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M. Garon, M. Rufiange, C. Casanova, P. Lachapelle; Elucidating the Light Adaptation Effect with the Photopic Hill . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1888.
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Purpose: In response to progressively brighter stimuli, the amplitude of the b-wave first increases (Phase 1), reaches a maximum value or Vmax (Phase 2) and then gradually decreases (Phase 3) to reach a plateau where the amplitude of the b-wave equals that of the a-wave (Phase 4), a phenomenon known as the Photopic Hill. We examined if this unique ERG luminance-response function could help us identify the still unknown retinal mechanisms at the origin of the enhancement of the cone ERG measured during the course of light adaptation which follows a period of dark-adaptation; the so-called light adaptation effect (LAE). Methods: Six normal subjects aged 17 to 23 participated in this study. Photopic ERGs (LKC UTAS E-3000 system, background: 30 cd.m-2; intensities: –0.8 to 2.84 log cd.sec.m-2 in 15 steps) were recorded prior (control) to and at 0 (t0) 5 (t5) and 10(t10) minutes following a 30 minute period of dark-adaptation. The magnitude of the LAE was determined by computing, for the same flash intensity, the tX /control amplitude ratio. Results: The magnitude of the LAE varied with the intensity of the stimulus. There were no significant gain in ERG amplitude brought with light adaptation for ERGs included in the first half of Phase 1 (7.8± 2.5 %) of the Photopic Hill and less than 5.2± 6.0 % gain during Phase 4. A gradual increase in amplitude gain was noted during second half of Phase 1, which culminated with a near 100 %, gain at the peak of Phase 2 [e.g. 57.0 ± 8.4 % of control (t0), 78.9 ± 6.2 % (t5) and 83.2 ± 5.4 % (t10) with no significant variation (p>.05) in the flash intensity needed to reach Vmax]. This was then followed by a progressive reduction to12.0± 7.3 % gain at the end of Phase 3. In contrast, the gain in a-wave amplitude resulting from the LAE remained at 10.6± 5.9 % irrespective of the intensity of the flash. Conclusions: Our results indicate that the magnitude cone ERG LAE is modulated by the intensity of the stimulus; the gain being maximal for intensities near the peak of the Photopic Hill. Given our previous claim that the retinal OFF pathway might initiate the descent of the Photopic Hill, and thus control the Vmax , our results would suggest that the powerful reduction in cone ERG amplitude noted at the onset light adaptation would result from an enhancement of the OFF pathway inhibition, presumably exacerbated by the preceding period of dark-adaptation.
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