Ganong's Review of Medical Physiology, 23rd Edition

5 mm
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.)
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.
MT/V5
LO
Parietal lobe
Occipital lobe
V7
V3A
V3
V2
V1
VP
V4v
V8
Cerebellum
OTHER CORTICAL AREAS
CONCERNED WITH VISION
CHAPTER 12 Vision 195
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 functions
are listed in Table 12–1. Studies of these areas have been
carried out in monkeys trained to do various tasks and then fitted
with implanted microelectrodes. In addition, the availability
of PET and functional magnetic resonance imaging (fMRI)
scanning has made it possible to conduct sophisticated experiments
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, concerned
primarily with motion, and a ventral or temporal pathway,
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 processing
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 (degree
of freedom from dilution with white). For any color there is
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.)
V7
V3A
V3
V1
V2
VP
V4v
V8