Sensitivity to a small test probe in the centre of a small, steady background is less than when the background is large (sensitization). When an equiluminous steady annulus is added to the region surrounding a small background, rod threshold takes several minutes to stabilize at its new, lower level. The after-effects of the small background follow a time course characteristic of cortical adaptation. 2. The sensitivity loss and time course of recovery after intense bleaching lights in the cone system depend markedly on the size of the retinal region bleached, although no such effect is observed in the rod system. If a steady annular surround is added to the region surrounding the bleached patch, threshold falls rapidly to the value it would have after a large-area bleach of the same intensity. 3. The interaction between bleaches and steady surrounds suggests that bleaches produce long-lasting signals in the cone receptors. 4. The different temporal properties of sensitization on rod backgrounds and sensitization after cone bleaches suggest that different mechanisms underlie the two phenomena. 5. In cone vision, if light is added to the area surrounding a small, steady background, the subsequent readjustment takes minutes to complete, as it does in rod vision. But in addition, for cones, a large proportion of the sensitivity loss caused by the small background can be rapidly restored, as it is with cone bleaches. 6. The above results, together with the known absence of sensitization in rod dark adaptation, are consistent with the hypothesis that sensitization occurs at least partly at the retinal level in the cone system, but not (or only weakly) in the rod system, and that there is an additional, probably cortical elevation, common to rod and cone systems, for small backgrounds, but not for small, brief bleaches.
