Eye Movements during Fixation

Neural Adaptation and Visual Fading

That the eyes move constantly has been known for centuries. In 1860, Hermann von Helmholtz pointed out that keeping one's eyes motionless was a difficult proposition and suggested that “wandering of the gaze” prevented the retina from becoming tired.

Animal nervous systems may have evolved to detect changes in the environment because spotting differences promotes survival. Motion in the visual field may indicate that a predator is approaching or that prey is escaping. Such changes prompt visual neurons to respond with neural impulses. Unchanging objects do not generally pose a threat, so animal brains—and visual systems,—did not evolve to notice them. Frogs are an extreme case because they produce no spontaneous eye movements in the absence of head movements. For a resting frog, such lack of eye movements results in the visual fading of all stationary objects. Jerome Lettvin and colleagues stated that a frog “will starve to death surrounded by food if it is not moving” (1968; p. 234). Thus, a fly sitting still on the wall will be invisible to a resting frog, but once the fly is aloft, the frog will immediately detect it and capture it with its tongue.

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