On the primary visual cortex

By the end of the eighteenth century, the gross structure of the cerebral cortex was beautifully illustrated in anatomical texts, but the cortex was portrayed as structurally homoge-

Fig. 5. Horizontal axis cylinder from the outer plexiform layer. a, terminal arborization as seen from the side; b, nerve fiber.

Fig. 6. Another terminal arborization of the same type.

Fig. 7. Nerve elements from the ox retina stained with chromium-silver according to the double impregnation method. A, semilunar amacrine cell whose enormously long branches arborize in the first sublayer; B, large amacrine cell with thick branches in the second sublayer; F, another amacrine cell, which is rather small and arborizes in the second sublayer; D, amacrine cell with a stellate cluster destined for the third sublayer; G, H, amacrine cells destined for the fourth sublayer; E, large amacrine cell destined for the fifth sublayer; C, special type of amacrine cell with very thin branches which spread preferentially in the first and fifth sublayers.

a, small ganglion cell destined for the fourth sublayer; b, ganglion cell whose branches form three superimposed plexes; c, small ganglion cell with branches arborizing in the first sublayer; d, middle-sized ganglion cell with branches in the fourth sublayer; f, ganglion cell which is similar to the multilayered cells (branching in three sublayers) in the reptile and bird; their branches form two plexes— one in the fourth sublayer and another in the second sublayer; e, giant ganglion cell destined for the third sublayer.

Fig. 8. Amacrine cells and ganglion cells from the dog retina. A, stellate amacrine cell destined for the first sublayer and a portion of the second sublayer; B, giant amacrine cell of the third sublayer; C, G, stellate amacrine cells destined for the second sublayer; F, small amacrine cell destined for the third sublayer; E, amacrine cell destined for the fourth sublayer; D, unstratified amacrine cell; a, ganglion cell whose upper cluster spreads in the second sublayer;

b, giant ganglion cell destined for the second sublayer; e, small ganglion cell whose cluster spreads in the fourth sublayer; f, middle-sized ganglion cell which arborizes in the first and in a portion of the second sublayers; g, ganglion cell which arborizes in the third and a portion of the fourth sublayers; i, two-layerd cell (cellule bistratifie).

Fig. 9. Ganglion cells from the dog retina. a, giant ganglion cell whose cluster spreads in the first and a portion of the second sublayers; b, small ganglion cell whose multiple processes disappear in the fifth sublayer; c, giant cell whose cluster seems to spread mainly in the second sublayer; e, giant ganglion cell of the second sublayer; d, g, small ganglion cells with clusters in the fourth sublayer; f middle-sized ganglion cells destined for the first sublayer; h, another ganglion cell destined for the second and partially for the first sublayer; i, unstratified ganglion cell; A, B, C, spongioblasts (amacrine cells); L, lower terminal arborization of a bipolar cell. (From Cajal, 1892.)

neous. One part of the cortex was depicted as looking like any other. The first recognition that the cerebral cortex is not uniform in structure was made by an Italian medical student, Francesco Gennari, working in the newly re-founded University of Parma (Gennari, 1782; Glickstein and Rizzolatti, 1984). Gennari packed brains in ice, which allowed him to make clean, flat cuts through them. He noted a thin white line, and sometimes two lines within the cortex, running parallel to and about halfway between the pial surface above and the white matter below. The line coalesces into a prominent single stripe in the caudal part of brain, "in that region near the tentorium." Gennari first saw the stripe in 1776 and described it in his monograph De Peculiari (1782) some 6 years later (Fig. 1.3).

Gennari's monograph was published in a limited edition and he came from what was then an obscure university, so although it was cited by some authors, it was often ignored. The same cortical stripe was discovered independently a few years later by the more eminent anatomist Vicq D'Azyr. The stripe was described in his Traité D'Anatomie (1786) 3 years later. It was the Austrian anatomist Obersteiner (1888) who found Gennari's earlier description of the white line and named it the stripe of Gennari.

Although regional variability in cortical structure was soon accepted, there was no agreement about possible differences in the functions of different cortical areas. Two of the major authorities at the beginning of the nineteenth century, Gall (Gall and Spurzheim, 1810-1819) and Flourens (1824), held opposing views. Gall and his followers, the cranioscopists/phrenologists, asserted that the cerebral cortex is made up of a number of individual areas, each associated with a specific personality characteristic. If a person has a good memory, for example, the memory area of the cortex is relatively enlarged. Enlargement of a cortical area is associated with corresponding change in the shape of the skull, hence a bump on the head. Person-

Figure 1.3. The first recognition of the presence of a fiber layer within the cerebral cortex (labeled l in the picture) which Gennari described as being "particularly prominent in that region near to the tentorium." (From Gennari, 1782.)

ality, ability, and character could be read by palpating the head.

The earliest experimentalists failed to confirm Gall's views. In a typical experiment, Flourens (1824) made lesions in the brains of birds and mammals and observed the resulting effects on the animals' behavior. Although Flourens was convinced that the cerebral cortex is responsible for sensation, movement, and thought, he could find no evidence that any of these functions is localized to a particular site on the cerebral cortex.

In later years, evidence began to accumulate in favor of functional localization in the cerebral cortex. A series of postmortem observations of focal injuries in the brains of patients who had lost the power of speech culminated in Broca's (1861) description of the lesion in the left frontal lobe of the patient "Tan," a man who had been unable to say any word other than tan for the past several years. The evidence for brain localization of speech was soon accepted, and within a few years experiments began to provide additional evidence that different areas of the cerebral cortex are specialized for different functions. The single most important experiment that led to modern understanding of the localization of motor and sensory functions in the cortex was done by Gustav Fritsch and Eduard Hitzig (1870). They electrically stimulated restricted regions of the frontal lobe of a dog and elicited movement of the face or limb on the opposite side of the body. Fritsch and Hitzig's discovery of a specifically motor area of the cortex was instrumental in prompting a search for other functions, including vision. There had been indications (Panizza, cited by Mazzarello and Della Sala, 1993) that lesions in the caudal part of the brain are associated with visual deficits, but the clearest and most influential evidence for the visual function of the occipital lobe was provided by Hermann Munk, professor of physiology in the Veterinary Institute in Berlin. Munk (1881) made lesions in the occipital lobe of dogs and monkeys. He reported that if he destroyed one occipital lobe, the monkeys became hemianopic. Bilateral lesions caused blindness (Fig. 1.4).

Munk's discovery focused the attention of clinicians and scientists on the role of the occipital lobe in vision. Salomon Henschen (1890) summarized the postmortem findings in a group of patients who had suffered from hemianopia as a result of a stroke. He compared these patients with a similar number who had sustained a comparable loss of brain tissue that had not become hemianopic. Henschen confirmed the location of the primary visual area, and he suggested a scheme for the way in which the visual fields are mapped on the primary visual cortex. Henschen recognized that the left hemisphere receives its input from the right visual field and the upper bank of the calcarine fissure from the upper retina, hence the lower visual field. But Henschen also suggested that the periphery of the visual field is projected

onto the caudal end of the striate cortex, with the fovea represented anteriorly. In this, he was in error.

Henschen's error is understandable, since the lesions in the brains that he studied were diffuse. What was needed to establish a more accurate spatial mapping was evidence of partial field defects, scotomas, caused by smaller, subtotal lesions of the striate cortex. Such lesions, regrettably, arise in wartime. One of the earliest clear pictures of the representation of the peripheral-central visual field representation was made by a young Japanese ophthalmologist, Tatsuji Inouye (Glickstein and Whitteridge, 1987; Inouye, 1909). Inouye was in the medical service of the Japanese Army during the Russo-Japanese war of 1904-1905. His responsibility was to evaluate the extent of visual loss in casualties of the war. Inouye used the opportunity to study the visual field defects caused by penetrating brain injuries. In that war the Russians used a newly developed rifle which fired small-caliber bullets at high velocity. Unlike most bullets used in previous wars, these bullets often penetrated the skull at one point and then exited at another, making a straight path through the brain. Inouye devised a three-dimensional coordinate system for recording the entry and exit wounds. He then calibrated the course of the bullet through the brain and estimated the extent of the damage it would have caused to the primary visual cortex or the optic radiations. Based on his study of visual field defects in 29 patients, Inouye produced a map of the representation of the visual fields on the cortex. The central fields were now placed correctly in the most caudal part of the striate cortex, with the peripheral visual fields represented anteriorly, and there was an over-representation of the central visual fields in the primary visual cortex.

Based on his studies of the visual field defects sustained by soldiers of the First World War, Gordon Holmes (1918a) produced a more accurate and detailed map of the representation of the visual fields on the striate cortex, which still forms the basis for interpreting partial visual loss in humans.

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