Journal of Neuroscience, Vol 13, 1916-1931, Copyright © 1993 by Society for Neuroscience
Development of local circuits in human visual cortex
A Burkhalter, KL Bernardo and V Charles
Department of Neurology and Neurological Surgery, Washington University School of Medicine, St. Louis, Missouri 63110.
How we see the world largely depends on the organization of neuronal
circuits in visual cortex. Physiological recordings in mammals indicate
that circuits develop over a period that extends well into early postnatal
ages (LeVay et al., 1980; Albus and Wolf, 1984). Our understanding of how
these circuits are assembled during development is still fragmentary (Katz
and Callaway, 1992). Here we describe the development of local connections
within visual cortex, using the fluorescent dye
1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate to trace
axonal projections in post-mortem human brains. Vertical (intracolumnar)
connections between layers 2/3 and 5, which link neurons representing the
same point in the visual field, develop prenatally at 26-29 weeks
gestation. In contrast, horizontal (intercolumnar) connections between
different points in the visual field develop later. They first emerge
prenatally at approximately 37 weeks gestation within layers 4B and 5.
After birth (> 40 weeks gestation) the fiber density increases rapidly,
showing a uniform plexus of connections at 7 weeks postnatal. The more
adult-like patchiness of the projection, however, emerges after 8 weeks
postnatal. Long-range horizontal connections within layer 2/3 develop after
the connections within layers 4B, 5, and 6. These connections emerge after
16 weeks postnatal, long after cytochrome oxidase blobs have developed, and
reach mature from sometime before 15 months of age. Unlike the patchy
horizontal projections within layers 4B and 5, which seem to develop
through a process of collateral elimination, long-range projections within
layer 2/3 are patchy from the outset and seem to develop with greater
topographical precision. The finding that 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 continuous 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 related to the processing channel for visual motion develop in
advance of those that may be more intimately related to the processing of
form, color, and precise stereoscopic depth.
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