Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 12Vision 195

covers much of the visual cortex (Figure 12–18) and is separate
from and independent of the grid of orientation columns.
About half the simple and complex cells receive an input
from both eyes. The inputs are identical or nearly so in terms
of the portion of the visual field involved and the preferred
orientation. However, they differ in strength, so that between
the cells to which the input comes totally from the ipsilateral
or the contralateral eye, there is a spectrum of cells influenced
to different degrees by both eyes.
Thus, the primary visual cortex segregates information
about color from that concerned with form and movement,
combines the input from the two eyes, and converts the visual
world into short line segments of various orientations.

OTHER CORTICAL AREAS

CONCERNED WITH VISION

As mentioned above, the primary visual cortex (V1) projects to

many other parts of the occipital lobes and other parts of the

brain. These are often identified by number (V2, V3, etc) or by

letters (LO, MT, etc). The distribution of some of these in the

human brain is shown in Figure 12–19, and their putative func-

tions are listed in Table 12–1. Studies of these areas have been

carried out in monkeys trained to do various tasks and then fit-

ted with implanted microelectrodes. In addition, the availability

of PET and functional magnetic resonance imaging (fMRI)

scanning has made it possible to conduct sophisticated experi-

ments on visual cognition and other cortical visual functions in

normal, conscious humans. The visual projections from V1 can

be divided roughly into a dorsal or parietal pathway, con-

cerned primarily with motion, and a ventral or temporal path-

way, concerned with shape and recognition of forms and faces.

In addition, connections to the sensory areas are important. For

example, in the occipital cortex, visual responses to an object are

better if the object is felt at the same time. There are many other

relevant connections to other systems.

It is apparent from the preceding paragraphs that parallel pro-

cessing of visual information occurs along multiple paths. In

some as yet unknown way, all the information is eventually pulled

together into what we experience as a conscious visual image.

COLOR VISION

CHARACTERISTICS OF COLOR

Colors have three attributes: hue, intensity, and saturation (de-

gree of freedom from dilution with white). For any color there is

FIGURE 12–18 Reconstruction of ocular dominance
columns in a subdivision of layer 4 of a portion of the right visual
cortex of a rhesus monkey. Dark stripes represent one eye, light
stripes the other. (Reproduced with permission from LeVay S, Hubel DH, Wiesel
TN: The pattern of ocular dominance columns in macaque visual cortex revealed by a
reduced silver stain. J Comp Neurol 1975;159:559.)

5 mm

FIGURE 12–19 Some of the main areas to which the primary visual cortex (V1) projects in the human brain. Lateral and medial views. See
also Table 8–1. (Modified from Logothetis N: Vision: A window on consciousness. Sci Am [Nov] 1999;281:99.)

LO

V7

MT/V5

Parietal lobe

Occipital lobe

V3A

V3

V2

V1

VP

V8

V4v

Cerebellum

V3

V1

V2

VP

V4v

V8

V3A

V7