Neurobiological Background - Hierarchical Neural Networks for Image Interpretation - page 18

(a) (b)
Fig. 2.1. Eye and visual pathway to the cortex. (a) illustration of the eye's anatomy; (b) visual
pathway from the eyes via the LGN to the cortex (adapted from [117]).
2.1 Visual Pathways
The human visual system captures information about the environment by detecting
light with the eyes. Figure 2.1(a) illustrates the anatomy of the eye. It contains an
optical system that projects an image onto the retina. We can move the eyes rapidly
using saccades in order to inspect parts of the visual field closer. Smooth eye move-
ments allow for pursuit of moving targets, effectively stabilizing their image on the
retina. Head and body movements assist active vision.
The iris regulates the amount of light that enters the eye by adjusting the pupil's
size to the illumination level. Accommodation of the lens focuses the optics to vary-
ing focal distances. This information, in conjunction with stereoscopic disparity,
vergence, and other depth cues, such as shading, motion, texture, or occlusion, is
used to reconstruct the 3D structure of the world from 2D images.
At the retina, the image is converted into neural activity. Two morphological
types of photoreceptor cells, rods and cones, transduce photons into electrical mem-
brane potentials. Rods respond to a wide range of wavelengths. Since they are more
sensitive to light than cones, they are most useful in the dark. Cones are sensitive
to one of three specific ranges of wavelengths. Their signals are used for color dis-
crimination and they work best under good lighting conditions. There are about 120
million rods and only 6.5 million cones in the primate retina. The cones are con-
centrated mainly in the fovea at the center of the retina. Here, their density is about
150,000/mm 2 , and no rods are present.
The retina does not only contain photoreceptors. The majority of its cells are
dedicated to image processing tasks. Different types of neurons are arranged in lay-
ers which perform spatiotemporal compression of the image. This is necessary be-
cause the visual information must be transmitted through the optic nerve, which
consists of only about 1 million axons of retinal ganglion cells.