MORPHOLOGY AND CONNECTIONS OF NEURONS IN AREA-17 PROJECTING TO THE EXTRASTRIATE AREAS MT AND 19DM AND TO THE SUPERIOR COLLICULUS IN THE MONKEY CALLITHRIX-JACCHUS

Neurons of area 17, the primary visual cortex, project to various anat omically and physiologically different extrastriate areas and subcorti cal regions. In the present investigation, we addressed the question o f whether the efferent neurons in area 17 can contribute to functional diversity between these regions. We approached this question by analy zing the dendritic morphology of neurons in area 17 projecting to area MT, area 19DM, and the superior colliculus in the new world simian pr imate Callithrix jacchus, because dendritic morphology is an important factor in determining physiological properties of nerve cells. Retrog rade transport of fluorochromes injected into the target regions, and intracellular injections of Lucifer yellow in the prelabelled neurons, revealed the following. 1) Morphologically identical large pyramidal cells in layer VI of area 17 project to all three targets. Some of the m possess axon collaterals to two or all three targets, suggesting tha t they provide common information to all three areas. 2) Pyramidal cel ls in layer IIIc projecting to area MT form a morphologically homogene ous population. 3) Three small to medium-sized pyramidal cell types in layers IIIa-c, spiny stellate cells in layer IIIc, and another large pyramidal cell type in layer VI project to area 19DM. 4) Pyramidal cel ls in the lower two-thirds of layer V in area 17 project to the superi or colliculus. In conclusion, we have shown that in Callithrix one eff erent pathway may originate from several cell types. However, with the exception of the large cells in layer VI, efferent cells projecting t o area MT, area 19DM, and the superior colliculus were morphologically distinct. This suggests that functional differences between brain reg ions could arise in part from morphological heterogeneity between and within the efferent cell populations. (C) 1995 Wiley-Liss, Inc.