Configuration, in serial reconstruction, of individual axons projecting from area V2 to V4 in the macaque monkey

The detailed morphology of long extrinsically projecting axons in the neocortex has been difficult to investigate and is in fact poorly understood. Some data, based on extracellular injections of Phaseolus vulgaris leucoagglutinin (PHA-L), are available for individual axons projecting from area V1 to area V2 or MT. Like geniculocortical projections, axons projecting from area V1 to area MT are readily identifiable (they typically have a bistratified termination pattern and large terminal specializations and are of large caliber), but those projecting from area V1 to V2 are more variable. To provide a broader basis for interpreting constant and variable features of axon morphology, we used high-resolution serial section reconstruction to analyze small populations of PHA-L-labeled axons projecting from area V2 to V4. Reconstruction of 20 axons suggests that this system is variable in terms of overall configuration and laminar distribution. Most terminal arbors are located at the border between layers 3 and 4, but some remain entirely within layer 3 or 4, some target preferentially the superficial layers (1, 2, and 3A), and some have collaterals in layer 5 or, rarely, layer 6. Arbor size is fairly constant among the three visual cortical projections examined so far (typically about 200 microns in diameter). In area V4, however, axons frequently have three or four separate arbors, which branch divergently (in one instance, over 2.6 mm x 3.0 mm). These features may be correlated with aspects of the particular functional organization of area V4, such as coarse topography, large receptive field size, and modularity. Axonal variability may also denote differences, morphological or physiological, among neurons of origin in area V2.