Mollon, Stockman & Polden (1987) reported an anomaly in the time-course of the light adaptation of the S-cone mechanism following the onset of an intense yellow bleaching light. Instead of recovering monotonically, S-cone increment threshold rises for several seconds before falling to its light-adapted steady-state value. As far as we aware, no mechanism has yet been proposed that provides a compelling explanation of this phenomenon.

As Mollon et al. showed, the anomaly must be mediated postreceptorally. We believe that the delayed suppression of S-cone sensitivity is not a property of the S-cone chromatic system per se, but instead reflects changes in the outputs of the L- and M-cones caused by the sluggish generation of an intermediate, active bleaching photoproduct within the L- and M-cone photoreceptors (some photoproducts are known to act much like real lights). We find that the time course of the rise and fall in S-cone threshold is consistent with the lifetime of an active photoproduct limited by two approximately first-order reactions with time constants of c. 5 and 20 s, which are likely to correspond to the rates of production and decay of the photoproduct. The S-cone thresholds are largely immune to the direct effects of photopigment depletion and other mechanisms of photoreceptor adaptation that affect the L- and M-cone thresholds (which recover monotonically). Consequently, this curious anomaly may provide a unique method of probing in vivo a part of the retinoid cycle that regenerates bleached photopigment in isolation from the usual photoreceptor adaptation mechanisms.