. Jackson GR, Scott IU, Kim IK, Quillen DA, Iannaccone A, Edwards JG. Diagnostic sensitivity and specificity of dark adaptometry for detection of age-related macular degeneration. Invest Ophthalmol Vis Sci. 2014; 55:1427–1431. DOI:10.1167/iovs.13-13745 

9. Bailliart J P. Le Scotome central provoque. Bull Soc Opht 1954; LIV: 13–16

10. J S Glaser, P J Savino, K D Sumers, S A McDonald, R W Knighton. The photostress recovery test in the clinical assessment of visual function. AJO 1977 Feb;83(2):255-60. doi: 10.1016/0002-9394(77)90624-9.
11. Newsome, DA, Negreiro, M. Reproducible Measurement of Macular Light Flash Recovery Time Using a Novel Device Can Indicate the Presence and Worsening of Macular Diseases. Current Eye Research, 34, 162–170, 2009 

12. Lamb TD, Pugh ENJ. Dark adaptation and the retinoid cycle of vision. Prog Ret Eye Res. 2004; 23:307–80. 
13. Cynthia Owsley, Gerald McGwin Jr, Mark E. Clark, Gregory R. Jackson, Michael A. Callahan, Lanning B. Kline, C. Douglas Witherspoon, and Christine A. Curcio. Delayed Rod-Mediated Dark Adaptation is a Functional Biomarker for Incident Early Age-Related Macular Degeneration Ophthalmology. 2016 February; 123(2): 344–351. doi: