 For example, the night sky looks darker than Eigengrau because of the contrast provided by the stars.Researchers noticed early on that the shape of intensity-sensitivity curves could be explained by assuming that an intrinsic source of noise in the retina produces random events indistinguishable from those triggered by real photons.Later experiments on rod cells of cane toads ("Bufo marinus") showed that the frequency of these spontaneous events is strongly temperature-dependent, which implies that they are caused by the thermal isomerization of rhodopsin.