Reciprocal heterotopic callosal connections between the two striate areas in Tupaia.

WGA-HRP injections were placed into area 17 close to the border with area 18 of Tupaia belangeri in order to study the callosal connections of the striate area in this animal. Most callosal neurons were found in the striate cortex (57.6-86.9%), some in the extrastriate area 18 (10.6-28.1%), and a few in even more temporal regions (2.5-14.3%). Concerning only the area 17, reciprocal homotopic connections could be observed as a strip along the area 17/18 border. Additionally, heterotopic callosal connections could be seen in regions representing the binocular visual field, especially the lower part. The area 17 cells were mostly located in the supragranular layers II and III (94.1-97.2%). But neurons could also be found in the infragranular layers, especially layer VI (2.6-5.2%) and in layer IV (0.2-1.1%). Homotopic projections were mostly seen in layers IIIc and V. The majority of the supragranular and infragranular neurons are pyramidal cells. However, a newly defined subpopulation of neurons, most probably stellate cells, were discovered forming a band in sublayer IIIc, very close to the layer III/IV border.