<P> Some animals use both of the above strategies . A starling, for example, has laterally placed eyes to cover a wide field of view, but can also move them together to point to the front so their fields overlap giving stereopsis . A remarkable example is the chameleon, whose eyes appear as if mounted on turrets, each moving independently of the other, up or down, left or right . Nevertheless, the chameleon can bring both of its eyes to bear on a single object when it is hunting, showing vergence and stereopsis . </P> <P> Binocular summation is the process by which the detection threshold for a stimulus is lower with two eyes than with one . There are various types of possibilities when comparing binocular performance to monocular . Neural binocular summation occurs when the binocular response is greater than the probability summation . Probability summation assumes complete independence between the eyes and predicts a ratio ranging between 9 - 25% . Binocular inhibition occurs when binocular performance is less than monocular performance . This suggests that a weak eye affects a good eye and causes overall combined vision . Maximum binocular summation occurs when monocular sensitivities are equal . Unequal monocular sensitivities decrease binocular summation . There are unequal sensitivities of vision disorders such as unilateral cataract and amblyopia . Other factors that can affect binocular summation include are, spatial frequency, stimulated retinal points, and temporal separation . </P> <P> Apart from binocular summation, the two eyes can influence each other in at least three ways . </P> <Ul> <Li> Pupillary diameter . Light falling in one eye affects the diameter of the pupils in both eyes . One can easily see this by looking at a friend's eye while he or she closes the other: when the other eye is open, the pupil of the first eye is small; when the other eye is closed, the pupil of the first eye is large . </Li> <Li> Accommodation and vergence . Accommodation is the state of focus of the eye . If one eye is open and the other closed, and one focuses on something close, the accommodation of the closed eye will become the same as that of the open eye . Moreover, the closed eye will tend to converge to point at the object . Accommodation and convergence are linked by a reflex, so that one evokes the other . </Li> <Li> Interocular transfer . The state of adaptation of one eye can have a small effect on the state of light adaptation of the other . Aftereffects induced through one eye can be measured through the other . </Li> </Ul>

How do last two stages of life overlap with each other in reality