Question

What's the visual field of a monkey, and how does it use its visual field to run away from predators??

Answer 1

Ans :

Visual feild :

The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point.

Perhaps the most distinguishing feature of primate vision is its high spatial acuity, or the ability to resolve fine detail. Among mammals humans rank highest in their visual acuity, which commonly exceeds 50 cycles/degree, and this is followed closely by apes and monkeys (Kirk and Kay, 2004). In fact, this aspect of primate vision is unmatched among mammals and only exceeded by a few species of large, predatory birds (Kirk and Kay, 2004).

Primates’ unusually high acuity stems from multiple adaptations, including the large size of the eye, its optics, the high density of retinal photoreceptors and ganglion cells in central vision, and the low amount of spatial pooling of photoreceptor signals onto individual ganglion cells . In simian primates (i.e. monkeys and apes), the most unique feature of the retina is the fovea mediating central vision.

The fovea is a pit in the inner retina caused by the local absence of cell processes, creating a window of optical clarity for light reaching photoreceptors situated along the outer circumference of the retinal epithelium . The high density of cone photoreceptors that populate the fovea is, quite remarkably, free of blood vessels .

Across primates, the fovea has a roughly constant size of somewhat less than 0.5 mm despite large variations in eye size. The fact that larger eyes do not have larger foveas may suggest that its size is limited by the diameter within which the overlying cell processes and vasculature can be cleared without harming the photoreceptors themselves . Some prosimian primates such as the nocturnal Galago have a more primitive region of photoreceptor concentration mediating central vision, termed the area centralis.

The increase in receptor density in this region is notably less than in the simian primate fovea, with only a 2–3 fold increase compared to a 20 or more fold increase in New and Old World monkey. Thus the spatial acuity of the galago (4.8–6.0 cycles/deg) is much lower than most monkeys and humans .

The evolution of high acuity in primates may have been gradual. One hypothesis holds that high acuity in early diurnal primates benefitted from the unusually large eyes of their nocturnal ancestors, which was itself an adaptation to low light conditions .

For a given cone density, larger eyes translate directly into higher acuity, since a given visual angle subtended on the retina is projected onto more photoreceptors. Another set of adaptations led to the intense concentration of photoreceptors and clearing of vasculature at the central part of the retina to create the foveal pit. How and when this came about is far from clear, but it may be linked to the requirements for visually guided insect predation, much like in some insectivorous birds.