Organization of cortical afferent and efferent pathways in the white matter of the rat visual system.

Fibers forming reciprocal connections between the dorsal lateral geniculate nucleus and primary visual cortex run in separate tracts in the white matter. The corticofugal fibers are organized into bundles which project through the coronal plane at an oblique angle. We have examined the organization of corticofugal fiber tracts within the white matter of the rat using cortical slices which encompassed the predicted trajectory of these fiber bundles. Field potentials were evoked in layer VI of visual cortex by focal stimulation of subcortical white matter using microbipolar electrodes. Two major responses were elicited: a short-latency and a longer-latency response. The short-latency response was elicited in the superficial strata of white matter with proximal stimulation sites and was obtained in deeper strata for more distant, lateral sites. The longer-latency response was associated with superficial strata in white matter at both proximal and distant stimulation sites. Based on the electrophysiological properties and the white matter location for eliciting these responses, it is likely that the short-latency response is due to antidromic activation of corticogeniculate fibers, whereas the longer-latency response probably arises from orthodromic activation of geniculocortical fibers. These findings provide an electrophysiological demonstration that cortical afferent and efferent pathways are segregated within the white matter and that they can be selectively activated by focal stimulation. The fact that the fiber bundle model successfully predicted the trajectory of corticofugal fibers provides additional support for this model of white matter organization. A double labeling technique which combined orthograde axonal transport and neuronal degeneration was used to examine the topographic arrangement of corticofugal fibers in the white matter.(ABSTRACT TRUNCATED AT 250 WORDS)