Using Silent Substitution to Track the Mesopic Transition From Rod- to Cone-Based Vision in Mice.

PURPOSE: To describe the activity of rods and cones in visually intact mice in mesopic conditions, and establish the relative importance of each photoreceptor type in defining the transition from rod to cone vision.

METHODS: Using mice (Opn1mwR) carrying a red-shifted cone opsin, we applied silent substitution methods to record light-adapted ERGs to flash stimuli visible only to rods or cones across a range of light levels (corneal irradiance 10⁹-10¹⁴ photons/cm²/s; ∼10⁰-10⁶ photoisomerisations/rod/s). We tested the impact of selectively changing the background light intensity as experienced by cones on the rod ERG (and vice versa) by adjusting the spectral composition of stimuli. The ERG parameters (b-wave amplitude and implicit time, oscillatory power) were extracted, and their relationship to background intensity and the effective irradiance for cones versus rods/melanopsin was established. We also attempted to record a melanopsin ERG by using modifications of the rod-isolating stimuli.

RESULTS: We saw the predicted decay and increase in rod- and cone-ERG amplitude, respectively, as a function of background intensity. There was only a single irradiance (10¹³ photons/cm²/s) at which both ERGs had high amplitude. Adjustments in the effective irradiance for rods/melanopsin did not impact the cone ERG except at the brightest backgrounds at which there was a melanopsin-dependent suppression of b-wave amplitude. Increasing effective irradiance for cones suppressed rod b-wave amplitude across all background intensities. In addition, we were unable to record a melanopsin ERG.

CONCLUSIONS: The cone measure of irradiance was particularly important in driving the transition from rod- to cone-based vision across mesopic light levels.