Optimal detection of flash intensity differences using rod photocurrent observations

The rod photocurrent contains two noise components that may limit the detec tability of flash intensity increments. The limits imposed by the low- and high-frequency noise components were assessed by computing the performance of an optimal detector of increments in flash intensity. The limits imposed by these noise components depend on the interval of observation of the pho tocurrent signal. When the entire photocurrent signal, lasting 3 or more se conds, is observed, the low- frequency component of the photocurrent noise (attributed to the quantal noise of the incoming light, as well as random i somerizations of enzymes within the phototransduction cascade) is the most significant limitation on detectability. When only the first 380 ms or less is observed, the high-frequency component of the noise (due to the thermal isomerizations of the cGMP-gated channel) presents a significant limit on the detectability of flashes.