Simon LeVay

Laminar patterns of geniculocortical projection in the cat

The cortical afferents from individual laminae of the dorsal lateral geniculate nucleus (LGN) were studied using both light and electron microscope autoradiography. In area 17, the A geniculate laminae (A and A1) had two main bands of projection, one extending from the bottom of IVc to the deepest cells in layer III, and one in layer VI. The C geniculate laminae projected in two dense bands to the upper and lower borders of layer IV, thus bracketing the A laminae projection, though with some overlap. In addition, the C laminae projected to the superficial half of layer I, which the A laminae did not. Conversely, while the A laminae projected to layer VI, the C laminae did not. The two sets of laminae also showed differences in the areas to which they projected. The A geniculate laminae projected to areas 17 and 18, whereas the C geniculate laminae had a more extensive projection, including areas 17, 18, 19 and other areas on the suprasylvian gyrus. The laminar organization of the projection to area 18 was similar to that found in area 17. At the electron microscopic level the geniculate terminals were found to make Grays type 1 synapses, for the most part onto dendritic spines. Labeled terminals were found in all the projection bands seen in the light microscope. The implications of these findings on the connectivity of cells in layer IV are discussed. The presence of labeled terminals in layer VI, which contains the cells of origin of the corticogeniculate pathway, suggests that the recurrent loop to the LGN is mediated monosynaptically. Finally, the afferents from each geniculate lamina were found to be segregated into patches, about 500 mum wide, which probably form the anatomical basis for ocular dominance columns.

The visual claustrum of the cat. I. Structure and connections

The cats dorsocaudal claustrum was studied in Golgi preparations, by electron microscopy, and by anterograde and retrograde tracer techniques. It receives a convergent retinotopic projection from several visual cortical ares, including areas 17, 18, 19, 21a and PMLS (posteromedial lateral suprasylvian area). The projection arises from spiny dendrite cells (pyramidal and fusiform) in the middle of cortical layer VI. As shown by a double label experiment, they form a separate population from those projecting to the lateral geniculate nucleus. There are also inputs from the lateral hypothalamus, from the nucleus centralis thalami, and probably from the locus coeruleus, but not from the sensory nuclei of the thalamus. Non-visual cortical areas do not project to the visual claustrum, but many of them are connected to other parts of the nucleus. For example, the splenial (cingulate) gyrus projects to a claustral zone just ventral to the visual area, and regions anterior to the visual area are connected with somatosensory and auditory cortex. The commonest cell type in the claustrum is a large spiny dendrite neuron whose axon leaves the nucleus after giving off local collaterals. Small spine-free cells, with beaded dendrites and a locally arborizing axon, are found also. Electron microscopy of the claustrum after ablation of the visual cortex showed degenerating type 1 axon terminals synapsing on spines and beaded dendrites, suggesting a direct cortical input to both cell types. The visual claustrum projects back to the visual cortex, to the same areas from which it receives an input. The return projection is predominantly ipsilateral, but there is, in addition, a small crossed projection. The claustrocortical axons terminate in all cortical layers but most heavily in layers IV and VI. The majority of the cells in the visual claustrum project to the cortex, and retinotopy is maintained throughout the entire corticoclaustral loop. No subcortical projections from the claustrum could be identified.

Mode of termination of retinotectal fibers in macaque monkey: An autoradiographic study

The distribution of retinotectal projections was studied in 4 macaque monkeys by examining the tectum autoradiographically 3-21 days after eye injection with radioactive proline or a proline-fucose mixture. Contrary to previous reports the optic fibers project to all regions of the tectum including a relatively sparse but nevertheless very clear projection to the anterolateral one-third, where the fovea is represented. Here the terminals were distributed within the superficial grey layer of the tectum at a depth extending from about 50 mum to 125 mum and in a patchy fashion, with a tendency to aggregation in clumps 0.1-0.5 mm wide from one or other eye. Further posteromedially, corresponding to more peripheral retinal regions, the input from the contralateral eye became more continuous superficially, with tongues extending more deeply in the superficial grey, apparently enclosing clumps of ipsilateral terminals. These deeper ipsilateral clumps occupied a rather well defined layer extending in depth from about 100 mum to about 175 mum. Still further posteromedially, in the temporal crescent representation, only the continuously distributed label from the contralateral eye was found. Continuous label was also seen in the optic disc region on the ipsilateral side; on the corresponding area contralaterally, label was absent. Both ipsilaterally and contralaterally, the pattern of input was roughly symmetrical about the representation of the horizontal meridian, which ran from anterolateral to posteromedial. The regional aggregations of input from one or other eye were to some extent reflected physiologically in a regional variation in eye dominance, though this was perhaps less than might have been expected from the marked heterogeneity of the inputs.

Effects of visual deprivation on polyribosome aggregation in visual cortex of the cat

Neurons in the visual cortex of 48 normal and visually deprived kittens and cats were examined electron microscopically for the presence of absence of polyribosomes in their perikaryal cytoplasm. In normal animals at most ages the ribosomes of cortical neurons were aggregated into polysomes, but during the second and third months of life -- a period corresponding approximately to the physiologically defined critical period -- variable numbers of cells were found which contained ribosomes only in the monomeric form. The affected cells were spiny stellate neurons in the fourth layer of the cortex. Even within the critical period, however, cells with dispersed ribosomes were not found in every animal examined.