Neuroscience Vol. 29, No. 1, pp. 95-107, 1989 Printed in Great Britain 0306-4522/89 $3.00 + 0.00 Pergamon Press plc 0 1989 IBRO zyxwvut VISUAL PATHWAYS TO THE CEREBELLUM: SEGREGATION IN THE PONTINE NUCLEI OF TERMINAL FIELDS FROM DIFFERENT VISUAL CORTICAL A REA S IN THE CAT J. G. BJAALIE* and P. BRODAL Anatomical Institute, University of Oslo, Karl Johansgt. 47, N-0162 Oslo 1, Norway Abstract-The cerebellum receives input from visual cortical areas via a relay in the pontine nuclei. We have compared the location in the pontine nuclei of terminal fields of fibres from visual areas 18 and 20, and the posteromedial lateral suprasylvian visual area. Due to individual variations in the precise location of terminal fields, comparisons were performed in individual animals. Horseradish peroxidase-wheat germ agglutinin conjugate was used as an anterograde tracer in combination with the Fink and Heimer method for visualization of anterograde degeneration. Most of the terminal fields of area 20 are widely separated from those of area 18. Fibres from the posteromedial lateral suprasylvian visual area and area 20 terminate close to each other but overlap of terminal fields is limited. Area 18 and the posteromedial area have in some places completely overlapping' terminal fields; in other places, however, there is only partial overlap or complete separation. Generally, segregation of terminal fields from different areas is most pronounced in the caudal part of the recipient zone of the pontine nuclei. The terminal fields of fibres from the three cortical areas studied appear as numerous patches arranged in a complicated mosaic that tend to form concentric lamellae around the ventromedial aspect of the peduncle. Within these lamellae, area 18 projects mainly to the innermost one, area 20 to the outermost, and the posteromedial area to an intermediate lamella. Whether terminal fibres from different areas are segregated (non-overlapping) or overlapping in the pontine nuclei is of relevance for the functional organization of the cerebrocerebellar pathway. Segregation of terminal fields from different areas would mean that the areas in question influence different sets of pontocerebellar neurons and thereby relay information to the cerebellum in separate channels. Overlap of terminal fields from different areas could mean that convergence on the same pontocerebellar neurons occurs (although convergence cannot be proved with the techniques employed in this study). This study indicates that information from visual areas is relayed at least in part in separate channels from the cortex to the cerebellum. The cerebral hemispheres of the cat contain more than a dozen separate visual areas (for a review, see Ref. 42). All the known visual cortical areas of the cat project to the pontine nuc]eil~7.12.13,24,28.29,31,32,39.43 and thereby give input to the cerebellum. The terminal fields in the pontine nuclei from areas 17, 18, 19,‘2,13.29.43 20,6 the anteromedial lateral suprasylvian visual area (AMLS),” and the anterior ectosylvian visual area3’ have been investigated in some detail. The termination in the pontine nuclei of fibres from parts of the posteromedial lateral suprasylvian visual area (PMLS) has also been determined, although in less detail3 (see also Ref. 41). Brodal13 described differences in the location of termination from area 19 compared to areas 17 and 18. Other authors, however, have emphasized the overlap of terminal fields from different visual areas in the pontine nuclei 3.24.38.41.43 Obviously, whether corticopontine cells in different *To whom correspondence should be addressed. Abbreviations: AMLS/PMLS, anteromedial/posteromedial lateral suprasylvian visual area; HRP-WGA, horse- radish peroxidase-wheat germ agglutinin; LS, lateral suprasylvian visual areas; TMB, tetramethylbenzidine. visual areas project to the same or different parts of the pontine nuclei is of relevance to the function of visual corticopontocerebellar connections. If cortico- pontine neurons in different areas project to precisely the same parts of the pontine nuclei, they would most likely contact the same pontocerebellar neurons and thereby allow integration of information from a wide range of visual cortical areas at a precerebellar level. If, on the other hand, different areas project to largely different parts of the pontine nuclei, there might be separate channels from different visual cortical areas to the cerebellum. The latter situation would even allow information from different visual areas to reach different targets at the cerebellar cortical level. To elucidate the question of overlap or non- overlap of terminal fields of pontine projecting neur- ons from different visual cortical areas, we compare in this paper the termination from areas 18, 20, and PMLS. We chose area 18 and PMLS since these areas are assumed to project to largely the same parts of the pontine nuclei3 (see also Ref. 41), and also since the terminal fields of PMLS have not been described in detail before. Area 20 was chosen since in a previous paper by Bjaahe,6 the impression was that the pontine terminal fields of this area appeared to be located 95