Antagonistic chromatic mechanisms in photoreceptors of the parietal eye of lizards.

Photoreceptors are the first in the chain of neurons that process visual information. In lateral eyes of vertebrates, light hyperpolarizes rod and cone photoreceptors that synapse onto bipolar and horizontal cells in the first synaptic layer of the retina. The sign of the photoreceptor signal is either conserved or inverted in bipolar cells, resulting in chromatically dependent depolarizing and hyperpolarizing responses to visual stimuli. Visual information is then conveyed to the second synaptic layer for encoding and transmission to the brain by ganglion cells. The parietal (third) eye of lizards does not contain bipolar cells or other interneurons. Photoreceptors synapse directly onto ganglion cells and yet, even in the absence of interneurons, antagonistic chromatic mechanisms modulate the ganglion cell responses. We report here that chromatic antagonism in the third eye originates in the chromatically dependent hyperpolarizing and depolarizing response of the photoreceptors to light. We also suggest that the antagonistic nature of these photoresponses may provide lizards with a mechanism for the enhanced detection of dawn and dusk.