DEVELOPMENT OF LOCAL CIRCUITS IN HUMAN VISUAL-CORTEX

How we see the world largely depends on the organization of neuronal c ircuits in visual cortex. Physiological recordings in mammals indicate that circuits develop over a period that extends well into early post natal ages (LeVay et al., 1980; Albus and Wolf, 1984). Our understandi ng of how these circuits are assembled during development is still fra gmentary (Katz and Callaway, 1992). Here we describe the development o f local connections within visual cortex, using the fluorescent dye -d ioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate to trace axonal projections in post-mortem human brains. Vertical (intracolumna r) connections between layers 2/3 and 5, which link neurons representi ng the same point in the visual field, develop prenatally at 26-29 wee ks gestation. In contrast, horizontal (intercolumnar) connections betw een different points in the visual field develop later. They first eme rge prenatally at almost-equal-to 37 weeks gestation within layers 4B and 5. After birth (>40 weeks gestation) the fiber density increases r apidly, showing a uniform plexus of connections at 7 weeks postnatal. The more adult-like patchiness of the projection, however, emerges aft er 8 weeks postnatal. Long-range horizontal connections within layer 2 /3 develop after the connections within layers 4B, 5, and 6. These con nections emerge after 16 weeks/postnatal, long after cytochrome oxidas e blobs have developed, and reach mature form sometime before 15 month s of age. Unlike the patchy horizontal projections within layers 4B an d 5, which seem to develop through a process of collateral elimination , long-range projections within layer 213 are patchy from the outset a nd seem to develop with greater topographical precision. The finding t hat intracolumnar connections develop before intercolumnar projections suggests that circuits that process local features of a visual scene develop before circuits necessary to integrate these features into a c ontinuous and coherent neural representation of an image. In addition, the sequential development of horizontal connections within layer 4B before those within layer 2/3 suggests that circuits that may be relat ed to the processing channel for visual motion develop in advance of t hose that may be more intimately related to the processing of form, co lor, and precise stereoscopic depth.