In the present study, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was injected into area V1 in order to demonstrate the detailed morphology of individual axons terminating in prestriate area MT. On the basis of 24 axon reconstructions, several representative (but not necessarily comprehensive) characteristics have been identified: (1) Most axons arborize in a patchy manner over a widespread territory, frequently greater than 1.0 mm and often up to 1.5 × 1.8 mm (dimensions uncorrected for shrinkage). (2) Terminal arbors are distributed to layers 3, 4, and 6. Those in layer 6 need not be in register with those in the upper layers. (3) Number and size of terminal arbors are variable. One axon may have 1–4 arbors in the middle layers; typically at least one of these will have a diameter of 200–250 µm, while the others may be less developed. There are from 1–3 arbors in layer 6, usually 50 µm (but sometimes up to 100 µm) in diameter. (4) Terminal boutons are of mixed morphology, but usually beaded and large (up to 3.0 µm). (5) In the white matter, many axons travel in the external sagittal stratum but some are part of the U-fiber system. Axons commonly branch, sometimes at depths up to 0.75–1.0 mm, below the gray matter of MT. In summary, these axons are not stereotyped, but rather vary in the number and size of their terminal arbors, as well as in their branching and overall geometry.

Connections from area V1 to MT have been associated with the magnocellular-dominated “processing channel.” As widespread arborizations and bistratified terminations are common to both striate axons in MT and to geniculocortical axons in layer 4Cα of primary visual cortex, these features might be correlated with magnocellular-specific processing requirements.

The present study uses the anterograde tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L), to investigate the detailed morphology of individual axons projecting from area V1 to prestriate area V2. Observations are derived from serial reconstructions of 45 axons. Axons are found to differ both in laminar distribution and in arbor size. The majority (25/45; 56%) terminate in the upper half of layer 4 and the lower part of layer 3. Terminal clusters typically measure about 200 μm in diameter (dimensions are uncorrected for shrinkage), and are either in one, two, or occasionally three patches. Patches are separated by 200−500 μm. Of these 25 axons, four also have minor collaterals to layer 5. Of the remaining 20 axons in our sample, eight have one or two terminal arbors (about 200 μm in diameter) mainly in layer 3; another eight have terminations, organized as a single field (about 350 μm in diameter), within layer 4; and four axons have much larger terminal fields (1.0−1.2 mm × 0.3 mm), in layers 3 nd 4. These morphological differences might constitute a gradient or, alternately, indicate distinct subgroups within the striate efferent population. Large terminal fields are asymmetrical, with their long axis oriented in an anterior-posterior fashion toward the depth of the lunate sulcus. Axons with two terminal arbors have a similar bias. As this arrangement is approximately perpendicular to the border of V1, we suggest that striate axons may be extended preferentially along the length of the stripelike compartments in V2. These compartments are also arrayed perpendicular to the border between areas V1 and V2. Reconstruction of small groups of 2–4 convergent axons demonstrates that axons with different morphology (i.e. large or small terminal fields) can occur within the same projection focus. Terminal arbors belonging to different axons can overlap, but tend not to be superimposed exactly.