Detailed and complete projection maps of the visual field to the whole of the dorsal lateral geniculate (LGNd) and medial interlaminar nucleus (MIN) of the cat have been prepared by plotting the receptive fields of single units in the two nuclei with tungsten-in-glass microelectrodes. The standard projection maps show the pattern of isoazimuths ("horizontals") and isoelevations ("verticals") in the two nuclei. Particular attention has been given to the projection of the upper visual field to the posterior part of the LGNd/MIN complex where the shape and relationship of the cellular laminae are changing rapidly. A separate projection of the visual field to the nucleus perigeniculatus (NPG) is also described, but the retinotopic organization of this projection is not as precise as for the LGNd and the MIN. Most of the cells in the NPG are binocularlyactivated and the receptive fields have ON/OFF centers. Further observations have been made concerning the distribution of the crossed and uncrossed fibers in the LGNd. In lamina B there is a thick dorsal zone of contralaterally-activated cells and below this is a zone in which ipsilaterally-activated cells are occasionally found. A few binocularly-activated cells were found in the vicinity of the interlaminar regions.The topographical representation of the retina upon the dorsal nucleus of the lateral geniculate body (LGNd) of the cat was first studied using anatomical methods by Brouwer, Zeeman and Houwer ('23) and Overbosch ('27). They made restricted retinal lesions and were able to show an area-to-area projection from the retina to the LGNd. However, the h t precise, though still partial, description of the representation of the visual field on the LGNd did not come until the more recent development of single unit recording techniques. By systematically plotting the visual directions of receptive fields of single neurons recorded in the nucleus Bishop, Kozak, Levick and Vakkur ('62) demonstrated a fairly exact point-to-point projection from the retina to the LGNd. They mapped the representation of the inferior visual field in the anterior half of the nucleus fairly thoroughly (cf. also Seneviratne and Whitteridge, '62; Bishop, '65) but left the remainder of the nucleus largely unexplored. The projection of the visual field to the

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The retinogeniculate pathways of several different genotypes of mink have been studied by the Nauta and Fink-Heimer methods. An abnormal retinogeniculate pathway has been found in all mink in which the retinal pigment is reduced. Most of the abnormally routed nerve fibers arise in the temporal retina and cross in the chiasm, instead of staying ipsilateral as is normal. Some abnormal fibers in some of the mink also appear to arise in the nasal retina close to the line of decussation, and these pass ipsilaterally instead of following their normal crossed pathway.A lack of pigment in the coat is not by itself associated with a pathway abnormality. In general, the size of the abnormal fiber component is related to the severity of the retinal pigment deficit. The abnormality is not related specifically to one gene or gene combination. We have found eight different gene combinations which produce a reduction of retinal pigment associated with a pathway abnormality.

Afferent connections of rat primary visual cortex (area 17 or V1 area) and the rostral and caudal parts of areas 18a and 18b were studied, by placing in each of the areas, small electrophoretic injections of enzyme horseradish peroxidase (HRP) or wheat germ agglutinated-HRP. The results indicate that: 1) each of the areas has a distinct pattern of distribution of afferent neurons in the ipsilateral visual thalamus - area 17 receives its principal thalamic input from the dorsal lateral geniculate nucleus, the caudal parts of areas 18a and 18b receive a major thalamic input from the lateral posterior nucleus and a minor input from the posterior nucleus, while the rostral parts of areas 18a and 18b receive a major input from the posterior nucleus, and a minor projection from the lateral posterior nucleus; 2) the rostral and caudal parts of areas 18a and 18b each receive an associational input from area 17; 3) the rostral parts of areas 18a and 18b each receive associational input from three different extrastriate regions, the caudal part of the same extrastriate area, and the rostral and caudal parts of the other extrastriate area, whereas the caudal parts of areas 18a and 18b receive associational inputs only from one or two extrastriate regions; 4) area 17, area 18b and rostral area 18a each receive a substantial associational input from lamina V of the caudal part of the frontal eye field (FEF) in the motor cortex; however the input from the FEF to caudal area 18a (if present) is very small; 5) The extrastriate areas studied receive associational input from the restrosplenial cingulate area 29d; however, the input from area 29d to area 17 appears to be very small. The distinct patterns of distribution of prosencephalic afferents suggest to us that multiple retinotopically organized areas described previously in the rat cortex (cf Montero 1981; Espinoza and Thomas 1983) represent functionally distinct areas.