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Dynamic and steady‐state light adaptation of mouse rod photoreceptors in vivo

1 Electroretinographic (ERG) methods were used to investigate the effects of background illumination on the responses of mouse rod photoreceptors in vivo. A paired‐flash procedure, involving the recording and analysis of the ERG a‐wave response to a bright probe flash presented after a brief test flash, was used to derive the rod response to the test flash in steady background light. A related, step‐plus‐probe procedure was used to derive the step response of the rods to backgrounds of defined strength. 2 Steady background light produced a maintained derived response that was graded with background strength. Determinations of the full time course of the derived weak‐flash response in steady background light, and of the effect of background strength on the flash response at fixed post‐test‐flash times, showed that moderate backgrounds reduce the peak amplitude and duration of the flash response. 3 The response to stepped onset of an approximately half‐saturating background (1.2 sc cd m−2) exhibited a gradual rise over the first 200‐300 ms, and an apparent subsequent relaxation to plateau amplitude within 1 s after background onset. Determinations of normalized amplitudes of the derived response to a test flash presented at 50 or 700 ms after background onset indicated substantial development of background‐induced shortening of the test flash response within this 1 s period. These findings indicate a time scale of ≈1 s or less for the near‐completion of light adaptation at this background strength. 4 Properties of the derived response to a stepped background and to test flashes presented in steady background light are in general agreement with photocurrent data obtained from mammalian rods in vitro and suggest that the present results describe, to good approximation, the in vivo desensitization of mouse rods by background light.