ANTAGONISTIC CHROMATIC MECHANISMS IN PHOTORECEPTORS OF THE PARIETAL EYE OF LIZARDS

PHOTORECEPTORS are the first in the chain of neurons that process visu al information. In lateral eyes of vertebrates, light hyperpolarizes r od and cone photoreceptors that synapse onto bipolar and horizontal ce lls in the first synaptic layer of the retina. The sign of the photore ceptor signal is either conserved or inverted in bipolar cells, result ing in chromatically dependent depolarizing and hyperpolarizing respon ses to visual stimuli. Visual information is then conveyed to the seco nd synaptic layer for encoding and transmission to the brain by gangli on cells. The parietal (third) eve of lizards does not contain bipolar cells or other interneurons. Photoreceptors synapse directly onto gan glion cells1-4 and yet, even in the absence of interneurons, antagonis tic chromatic mechanisms modulate the ganglion cell responses5,6. We r eport here that chromatic antagonism in the third eye originates in th e chromatically dependent hyperpolarizing and depolarizing response of the photoreceptors to light. We also suggest that the antagonistic na ture of these photoresponses may provide lizards with a mechanism for the enhanced detection of dawn and dusk.