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A.S. Rajagopalan, K.R. Alexander, G.A. Fishman; Changes in the Derived Parameters of Cone Phototransduction During Light Adaptation . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5688.
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Purpose: Both the a–wave and b–wave of the cone ERG are known to increase in amplitude during continued exposure to an adapting field, but the precise mechanism is uncertain. Given that the cone a–wave may contain contributions from postreceptoral neurons, the purpose of the present study was to determine whether continued light adaptation alters the properties of the cone photoreceptor response itself. Methods: ERG responses were obtained from two visually normal subjects, ages 25 and 60 years. Following 45 min of dark adaptation, an achromatic, rod–desensitizing adapting field of 3.7 log td was presented, and ERG responses were obtained every 15 s, alternately in the presence of the adapting field and 300 ms after the adapting field offset, to determine whether the latter provides a measure of the "dark–adapted" cone response. The adapting field was turned back on immediately after the ERG response was obtained. Test flashes of 2.8 and 4.0 log td–s were presented in separate sessions. Amplitudes and implicit times of the a–wave and b–wave were measured as per convention. The a–wave responses to the two flash luminances were fit with a delayed Gaussian model of the activation phase of cone phototransduction to derive Rmp3, the maximum response amplitude, and S, a sensitivity parameter. Results: The amplitude of the b–wave increased systematically with time after the initial onset of the adapting field, regardless of whether the test flash was presented against the adapting field or shortly after its temporary offset. Rmp3 increased and S decreased with time for both adaptation conditions, but the time course of the change was shorter and the proportional change in amplitude was less than for the b–wave. The changes in the derived parameters of phototransduction paralleled those of the a–wave. Conclusions:The increase in the amplitude of the cone b–wave during light adaptation has only a small contribution from changes at the photoreceptor level. Further, the cone ERG response measured shortly after the offset of a rod–desensitizing adapting field is subject to the same adaptational processes as responses measured against a steady background during the course of light adaptation.
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