Molecular biological, histological and flicker electroretinographic results have established that mice have two cone photopigments, one peaking near 350 nm (UV-cone pigment) and a second near 510 nm [midwave (M)-cone pigment]. The goal of this investigation was to measure the action spectra and absolute sensitivities of the UV-cone-and M-cone-driven b-wave responses of C57BL/6 mice. To achieve this goal, we suppressed rod-driven signals with steady or flashed backgrounds and obtained

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... esponse relations for conedriven b-waves elicited by narrowband flashes between 340 and 600 nm. The derived cone action spectra can be described as retinal 1 pigments with peaks at 355 and 508 nm. The UV peak had an absolute sensitivity of ϳ8 nV/(photon m 2 ) at the cornea, approximately fourfold higher than the M peak. In an attempt to isolate UV-cone-driven responses, it was discovered that an orange conditioning flash ( Ͼ 530 nm) completely suppressed ERG signals driven by both M pigment-and UV pigment-containing cones. Analysis showed that the orange flash could not have produced a detectable response in the UV-cone pathway were their no linkage between M pigmentand UV pigment-generated signals. Because cones containing predominantly the UV and M pigments have been shown to be located largely in separate parts of the mouse retina (Szel et al., 1992) , the most probable linkage is coexpression of M pigment in cones primarily expressing UV pigment. New histological evidence supports this interpretation (Gloesman and Ahnelt, 1998) . Our data are consistent with an upper bound of ϳ3% coexpression of M pigment in the cones that express mostly the UV pigment.