Motion and vision: why animals move their eyes

Nearly all animals with good vision have a repertoire of eye movements. The majority show a pattern of stable fixations with fast saccades that shift the direction of gaze. These movements may be made by the eyes themselves, or the head, or in some insects the whole body. The main reason for keeping gaze still during fixations is the need to avoid the blur that results fro m the long response time of the photoreceptors. Blurbegins to degrade the i mage at a retinal velocity of about 1 receptor acceptance angle per respons e time. Some insects (e.g. hoverflies) stabilise their gaze much more rigid ly than this rule implies, and it is suggested that the need to see the mot ion of small objects against a background imposes even more stringent condi tions on image motion. A third reason for preventing rotational image motio n is to prevent contamination of the translational flow-field, by which a m oving animal can judge its heading and the distances of objects. Some anima ls do let their eyes rotate smoothly, and these include some heteropod moll uscs, mantis shrimps and jumping spiders, all of which have narrow linear r etinae which scan across the surroundings. Hymenopteran insects also rotate during orientation flights at speeds of 100-200 degrees s(-1). This is jus t consistent with a blur-free image, as are the scanning speeds of the anim als with linear retinae.