The connections of layer 4 subdivisions in the primary visual cortex (V1) of the owl monkey

The primary visual cortex (V1) of primates receives signals from parallel l ateral geniculate nucleus (LGN) channels. These signals are utilized by the laminar and compartmental [i.e. cytochrome oxidase (CO) blob and interblob ] circuitry of V1 to synthesize new output pathways appropriate for the nex t steps of analysis. Within this framework, this study had two objectives: (i) to analyze the connections between primary input and output layers and compartments of V1; and (ii) to determine differences in connection pattern s that might be related to species differences in physiological properties in an effort to link specific pathways to visual functions. In this study w e examined the intrinsic interlaminar connections of V1 in the owl monkey, a nocturnal New World monkey, with a special emphasis on the projections fr om layer 4 to layer 3. Interlaminar connections were labeled via small iont ophoretic or pressure injections of tracers [horseradish peroxidase. biocyt in, biotinylated dextrine amine (BDA) or cholera toxin subunit B conjugated to colloidal gold particles]. Our most significant finding was that layer 4 (4C of Brodmann) can be divided into three tiers based upon projections t o the superficial layers. Specifically, we find that 4 alpha (4C alpha), 4 beta (4C beta) and 4ctr send primary projections to layers 3C (4B), 3B beta (4A) and 3B alpha (3B), respectively. Examination of laminar structure wit h Nissl staining supports a tripartite organization of layer 4. The cortica l output layer above layer 3B alpha (3B) (e.g. layer 3A) does not appear to receive any direct connections from layer 4 but receives heavy input from layers 3B alpha (3B) and 3C (48). Some connectional differences also were o bserved between the subdivisions of layer 3 and the infragranular layers. N o consistent differences in connections were observed that distinguished CO blobs from interblobs or that could be correlated with differences in visu al lifestyle (nocturnal versus diurnal) when compared with connectional dat a in other primates. Re-examination of data from previous studies in squirr el and macaque monkeys suggests that the tripartite organization of layer 4 and the unique projection pattern of layer 4ctr are not restricted to owl monkeys, but are common to a number of primate species.