Convergence and branching patterns of round, type 2 corticopulvinar axons

Corticopulvinar connections consist of at least two morphologically distinct subpopulations. In one subgroup (E, type 1), axons have an "elongated" terminal field and thin, spinous terminations; in the other (R, type 2), axons have a small, round arbor and large, beaded terminations. Previous work (Rockland, 1996) indicates that E-type axons from several occipitotemporal areas branch extensively within and sometimes between pulvinar subdivisions, but that R-type axons tend to have spatially delimited arbors. The present report is a further investigation of R-type axons from areas V1 and MT and was initiated to test the generality of the previous findings. There are four main results: 1) By serial section reconstruction of anterogradely labeled axons, 10 of 25 axons originating in area V1 had two or three spatially separate arbors (8 and 2 axons, respectively). Sixteen axons analyzed from area MT, however, all had single arbors, although the arbors were often formed by the convergence of widely separate branches. 2) Multiple (at least 2-5) R-type corticopulvinar axons, from V1 or from MT, can converge in a single focus. 3) R-type axons originating from both areas V1 and MT can branch to other structures; namely, the superior colliculus, the pretectal area, and/or the reticular nucleus of the thalamus. 4) Finally, corticopulvinar terminations from area V1 are predominantly R-type, whereas those from MT are more predominantly E-type. These results thus provide additional evidence of the special relationship of area V1 to the pulvinar. They also emphasize that the idea of corticopulvinocortical "feedback loops," although convenient as a shorthand nomenclature, does not adequately convey the full complexity of the system.