Transduction in photoreceptors: determination of the pigment transition or state coupled to excitation.

Several recent reports have shown that light absorption by metarhodopsin does not contribute to the excitation of invertebrate photoreceptors at low intensities. Where the pigment transition scheme is known, this result may be used to exclude some or most of the transitions and states of the pigment as sources of the coupling to excitation. The methodology of this approach is described and illustrated.