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D.L. McCulloch, C.A. Chaplin, R.E. Hamilton; The ‘Photopic Hill’ is Absent in the Electroretinogram (ERG) of Young Human Infants . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4733.
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© ARVO (1962-2015); The Authors (2016-present)
With standard ERG stimuli, the photopic systems of the retina appear more mature at birth and develop more slowly than the scotopic systems (1). In the mature retina, the photopic ERG to brief flashes consists of a cone–generated a–wave followed by a b–wave generated proximally with by the on and off pathways. For moderate flash levels, the timing of the on and off pathway contributions coincide causing a maximal b–wave amplitude. At higher flash levels, on and off pathway contributions interfere destructively creating the apparent paradox of reduced b–wave amplitudes for stronger stimulation. This phenomenon, the ‘photopic hill’, has not been reported in human infants.
Full field ERGs were recorded with dilated pupils on a standard photopic background (26.6 cd/m2) to a series of flashes ranging from sub–threshold to 1000 cd.s/m2 in four infants under eight weeks of age and 10 adult volunteers. Clinical data from fellow eyes was examined for evidence of the photopic hill in older infants.
For a–wave amplitude, the luminance–response curve shows a monotonic saturating function in all subjects. Infant a–waves have small amplitudes, low gain and low sensitivity demonstrating immaturity. In adults the b–wave amplitudes are optimal for flash luminance between 1.3 and 2.4 cd.s.m2. At higher flash levels, b–waves fall to a plateau 51(±14)% below the peak amplitude, thus demonstrating the ‘photopic hill’. Before eight weeks of age, b–wave amplitudes are small and increase monotonically with no evidence of the ‘photopic hill’. In the fellow eyes of clinical patients, the ‘photopic hill’ of the b–wave is present and within the adult range after one year of age.
The ‘photopic hill’ is absent from the b–wave luminance response function in young infants, similar to the effect of blocking the off pathways(2). This suggests a relative delay in the maturation of the retinal off pathways in the early weeks of life. (1) Westall CA, Panton CM, Levin AV. Doc Ophthalmol. 96:355–359, 1999.
(2) Uneo S, Kondo M, Niwa Y, Terasaki H, Miyaki Y. IOVS, 45:1033–1040.
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