The brain's visual retinotopic cortical maps:
Although Allman eloquently covers visual processing in Evolving Brains, at this point in my research, the material is a bit over my head, to say the least. If this is your area of interest, however, then you are urged to check out Evolving Brains. Meanwhile, some basic information will provided here about cortical maps involved in visual processing.
a concise explanation is found of the visual cortex in the Wikipedia entry for "Human brain." It reads: "In visual areas, the maps are retinotopic—that is, they reflect the topography of the retina, the layer of light-activated neurons lining the back of the eye." The entry explains that "the fovea—the area at the center of the visual field—is greatly overrepresented compared to the periphery. The visual circuitry in the human cerebral cortex contains several dozen distinct retinotopic maps, each devoted to analyzing the visual input stream in a particular way. The primary visual cortex (Brodmann area 17), which is the main recipient of direct input from the visual part of the thalamus, contains many neurons that are most easily activated by edges with a particular orientation moving across a particular point in the visual field. Visual areas farther downstream extract features such as color, motion, and shape."
Much of the brain's visual processing takes place in the occipital lobe. For orientation as to where these visual areas area, please reference the illustration to the left. Brodmann's area 17 is the primary visual cortex (V1). Brodmann's area 18 represents the secondary visual cortex (V2) and Brodmann's area 19 represents the associative visual cortex (V3). a larger red circle is drawn around these three areas. Each one of these areas is a retinotopic map. As mentioned earlier when discussing the temporal lobe, Brodmann's areas 20 and 21 in the temporal lobe (also circled in red) represent the inferotemporal visual cortex, which includes neurons sensitive to the images of faces and sends visual information to the amygdala.
Allman, in Evolving Brains, details how the primary visual cortex, designated V1, was initially mapped:
In the Russo-Japanese War of 1905, many Japanese soldiers sustained bullet wounds that penetrated through the posterior part of their brains. Because of the higher muzzle velocity and the smaller bullet size of rifles developed in the late nineteenth century, these weapons tended to produce more localized brain injuries than were inflicted in earlier wars, and improved care of the wounded also resulted in higher rates of survival. Many of the wounded soldiers were partially blinded by these injuries, and Tatsuji Inouye, an ophthalmologist, was asked by the Japanese government to evaluate the extent of their blindness as a means to determine their pension benefits. Inouye found that the part of the visual field in which these soldiers were blind corresponded to the locations of their brain injuries as determined by the sites of the bullet's entry and exit through the head. By combining the visual field deficits from different soldiers he was able to deduce the topographic organization of the primary visual cortex. Inouye's map revealed that much more cortex was devoted to the representation of the central part of the retina than to the periphery. This is the portion of the retina with the highest acuity, and it is our most important means for probing our environment for information, and the part you are using to read this book. Inouye's map of the primary visual cortex has been confirmed by modern brain-imaging techniques.
In The Modular Brain: How New Discoveries in Neuroscience are Answering Age-Old Questions about Memory, Free Will, Consciousness, and Personal Identity (1994), Richard M. Restak provides an example of how damage to one area of the brain, in only one hemisphere, can dramatically change how we perceive the world. Restak discusses how Michael Gazzaniga conducted an experiment that involved a subject who had lost sight off to his left because of brain damage to the visual area on the right side of his brain. The subject "was asked to imagine himself looking toward California from New York and naming the states in between. He named only ten states, all located to the right of his imaginary vantage point. He omitted the states to the left, corresponding to the visual field mediated by his right brain lesion. What he could not see in the real world as a result of his brain damage, he could neither picture in imaginal space nor speak about."
a readable description is found of the various visual cortical areas in the human brain in an intermediate-level module titled "The Eye" in The Brain from Top to Bottom. An advanced-level module in this same resource provides another good description of the various visual cortexes. The image to the right is taken from the advanced-level module (image links to source).
As a last note on the vastly complex visual cortical areas, in Evolving Brains, Allman notes the differences in the visual cortex that distinguish earlier mammals from primates and humans. He writes that "opossums and hedgehogs, which in many respects resemble the early mammals that lived more than 60 million years ago, have rather limited visual capacities and a small number of the visual cortical areas. In these mammals, the cortical maps of the retina are relatively uniform in that the amount of cortical space devoted to the more central part of the visual field in front of the animal is not much greater than the cortex devoted to the more peripheral parts of the visual field. By contrast, primates have extremely well developed visual capacities and have a large number of cortical maps devoted to visual perception and memory. Within most of these maps there is a strong emphasis of the representation of the central part of the visual field and a much smaller representation of the peripheral parts of the visual field."
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