Cortical connections within the occipital lobe (areas 17, 18 and 19) of the rhesus
monkey are investigated with the autoradiographic and horseradish peroxidase procedures.
Two efferent systems, each with a specific laminar organization, are observed. (1)
Rostrally directed connections, from area 17 to 18, area 18 to 19 and area 19 to the
inferotemporal region (area TE), originate from neurons in layer IIIc (and, in area
19, from a small complement of neurons in layer Va), and terminate in and around layer
IV. (2) In contrast, connections in the reverse direction ('caudally directed' connections),
from area TE to 19, area 19 to 18, and area 18 to 17, originate from neurons in layers
Vb, VI and, to a lesser extent, IIIa, and terminate mainly in layer I. In addition,
the laminar organization of several intrinsic and callosal connections are observed.
In trinsic connections within areas 18 and 19 originate from neurons in layers IIIc
and, to a lesser extent, Va, and terminate in vertical bands in layers I to IV. Callosal
connections from areas 18, 19, and the caudal inferotemporal region originate from
neurons mainly in layer IIIc. From areas 18 and 19, these callosal connections terminate
in vertical bands in layers I through IV. Thus, different cortical projection systems
are characterized by specific laminar distributions of efferent terminations as well
as of their neurons of origin.