Figure 2.

Calibration of the light stimulus by patch-clamp recordings

The photovoltage responses to a series of 70–150 flashes of light of fixed density (0.44 photons μm⁻²) were recorded during the early day (ZT00–06) for 14 rods (1 rod retina^–1). The mean number of events per flash (m), that is, the mean number of isomerizations per rod per flash (R*), was determined by two different methods. The value of m was estimated from the natural logarithm of the observed probability of failure (P₀; eqn 04) and from the ratio of the square of the peak of the ensemble mean (μ²) to the square of the peak ensemble variance (σ²; eqn 05). The figure illustrates the comparison of the calibration methods. Proximity to the dashed line (unity) indicates agreement between the two methods for individual rods. Estimates of m averaged 0.39 ± 0.04 (eqn 04) and 0.47 ± 0.05 (eqn 05; mean ± SEM). The value of m was set at 0.44R* at this attenuation.