Neuronal Pathways of Vision - Vision with Direction - page 3

Temporal view

Optic radiations

Primary visual cortex (V1)

Cornea

Nasal retina

Optic chiasm

Nasal view

Temporal retina

Lens

LGN

Nasal view

Lateral geniculate nucleus

Optic nerve

Optic tract

Nasal retina

Pupil

V2

Temporal view

Fig. 1.1. The anatomic pathways of the visual signals

The spatial 2D image formed on the retina represents the light pattern reflected
from a thin 1 plane in the 3D spatial world which the eye observes. This is so, thanks
to the deformable lens sitting behind the cornea , a transparent layer of cells that first
receives the light. The thickness of the cornea does not change and can be likened
to a lens with fixed focal length in a human-made optical system, such as a camera.
Because the lens in the eye can be contracted or decontracted by the muscles to which
it is attached, its focal length is variable. Its function can be likened to the zooming
of a telephoto objective. Just as the latter can change the distance of the plane to be
imaged, so can the eye focus on objects at varying distances. Functionally, even the
cornea is thus a lens, in the vocabulary of technically minded. Approximately 75%
of the refraction that the cornea and the eye together do is achieved by the cornea
(Fig. 1.1). The pupil , which can change the amount of light passing into the eye, can
be likened to a diaphram in a camera objective.
The light traverses the liquid filling the eye before it reaches the retinal surface
attached to the inner wall of the eyeball. The light rays are absorbed, but the sen-
sitivity to light amount, that is the light intensity, 2 of the retinal cells is adapted in
various ways to the intensity of the light they usually receive so as to remain opera-
tional despite an overall decrease or increase of the light intensity, e.g., on a cloudy
or a sunny day. A ubiquous tool in this adaptation is the pupil, which can contract
or decontract, regulating the amount of light reaching the retina. There is also the

1 The thickness of the imaged 3D plane can be appreciated as thin in comparison with its
distance to the eye.
2 The light consists of photons, each having its own wavelength. The number of photons
determines the light intensity. Normally, light contains different amounts of photons from
each wavelength for chromatic light . If however there is only a narrow range of wavelengths
among its photons, the light is called monochromatic , e.g., laser light.