Origin and functional impact of dark noise in retinal cones

Spontaneous fluctuations in the electrical signals of the retina's photorec eptors impose a fundamental limit on visual sensitivity. While noise in the rods has been studied extensively, relatively little is known about the no ise of cones. We show that the origin of the dark noise in salamander cones varies with cone type. Most of the noise in long wavelength-sensitive (L) cones arose from spontaneous activation of the photopigment, which is a mil lion-fold less stable than the rod photopigment rhodopsin. Most of the nois e in short wavelength-sensitive (S) cones arose in a later stage of the tra nsduction cascade, as the photopigment was relatively stable. Spontaneous p igment activation effectively light adapted L cones in darkness, causing th em to have a smaller and briefer dim flash response than S cones.