Rod-cone Interaction in Flicker Perimetry

We have assessed the influence of the rod system on cone flicker sensitivity during flicker perimetry. For temporal frequencies above 18 Hz extrafoveal cone-mediated flicker thresholds for a white test stimulus are as much as 1.5 log units lower when measured against a large background light that saturates the rods than when measured in darkness. Following a Ganzfeld bleach extrafoveal cone flicker thresholds are at their minimum once the cones have recovered their sensitivity, but then thresholds rise as the rods begin to recover from the bleach. Our results indicate that the flicker sensitivity of the extrafoveal cone system at high temporal frequencies is influenced by the rods surrounding the flickering test stimulus. The rods reduce flicker sensitivity maximally in the dark adapted state, and their suppressive influence is minimised only by strong rod bleaches or by large backgrounds that saturate the rod system.