Eye rotations are a complicated and integral part of human vision. They occur both voluntarily and involuntarily, and allow a person to fixate on features in the world, even as his head or target features are moving. One of the main reasons for eye movement is to position the feature of interest on the fovea. Recall from Section 5.2 that only the fovea can sense dense, color images, and it unfortunately spans a very narrow field of view. To gain a coherent, detailed view of a large object, the eyes rapidly scan over it while fixating on points of interest. Figure 5.15 shows an example. Another reason for eye movement is that our photoreceptors are slow to respond to stimuli due to their chemical nature. They take up to ms to fully respond to stimuli and produce a response for up to ms. Eye movements help keep the image fixed on the same set of photoreceptors so that they can fully charge. This is similar to the image blurring problem that occurs in cameras at low light levels and slow shutter speeds. Additional reasons for eye movement are to maintain a stereoscopic view and to prevent adaptation to a constant stimulation. To support the last claim, it has been shown experimentally that when eye motions are completely suppressed, visual perception disappears completely . As movements combine to build a coherent view, it is difficult for scientists to predict and explain how people interpret some stimuli. For example, the optical illusion in Figure 5.16 appears to be moving when our eyes scan over it.