a condition known as anomalous retinal correspondence, the brain associates the fovea of one eye with an extrafoveal area of the other eye. Other animals The fovea is also a pit in the surface of the retinas of many types of fish, reptiles, and birds. Among mammals, it is found only in simian primates. The retinal fovea takes slightly different forms in different types of animals. For example, in primates, cone photoreceptors line the base of the foveal pit, the cells that elsewhere in the retina form more superficial layers having been displaced away from the foveal region during late fetal and early postnatal life. Other foveae may show only a reduced thickness in the inner cell layers, rather than an almost complete absence. Most birds have a single fovea, but hawks, swallows, and hummingbirds have a double fovea. The second is called the temporal fovea, which enables them to track slow movements. The density of cones in a typical bird's fovea has 400,000 cones per square millimeter, but some birds can reach a density of 1,000,000 cones per square millimeter (e.g., Common Buzzard). Additional images See also Eye movement Gaze-contingency paradigm Macular degeneration Foveated imaging References Human eye anatomy