In his experiment, Whitteridge cut the right optic tract. For information to get from either eye to the right visual cortex, it now has to go to the left visual cortex and cross in the corpus callosum. Cooling either of these areas blocks the flow of nerve impulses.
STUDIES OF THE PHYSIOLOGY OF THE CALLOSUM
One of the first neurophysiological examinations of the corpus callosum was made a few years after Myers' experiments by David Whitteridge, then in Edinburgh. Whitteridge realized that for a band of nerve fibers to join homologous, mirror-symmetric parts of area 17 made no sense. No reason could possibly exist for wanting a cell in the left hemisphere, concerned with points somewhere out in the right field of vision, to be connected to a cell on the other side, concerned with points equally far out in the left field. To check this further Whitteridge surgically severed the optic tract on the right side, just behind the optic chiasm, thus detaching the right occipital lobe from the outside world—except, of course, for any input that area might receive from the left occipital lobe via the corpus callosum, as you can see from the illustration on this page. He then looked for responses by shining light in the eyes and recording from the right hemisphere with wire electrodes placed on the cortical surface. He did record responses, but the electrical waves he observed appeared only at the inner border of area 17, a region that gets its visual input from a long, narrow, vertical strip bisecting the visual field: when he used smaller spots of light, they produced responses only when they were flashed in parts of the visual field at or near the vertical midline. Cooling the cortex on the opposite side, thus temporarily putting it out of commission, abolished the responses, as did cooling the corpus callosum. Clearly, the corpus callosum could not be joining all of area 17 on the two sides, but just a small part subserving the vertical midline of the visual field.
Anatomical experiments had already suggested such a result. Only the parts of area 17 very close to the border between areas 17 and 18 sent axons across to the other side, and these seemed to end, for the most part, in area 18, close to its border with area 17. If we assume that the input the cortex gets from the geniculates is strictly from contralateral visual fields—left field to right cortex and right field to left cortex—the presence of corpus-callosum connections between hemispheres should result in one hemisphere's receiving input from more than one-half the visual fields: the connections should produce an overlap in the visual-field territories feeding into the two hemispheres. That is, in fact, what we find. Two electrodes, one in each hemisphere near the 17-18 borders, frequently record cells whose fields overlap by several degrees.
Torsten Wiesel and I soon made microelectrode recordings directly from the part of the corpus callosum containing visual fibers, the most posterior portion. We found that nearly all the fibers that we could activate by visual stimuli responded exactly like ordinary cells of area 17, with simple or complex properties, selective for orientation and responding usually to both eyes. They all had receptive fields lying very close to the vertical midline, either below, above, or in the center of gaze, as shown in the diagram on this page.
Perhaps the most esthetically pleasing neurophysiological demonstration of corpus-callosum function came from the work ofGiovanni Berlucchi and Giacomo Rizzolatti in Pisa in 1968. Having cut the optic chiasm along the midline, they made recordings from area 17, close to the 17-18 border on the right side, and looked for cells that could be driven binocularly. Obviously any binocular cell in the visual cortex on the right side must receive input from the right eye directly (via the geniculate) and from the left eye by way of the left hemisphere and corpus callosum. Each binocular receptive field spanned the vertical midline, with the part to the left responding to the right eye and the part to the right responding to the left eye. Other properties, including orientation selectivity, were identical, as shown in the illustration on the facing page.
This result showed clearly that one function of the corpus callosum is to connect cells so that their fields can span the midline. It therefore cements together the two halves of the visual world. To imagine this more vividly, suppose that our cortex had originally been constructed out of one piece instead of being subdivided into two hemispheres; area 17 would then be one large plate, mapping the entire visual field. Neighboring cells would of course be richly interconnected, so as to produce the various response properties, including movement responses and orientation selectivity. Now suppose a dictareally happened, since the brain had two hemispheres long before the cerebral cortex evolved.
This experiment ofBerlucchi and Rizzolatti provides the most vivid example I know of the remarkable specificity of neural connections. The cell illustrated on this page, and presumably a million other callosally connected cells like it, derives a single orientation selectivity both through local connections to nearby cells and through connections coming from a region of cortex in the other hemisphere, several inches away, from cells with the same orientation selectivity and immediately adjacent receptive-field positions—to say nothing of all the other matching attributes, such as direction selectivity, end-stopping, and degree of complexity. Every callosally connected cell in the visual cortex must get its input from cells in the opposite hemisphere with exactly matching properties. We have all kinds of evidence for such selective connectivity in the nervous system, but I can think of none that is so beautifully direct.
Visual fibers such as these make up only a small proportion ofcallosal fibers.
In the somatosensory system, anatomical axon-transport studies, similar to the radioactive-amino-acid eye injections described in earlier chapters, show that the corpus callosum similarly connects areas of cortex that are activated by skin or joint receptors near the midline of the body, on the trunk, back, or face, but does not connect regions concerned with the extremities, the feet and hands.
Every cortical area is connected to several or many other cortical areas on the same side. For example, the primary visual cortex is connected to area 18 (visual area 2), to the medial temporal area (MT), to visual area 4, and to one or two others. Often a given area also projects to several areas in the opposite hemisphere through the callosum or, in some few cases, by the anterior commissure. We can therefore view these commissural connections simply as one special kind of cortico-cortico connection. A moment's thought tells us these links must exist: if I tell you that my left hand is cold or that I see something to my left, I am using my cortical speech area, which is located in several small regions in my left hemisphere, to formulate the words. (This may not be true, because I am left handed.) But the information concerning my left field of vision or left hand feeds into my right hemisphere: it must therefore cross over to the speech area if I am going to talk about it. The crossing takes place in the corpus callosum. In a series of studies beginning in the early i96os, Roger Sperry, now at Cal Tech, and his colleagues showed that a human whose corpus callosum had been cut (to treat epilepsy) could no longer talk about events that had entered through the right hemisphere. These subjects provided a mine of new information on various kinds of cortical function, including thought and consciousness. The original papers, which appeared in the journal Brain, make fascinating reading and should be fully understandable to anyone reading the present book.
This experiment by Berlucchi and Rizzolatti beautifully illustrates not only the function of the visual part of the callosum but also the high specificity of its connections between cells of like orientation and bordering receptive fields. Berlucchi and Rizzolatti cut the chiasm of a cat in the midline, so that the left eye supplies only the left hemisphere, with information coming solely from the right field of vision.
Similarly, the right eye supplies only the right hemisphere, with information from the left visual field. After making the incision, they recorded from a cell whose receptive field would normally overlap the vertical midline. They found that such a cell's receptive field is split vertically, with the right part supplied through the left eye and the left part through the right eye.
The receptive fields of fibers in the corpus callosum lie very close to the vertical midline. The receptive fields here were found by recording from ten fibers in one cat.