Rod pathways: the importance of seeing nothing

Anatomical and physiological studies of the mammalian retina have revealed two primary pathways available for the transmission of rod signals to the ganglion cells: one via ON rod bipolars, amacrine II cells, and ON and OFF cone bipolars, which is exquisitely designed for the transmission of single-photon absorption events; and a second via rod-cone gap junctions, and ON and OFF cone bipolars, which is designed for the transmission of multiple photon-absorption events at higher light levels. Psychophysical and electroretinographic (ERG) studies in normal observers and in two rare types of observer, who are devoid of either rod or cone function, support an analogous duality in the human visual system, the clearest signature of which is a loss of flicker visibility and ERG amplitude at frequencies near 15 Hz that results from destructive interference between sensitive 'slow' and insensitive 'fast' rod signals. The slow rod signal is most probably derived from the ON rod bipolar pathway and the fast signal from the rod-cone gap junction and cone pathways. Evidence has emerged recently for a third, insensitive rod pathway between rods and OFF cone bipolars, but it has so far only been observed clearly in rodents.