movement. it is thought that the brain does this by temporarily recording a copy of the command for the eye movement, and comparing this to the remembered image of the target. this is called spatial updating. neurophysiologists, having recorded from cortical areas for saccades during spatial updating, have found that memory-related signals get remapped during each saccade. trans-saccadic perception it is also thought that perceptual memory is updated during saccades so that information gathered across fixations can be compared and synthesized. however, the entire visual image is not updated during each saccade. some scientists believe that this is the same as visual working memory, but as in spatial updating the eye movement has to be accounted for. the process of retaining information across a saccade is called trans-saccadic memory, and the process of integrating information from more than one fixation is called trans-saccadic integration. 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) 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 cell 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, has a higher metabolic activity, and has less vasculature obstruction, for greater visual acuity. because of this, 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. experiments show that, during saccadic eye oscillations (which occupy up to 12% of avian viewing time), the pecten oculi acts as an agitator, propelling perfusate (natural lubricants) toward the retina. thus, in birds, saccadic eye movements appear to be important in retinal nutrition and cellular respiration.