Uniocular occlusion (dark patching) of adults for a period of days produces marked reduction in the directional sensitivity of the eye (Stiles-Crawford effect). The effect reaches a maximum 3 to 5 days after the onset of patching. Recovery occurs in a comparable period. The second eye is not affected, and a simple light diffuser fails to produce the same effect. Coupled with alteration in directional sensitivity are an overall increase in sensitivity and modest resolution and perceived colour changes. These findings, with other research, suggest that directional sensitivity is at least partially mediated by light and that orientation of photoreceptors is an active process.