Vision 2b


A. Central and Peripheral Vision

Central vision:

This is the vision as captured by the fovea. Most of the photoreceptors there are cones and practically every cone has its own nerve to the brain. Therefore the image is sharp and in colour. The whole eye is built to project the image sharp (in focus) onto the fovea.

Peripheral Vision:

The remainder but largest part of the retina is covered by the rods which are light sensitive but not colour sensitive. In fact, the rods are more light sensitive than the cones. For example, at night when you are outside, and you look at a faint star, you may see it better when you don't look at it straight but at a slight distance from the star.

Warning System:

The rods in the peripheral retina, and the nervous system attached to them, are especially sensitive to movements. Therefore, when something moves, in the "corner" of the eye, it attracts our attention, we turn our eye towards the source of the movement and see it sharp and in colour.

Tunnel Vision:

In some patients, who have their peripheral vision destroyed, the lack of a peripheral vision is striking. They can still see clearly and in colour with their fovea but they are often involved in road accidents, as they have not seen other cars moving into their path from other directions. They lack an early warning system.



B. The Visual Pathways


The visual pathways consist of four parts:

a. the optic nerve

b.the optic chiasm

c. the optic tract

d. the optic radiation

a. The optic Nerve:

runs from the retina to the optic chiasm. It consists of nerve axons from both the temporal and the nasal retina.

b. The optic Chiasm:

In this chiasm (=crossing), the axons from the nasal retina from both eyes cross to the other side whereas the axons from the temporal retina's do not cross (cave the pituitary gland).

c. The optic Tract:

Therefore, the optic tract on the right side (blue) contains the axons from the right temporal retina and the nasal left retina. The left optical tract (red) contains the other axons.

d. The optic Radiation:

The axons end in the lateral geniculate body into a new series of axons that radiate towards the visual cortex.