Comparative Physiology
Saccades are a widespread phenomenon across animals with image-forming visual systems. They have been observed in animals across three phyla, including animals that do not have a fovea (most vertebrates do not) and animals that cannot move their eyes independently of their head (such as insects). Therefore, while saccades serve in humans and other primates to increase the effective visual resolution of a scene, there must be additional reasons for the behavior. The most frequently suggested of these reasons is to avoid blurring of the image, which would occur if the response time of a photoreceptor is longer than the time a given portion of the image is stimulating that photoreceptor as the image drifts across the eye.
In birds, saccadic eye movements serve a further function. The avian retina is highly developed. It is thicker than the mammalian retina and has a higher metabolic activity, but it lacks proper vasculature. Therefore, the retinal cells must obtain nutrients via diffusion through the choroid and from the vitreous humor. The pecten is a specialised structure in the avian retina. It is a highly vascular structure that projects into the vitreous humor. Experimentally, it has been shown that during saccadic eye oscillations (which occupy up to 12% of avian viewing time), the pecten acts as an agitator, propelling perfusate towards the retina. Thus, in birds, saccadic eye movements appear to be important in retinal nutrition and respiration.
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