| Summary: | <p>The central connexions of the visual system of the cat were investigated using neuroanatomical techniques, particular emphasis being given to the interrelationships of the visual cortox and the superior colliculus. Lesions were made in the retina by means of needle electrodes and in the cerebral cortex by devascularization. The resultant degeneration was studied either after a survival period of a few days by the silver impregnation technique of Nauta and Gygax (1954) for the fibres or after a survival period of some weeks by thionin staining for retrograde cellular changes. Reconstructions were made of every retina and brain in order to compare the site and extent of the lesion with the site and extent of the degeneration.</p> <p>Preliminary investigations, in which one eye was completely removed, showed that the retina projects to several sites. On the ipsilateral side these are lamina Al, the dorsal and ventral components of the central interlaminar nucleus and the central part of the medial interlaminar nucleus of the dorsal nucleus of the lateral geniculate body, the ventral nucleus of the lateral geniculate body, the whole extent of the pretectal area (particularly the nucleus of the optic tract) and the strata opticum, griseum superficial and griseum intermediale of the superior colliculus. On the contralateral side terminal degeneration is found in lamina A and B, the dorsal and ventral components of the central interlaminar nucleus and the medial and lateral parts of the medial interlaminar nucleus of the dorsal lateral geniculate body, the ventral lateral gesticulate nucleus, the pretectal area and nucleus of the optic tract, the same layers of the superior colliculus, and in the lateral and medial terminal nuclei of the accessory optic tract. The density of the terminal degeneration in the superior colliculus is considerably greater on the contralateral side.</p> <p>The details of the representation of the retinal quadrants upon the dorsal lateral geniculate nucleus and the superior colliculus were studied by placing small electrolytic lesions in different parts of the retina. That part of the retina lateral to the area centralist the temporal retina, projects to the above sites on the ipsilateral side, while fibres from the retina on the nasal side of the area central is are distributed only to the contralateral nuclei. The superior quadrants of the retina project to the anterior part of the lateral geniculate nucleus and to the postero-lateral part of the superior colliculus, while the inferior quadrants project posteriorly and antero-medially respectively. In the lateral gesticulate nucleus the horizontal retinal meridian is represented obliquely from postero-lateral to antero-medial just behind the middle of its antero-poaterior extent as viewed from the dorsal aspect, while on the colliculus this meridian is disposed obliquely from postero-medial to antero-lateral; the representations of the vertical meridian are coextensive with the medial and anterior margins of the lateral geniculate body and superior colliculus respectively. Very small lesions in the immediate vicinity of the area contrails have shown that it projects to a little behind the middle of the medial margin of the lateral geniculate body and to the antero-lateral edge of the superior colliculus , and also that the degeneration from such lesions occupies a disproportionately large area compared with similar lesions at the periphery of the retina. There is also a well defined organization din the retinal projection to the inter laminar nuclei of the lateral geniculate body. In the central interlaminar nucleus the terminal degeneration found in both its dorsal and ventral components on each side always adjoins that present in the main laminae. In the medial interlaminar nucleus the upper and lower parts of the retina project respectively anteriorly and posteriorly; the central area is represented in the more dorsal &arts; of the nucleus, while peripheral retinal lesions cause degeneration in the ventral half. As the temporal hemiretina projects to these nuclei on the ipsilateral side, and the nasal on the contralateral , it is clear that there is an additional representation of the entire retina in each of the nuclei.</p> <p>The organization of the projection of the lateral geniculate nucleus upon the cortex has been defined by correlating the site and extent of retrograde cellular changes in the thalamus following lesions of various sizes in the visual cortex as recently defined. The nucleus projects ipsilaterally to areas 17, 18 and 19 in such a way that its anterior and posterior parts project to the corresponding portion of these areas; in the medio-lateral dissension, however, there is a reversal in the thalamo-cortical relationship, in that medial and lateral parts of the nucleus project to the lateral and medial portions respectively of the cortex. The cellular degeneration in the central interlaminar nucleus is found to be co—extensive with that in the main laminae. Cellular degeneration is found in the medial Inter laminar nucleus only when the lesion extends beyond area l?t but whether this nucleus is connected with area 18 or with area 19 or both has not been determined. The retina is therefore represented in the visual cortex with the superior quadrants anteriorly and the inferior quadrants posteriorly; the area centralis is represented near the junction of the lateral and postlateral gyri; and the periphery of the retina along the anterior, medial and posterior borders of the visual cortex. The representation of the area centralis is relatively greater than that of the rest of the retina. The finding of a separate retinal representation in the medial interlaminar nucleus together with the differential projection of this nucleus to area 18 or area 19 (or both) may provide the anatomical basis for the additional retinal representation recently found in these areas.</p> <p>Following lesions of the visual cortex terminal fibre degeneration is found in several subcortical nuclei; of particular relevance are the dorsal and ventral nuclei of the lateral geniculate body, the posterior thalamic and pulvinar complex, the protection and nucleus of the optic tract, and the superior colliculus. The nucleus of the optic tract receives fibres from all parts of the visual cortex, but the rest of the pretectum is found to be related only to a small inferior part of the postlateral gyrus. The visual cortex projects upon those layers of the superior colliculus which receive retinal fibres and to the stratum zonale in a well organized manner, the anterior part of the cortex being related to the postero-lateral half of the colliculus and the posterior cortex to the antero-medial half. Furthermore, any part of the cortex related to a specific part of the retina sends fibres to that part of the tectum which is directly related to the same part of the retina. Fibres are found to project to the deeper layers of the superior colliculus from other neocortical areas. The degeneration in the lateral geniculate nucleus after lesions in the visual cortex is localized to those areas which project to the same parts of the cortex.</p> <p>It is concluded that the retina is related to the superior colliculus in a highly organised fashion and that the visual cortex has a similar relationship to the tectum, such that any part of the cortex representing a specific area of the retina projects to that part of the superior colliculus which receives the direct projection of that area of the retina. The retina also projects in an organised manner on the Main laminae of the lateral geniculate body and is represented a second time in the medial inter laminar nucleus; the former part of the lateral geniculate nucleus projects to area 17 in an organized fashion, while the second projects to visual cortex outside area 17.</p> <p>The possible significance of the results is discussed. The phylogenetically older midbrain has developed close relations with the cortex as this latter structure developed in the mammal; there are, in fact, many similarities between the sub-mammalian optic tectum and the mammalian cortex, both in structure and in the distribution of retinal afferents to them. Although the functional significance of the tectum is poorly understood, some of its possible roles are examined in the light of the present findings. These include pattern discrimination, eye movement, accommodation and visual withdrawal. Also, considering the well defined organization of the projection of the optic tectum upon the nucleus of origin of the centrifugal fibres to the retina of the pigeon, the possibility of a similar relationship of the superior colliculus in the cat is mentioned. It is emphasized that the superior colliculus is related to many systems other than the visual, and its relationship to the reticular formation may be among its most important. The incidental findings in the present study that there are two distinct retino-thalamo-cortical pathways, one to primary visual cortex through the main laminae of the lateral geniculate body and the other to the second visual area through the medial inter laminar nucleus is discussed. The finding of a reciprocal visual cortico- geniculate pathway provides an anatomical basis for a feedback system which may be of significance in attention and habituation. Finally, the question of homology between the visual cortex of the cat and of the primate is discussed.</p>
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