No abstract

Projections to and from the visual sector of the thalamic reticular nucleus were studied in the prosimian primate genus Galago by anterograde and retrograde transport of WGA-HRP injected into the dorsal lateral geniculate nucleus (GLd), pulvinar nucleus, and their cortical targets. Contrary to the idea that thalamic connections with the reticular nucleus are not delimited sharply between nuclei associated with the same modality, our results show a distinct laminar segregation of the projections from the GLd and pulvinar nuclei. The GLd is connected reciprocally with the lateral {frsol|2/3} of the caudal part of the reticular nucleus, and the striate cortex sends projections to the same lateral tier. Both sets of projections are organized topographically, lines of projection taking the form of slender elongated strips that run from caudo-dorsal to rostro-ventral within the nucleus. The pulvinar nucleus, which projects to several areas of the temporal, parietal, and occipital lobes, including the striate cortex, is connected reciprocally with the medial {frsol|1/3} of the caudal part of the reticular nucleus. Every injection into the pulvinar nucleus labelled a wide area of the medial tier, with no indication of visuotopic organization. The projections from the middle temporal area, one of the principal targets of the pulvinar nucleus, also terminate only in the medial tier of the visual sector. And we would expect that, in general, a thalamic nucleus and its cortical target would project to the same part of the reticular nucleus. The case of the striate area is an exception but only in the sense that it projects to the pulvinar nucleus as well as GLd. Thus an injection into a single locus in area 17 produces two parallel strips in the visual sector of the reticular nucleus, but both are in the lateral tier. We propose that each strip arises from a separate population of cells with cortical layer VI, one with an allegiance to the GLd and the other to the pulvinar nucleus.

The projections of the superficial layers of the superior colliculus to the pulvinar nucleus in Tupaia were reexamined by injecting WGA-HRP into the tectum. The main result was finding two different patterns of terminations in the pulvinar nucleus: a zone remote from the lateral geniculate nucleus, which occupies the dorsomedial and caudal poles of the pulvinar nucleus, was almost entirely filled with terminals in every case irrespective of the location of the injection site; and a second division of the pulvinar nucleus, adjacent to the lateral geniculate nucleus, contained irregular patches--much more densely populated--and the distribution of patches varied from case to case. We call the first projection "diffuse" and the patchy projection "specific." Next we injected several divisions of the extrastriate visual cortex to find the cortical target of each pathway. The diffuse path terminates in the ventral temporal area (Tv). The specific path terminates in the dorsal temporal area (Td) and area 18. We speculated about the significance of the two pathways: the specific path may be responsible for the preservation of vision after removal of the striate cortex; the diffuse path may have an important place in the evolution of the visual areas of the temporal and occipital lobe. We argued that the target of the diffuse path is in a position to relate limbic and visual impulses and relay the product of such integration to the other visual areas, striate as well as extrastriate cortex.

Thalamic projections to the visual cortex of the grey squirrel were studied by retrograde degeneration in the lateral geniculate and the pulvinar nuclei. The lateral geniculate was found to project to architectonic area 17 which also corresponds to visual area I as defined by its retinotopic organization. The projection is spatially organized in a precise way, and for every cortical point there is a corresponding column in the lateral geniculate which extends from border to border. For the binocular sector of area 17 the lateral geniculate column lies in the trilaminar part of the lateral geniculate, while for the uniocular sector the column lies in the bilaminar sector of the lateral geniculate. The pulvinar projects to several architectonic areas, areas 18 and 19 and to two or more temporal areas below area 19. This projection is roughly topographic but follows the sustaining pattern. When the squirrel is compared to the tree shrew and hedgehog what emerges is a conception of those changes in the visual system which arose as a result of adaptation to an arboreal habitat.In an earlier study (Hall et al., '71) the method of evoked potentials was used to map the visual cortex of the squirrel with the result that three topographic representations of the visual field were defined, visual areas I, I1 and 111. Each of these subdivisions corresponded precisely with an architectonic area: V I corresponded to area 17, V I1 to area 18, and V I11 to area 19. Responses to light were also elicited in the temporal cortex lying between area 19 and the rhinal fissure. The experiments reported here were undertaken to relate the projections from the visual thalamus to these areas. The present paper also presents in greater detail the basis for the architectonic subdivision into 17, 18 and 19 and includes architectonic evidence for subdividing the temporal cortex.By the "visual thalamus" we include the pulvinar nucleus in addition to the dorsal lateral geniculate. This extension of the concept of the visual thalamus has depended in general on our comparative studies of the mammalian visual system, and in particular on studies in several species which show that the pulvinar J. COMP. NEUR., 145: 273-306.serves as a visual center relaying impulses from the optic tectum to the extrastriate visual cortex ( H a r t h g et al., '72b).The specific aims of the present experiments were, first, to determine whether the projection of the lateral geniculate in the squirrel is restricted to area 17; second, to find out how the lateral geniculate projections are organized; and finally, to determine the extent of the pulvinar projection, and especially whether the target of the pulvinar includes areas 18, 19 and the temporal regions below 19.We have tried in several places (Diamond and Hall, '69; Hall et al., '71) to justify the effort devoted to the study of the squirrel, and for the present purposes it may suffice to say that one of our main interests is in comparing these results with the results of similar investigations in a second ...