Cortical connections of area 18 (V-II) and part of the dorsolateral visual area (DL) were determined in squirrel monkeys with single and multiple injections of the sensitive bidirectional tracer, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Injections were placed into portions of area 18 or DL on the dorsolateral surface of the brain, patterns of label were examined in brain sections cut parallel to the surface of physically flattened cortex, and comparisons were made with alternate brain sections reacted for cytochrome oxidase (CO) or stained for myelinated fibers. Major results are as follows. (1) Area 18 was identified by a characteristic alternation of dense and light CO bands crossing its width; the middle temporal visual area (MT) was CO dense; the dorsolateral area (DL) was less reactive, with rostral DL (DLR) lighter than caudal DL (DLC); area 17 had clear CO puffs in the supragranular layers. (2) Intrinsic connections revealed in area 18 included a narrow 100–200 μm fringe of less dense label around each injection core, label unevenly distributed in small clumps within 1–2 mm of injection sites, and clumps of transported label up to 6 mm from injection sites. (3) Single and multiple injections in area 18 produced patterns of labeled cells and terminations in area 17 that ranged from lattice- to puff-like in surface-view distribution. With multiple area 18 injections, regions of area 17 could be found where transported label was concentrated in CO puffs, avoided the CO puffs, or overlapped both puff and interpuff divisions of cortex. The labeled regions of area 17 were somewhat larger than the injection sites, suggesting some convergence from area 17 to area 18. (4) The major rostral connections of area 18 were with caudal DL (DLC). Rostral DL (DLR) was largely free of transported label. Single injection sites in area 18 resulted in several large clumps of label separated by regions of sparse or no label in DLC. Injections in lateral area 18 produced lateral foci of label in DL, while more medial injections produced more medial foci. However, following multiple injections into area 18 that included the representation of central vision, a continuous 2–3-mm-wide band of infragranular labeled cells extended from area 18 caudally to MT rostrally in the presumed location of central vision in DLC and DLR. (5) Injections in area 18 produced foci of label in MT. Label was more dorsal in MT with more dorsal injection sites in area 18. (6) Injections in area 18 resulted in sparse label in cortex within the inferior temporal sulcus and in cortex in the location of the frontal eye field. (7) Callosal connections of area 18 were with areas 17, 18, DL, and sparsely with MT. Multiple injections in area 18 produced a narrow, dense strip of label along the contralateral 17/18 border. Most of this label was in area 18, but small protrusions of label extended into area 17, and small separate foci of label were found displaced slightly into area 17. Fingers of callosal connections extended rostrally from the caudal border to cross up to half of the width of medial area 18 and the entire width of lateral area 18 where central vision is represented. Patchy callosal connections were found with DLC. (8) Injections in caudal DL confirmed the observation from area 18 injections that major connections of DLC are with area 18. Injections in DLR produced scattered, small foci of label in area 18 near the rostral border, as well as puffs of intrinsic connections, connections with MT, and with cortex rostral to area 18 medially.
The major conclusion stemming from the present results is that the DL region consists of at least two fields, with the caudal portion, DLC, receiving major inputs from area 18, and the rostral portion, DLR, having little input from area 18.