Cortical connections of area 17 in tree shrews.

In order to better understand the organization of extrastriate cortex in tree shrews, injections in area 17 of wheat germ agglutinin or tritiated proline were used to reveal an intrinsic pattern of connections, ipsilateral connections with area 18 and two other subdivisions of cortex, and callosal connections with areas 17 and 18 of the opposite cerebral hemisphere. Areal patterns of connections were best seen in sections cut parallel to the surface of flattened cortex. Within area 17, periodic foci of labeled terminations and cells extended from and surrounded injection sites as described by Rockland et al. ('82). Single injections produced multiple foci of labeled terminations and cells in area 18. The foci tended to fuse into short bands that sometimes crossed the width of area 18. Double injections produced more foci, and multiple injections tended to produce more continuous regions of label. An overall retinotopic pattern was evident with rostral area 17 connected to rostral area 18 and caudal area 17 connected to caudal area 18. Terminations extended through layers II-VI, with some increase in density in layer IV. Cells in area 18 projecting back to area 17 were in layers III and V. The injections also allowed identification of previously undefined subdivisions of visual cortex in temporal cortex immediately adjoining area 18. Dense reciprocal connections were observed in a 13 mm2 oval of cortex on the lateral border of the middle section of area 18 that we define as the temporal dorsal area, TD. Connections indicate a crude topographic organization with lower field represented rostrally and upper field caudally. Inputs were most dense in the middle cortical layers, and labeled cells were supragranular, and less frequently, infragranular. A 10-mm2 oval of cortex near the posterior edge of the hemisphere, the temporal posterior area (TP), contained labeled cells after area 17 injections, but terminal labeling was only obvious in the dorsal part. Single injections sometimes produced quite separate dorsal and ventral zones of label in TP, suggesting a small separate dorsal division. A crude retinotopic order appears to exist within ventral TP, with the lower field most ventral. Labeled cells were largely supragranular. A fourth zone of ipsilateral connections was in posterior limbic cortex bordering area 17 on the ventromedial surface of the cerebral hemisphere. The callosal connections were reciprocal and included regions 1 mm wide on either side of the area 17 and area 18 border. Callosal connections were rougly homotopic. Callosal terminations included superficial layers, and projecting cells were both supragranular and infragranular.(ABSTRACT TRUNCATED AT 400 WORDS)