Purpose: For decades, the retina was believed to be the only photosensitive tissue in
mammals. Recently, however, Xue et al. reported that the isolated iris muscle is
photosensitive in a variety of mammalian species and directly constricts in response to
extremely bright light due to expression of the photopigment melanopsin in iris muscle cells
(Xue et al., Nature 2011). In agreement, Xue et al. also identified enhanced ipsilateral pupil
constriction relative to contralateral constriction, but with greater sensitivity than in isolated …