What is the function of the cone-rich rim of the retina?

Although there is good histological evidence for a rim of cones extending round the margin of the human retina at the ora serrata, the function of these cones is unknown, and indeed it is not known whether they are functional at all. Four possibilities are discussed here: (i) the cones of the ora serrata may alert us to sudden movements in the far peripheral field, (ii) their signal may be used in estimating optic flow during locomotion, (iii) they may integrate light scattered within the globe of the eye or passing through the sclera, for purposes of colour constancy, or (iv) they may drive circadian rhythms. We report two experiments designed to detect psychophysical correlates of the cone rim. Under the conditions we have used, neither flicker detection nor colour naming show, near the limit of vision, a discontinuity that would correspond to the cone-rich rim.