Green anoles (Anolis carolinensis) are common lizards in the southeast of the United States. They are arboreal (in trees) and diurnal (daytime) predators that use vision for navigation, hunting, and communication.
Green anoles have two foveas (higher-resolution retinal specializations) per eye. These structures appear to reflect two distinct modes of vision: longer-distance high-resolution image formation (central fovea) and binocular depth perception for closer objects. When stationary, green anoles tend to angle their heads and rotate their eyes to center areas of interest in the visual field of a single eye. In this mode, their eyes often move independently and one eye might close while the other inspects a target. When preparing to move, green anoles will turn their heads to point directly at the surface they are moving to or the prey they are hunting. The target object is thus centered in their binocular visual field and processed by their temporal fovea.
The central fovea is dramatically differentiated from the surrounding retina and takes up a substantial portion of the back of the eye.
The pit of the central fovea is far more pronounced than what is observed in primate fovea. Note the complete absence of all cell nuclei at the center of the fovea. The pit is surrounded by a dramatic thickening of the retina that reflects a much higher density (per retinal area) of photoreceptors, bipolar cells, and retinal ganglion cells.
The temporal fovea is positioned to receive information for the region of the green anole’s visual field with the most binocular overlap. This fovea is marked by a higher areal density of photoreceptors, bipolar cells, and retinal ganglion cells. However, the increase in density is only a fraction of what is observed in the central fovea.
The optic tectum (large layered M) is the largest recipient of retinal input in the green anole brain. Aside from performing its own sensory-motor transformations, the optic tectum projects to the nucleus rotundus which, in turn, delivers visual information to the telencephalon.
The dorsal lateral geniculate nucleus (dLGN) receives retinal input and projects to the dorsal cortex. This pathway is homologous to the retinogeniculate pathway that underlies conscious vision in humans.
The dorsal cortex (above the thin blue line) receives visual input from the dLGN of the green anole. These retinogeniculate inputs are surrounded by visual information following the retinotectal nucleus rotondus pathway.
Aside from the retinas, two other parts of the green anole brain are photoreceptive; the pineal gland and the parietal eye. The parietal eye is centered in the middle of the head; behind the median aperture of the skull. The parietal eye includes a small lens and retina but is unlikely to be able to resolve more than the direction of light.