Corticofugal projections from the visual cortices to the thalamus, pretectum and superior colliculus in the cat.

Abstract

AbstractSmall lesions were placed in visual cortical areas 17, 18, 19, 20, 21, 7, and Clare‐Bishop in the cat, and the sites of terminal degeneration seen with Fink‐Heimer technique were plotted in thalamus, pretectum and superior colliculus. No degeneration was found in these sites after area 20 lesions; lesions in the other cortical areas gave different patterns of degeneration. Two major patterns were present, one from lesions in 17–18, one from lesions in 21, 7 and C‐B, with degeneration from 19 forming a transition between the two groups.Areas 17, 18 and 19 project to the dorsolateral geniculate nuclear complex (LGNd); areas 21, C‐B and 7 do not. Area 17 projects to the laminar part, area 18 to both laminar and interlaminar (NIM) parts, and 19 only to NIM. The corticogeniculate projections from all three areas are topically organized anteroposteriorly, and at least that from area 17 is topically organized mediolaterally.Areas 17 and 18 project topically to a columnar locus of the medial pulvinar (=lateral posterior) nucleus which ventrally includes that area known as the posterior nucleus. Area 19 has a double columnar projection to this part of the thalamus, one in the medial and one in the lateral pulvinar area. The medial column lies medial to that from 17–18, and appears to overlap the termination of the ascending projection from the superior colliculus. Cortical areas 21, C‐B and 7 also have a double projection to the pulvinar.These findings indicate that the corticorecipient neurons in both medial and lateral sectors of the pulvinar are organized so that dorsal neurons are activated by stimuli in upper visual fields (lower retina) and ventral neurons by stimuli in lower fields (upper retina).Areas 17 and 18 project to the external layer of the ventrolateral geniculate (LGNv) nucleus, 19 to both external and internal layers, and 21, C‐B and 7 to internal layer only.The pretectal projection from 17–18 is limited to its caudal pole chiefly in the posterior pretectal nucleus (NPP), and also in the nucleus of the optic tract (NOT). Area 19 fibers terminate in NPP, NOT and also in the reticular part of the anterior pretectal nucleus (NPAr). Those from 21 and C‐B end primarily in NPAr, and from area 7 in both reticular and compact parts of NPA. These corticopretectal systems all appear to be organized topically.Areas 17, 18 and 19 have a double termination in the superior colliculus, a focal pattern in the superficial layers (chiefly lamina II), and a diffuse pattern in deeper layers (laminae IV, V, VI). The superficial pattern only provides the retinotopical matching with the optic afferents. All other cortical areas project diffusely to the deep layers. After lesions in 21 and C‐B, the superficial foci are larger and centered in lamina III; after area 7 lesions this focal degeneration is centered in laminae III and IV and spread over much of the width of the colliculus. Degeneration to pontine nuclei and inferior olive was not examined.