The equivalent light hypothesis for retinal degeneration:

Many mutations that underlie the disease produce a common physiological change: the persistent activation of the transduction cascade. This, like constant real light, triggers photoreceptor degeneration. Light, Ca2+, and Photoreceptor Death: New Evidence for the Equivalent-Light Hypothesis from Arretin Knockout Mice, Gordon L. Fain and John E. Lisman, IOVS, November 1999, Vol. 40, No. 12 [.PDF]. Photoreceptor Degeneration in Vitamin A Deprivation and Retinitis Pigmentosa: the Equivalent Light Hypothesis, Gordon L. Fain and John E Lisman, Exp. Eye Res. (1993) 57, 335-340 [.PDF].