Retinal influences specify cortico‐cortical maps by postnatal day six in rats and mice

Studies of callosal projections in striate cortex show that the retina is involved in the development of topographical connections. In normal animals callosal fibers connect retinotopically corresponding, nonmirror‐symmetric cortical loci, whereas in animals bilaterally enucleated at birth, callosal fibers connect topographically mismatched, mirror‐symmetric loci. Moreover, in rodents the overall pattern of visual callosal connections is adult‐like by postnatal day 12 (P12). In this study we delayed the onset of retinal deafferentation in rats and mice in order to determine the period when retinal influences are critically needed for the development of retinotopically matched callosal linkages. Callosal maps were revealed by placing small injections of retrogradely and anterogradely transported tracers into different loci of lateral striate cortex. We found that the patterns of callosal linkages in rats enucleated at P12, P8, and P6 were nonmirror‐symmetric, as in normally reared rats. In contrast, the patterns of linkages in rats enucleated at P4 closely resembled the mirror‐symmetric pattern seen in rats enucleated at birth (P0). A similar reversal in topography (from symmetric to nonsymmetric) occurred in mice when enucleation was delayed from P4 to P6. These findings indicate that retinal input prior to P6, but not prior to P4, is sufficient for specifying normal callosal topography. Moreover, they suggest that development of retinotopically matched callosal linkages depends critically on retinal influences during a brief period between P4 and P6, when callosal connections are still very immature. J. Comp. Neurol. 459:156–172, 2003. © 2003 Wiley‐Liss, Inc.