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Basic Science of Amblyometry
The importance of Brightness Sense
Brightness sense behaves relative at both the monocular and binocular levels. In reference to the monocular level, Sadun1 states, “In a monochromatic setting a shade of gray is identified on a relative basis.” He explains, “a viewer will always describe objects that reflect the most light as white and those that reflect moderate or minimal amounts of light as gray or black by comparing each to the others.” In other words, a grey jacket will appear white when that object has the greatest reflectance in the environment. When that grey jacket is side by side with a white jacket, that grey jacket will then appear grey. Sudan goes on to explain, “The ability of the eye to normalize the gray scale over a vast range of illumination is termed "brightness constancy" and “Thus it appears that for survival value, information regarding relative intensities (reflectance) is more important than information about absolute intensities.”
At the binocular level, brightness appreciation in the right and left eyes are extremely similar in the normal human1,2. The question arises, why is the brightness sense of the two eyes so similar, what purpose does it serve, was there an evolutional natural selection driving force? For stationary targets, using sequential brightness comparisons of the two eyes, Sudan found in normal subjects that the brightness senses varied by only 9% between the two eyes; using 2 standard deviations, he defined a normal score as less than 18% difference between the two eyes. For moving objects, the Pulfrich phenomenon demonstrates that unbalancing the brightness sense between the two eyes drastically disrupts motion stereopsis. By dimming, the light by no more than 0.3 log units to one eye causes the perceived path of an oscillating object to change from the frontal plane to an elliptical orbit, which results in misjudging the location of moving objects. For the moving baseball, it has been shown by Hofeldt, et al2 that an imposed brightness imbalance between the two eyes as little a 0.3 log units significantly reduces the ability of the player to hit a baseball. Dimming the light to both eyes the same amount has no effect on baseball hitting, only binocular brightness imbalance changes the perception of object location. For early man and probably other animals, surviving attacking predictors and flying projectiles a precise motion stereopsis system would have been essential; the above evidence supports binocular relative brightness sense balance as an important component of that system. Understanding and manipulation this brightness based motion stereopsis system provides for identifying and measuring diseases that disrupt the delicate bilateral brightness balance.
1. Sadun A.A. (1989) Brightness Sense Testing. In: Wall M., Sadun A.A. (eds) New Methods of Sensory Visual Testing. Springer, New York, NY, pp14-28.
2. Hofeldt AJ, Hoefle FB, Bonafede B.: Baseball hitting, binocular vision, and the Pulfrich phenomenon. Arch Ophthalmology,114(12):1490-1494,1996.
3. Hofeldt, TS, Hofeldt, AJ: Measuring Colour Rivalry Suppression In Amblyopia. British Journal of Ophthalmology. Vol 83, N0 11, pp1283-1286,1999.
4. Gise, R, Kane S, Detection of Amblyopia in Young Children via Retinal Rivalry Using a Video Game Styled Interface on a Tablet Device, ARVO Meeting, Program 3819 (Paper) May 3, 2016.
5. Kane, S. Population Based Study to determine the efficacy of Retinal Rivalry performed in a Video Game Format for Screening a Large School Aged Population for Amblyopia, unpublished to date
6. Hofeldt, AJ, Hofeldt, GT, Weiss, MJ: Pulfrich Psychophysical Stereo-Illusion and Visual-Evoked Potentials in Epiretinal Membrane of the Macula. Journal of Neuro-Ophthalmology, 16(1): pp36-40, 1996.
7. Cummins, D; MacMillan, E S; Heron, G, Dutton, G: Simultaneous Interocular Brightness Sense Testing in Ocular Hypertension and Glaucoma, Arch Ophthalmology;112(9):1198-1203, 1994.
Diseases Affecting Brightness balance (numbers refer to references above)
Epiretinal Membrane of Macula (6)
Optic Neuritis (1)
Compressive Optic Neuropathy (1)
Anterior Ischemic Optic Neuropathy (1)
Ocular Hypertension and Glaucoma (7)