Efferent systems of the rabbit visual cortex: laminar distribution of the cells of origin, axonal conduction velocities, and identification of axonal branches.

Several efferent systems of visual area I in Dutch rabbits were studied with anatomical (horseradish peroxidase) and physiological (antidromic) methods. Anatomical studies provided information regarding the laminar origin of the projections to the contralateral hemisphere, visual area II, the dorsal lateral geniculate nucleus, and the superior colliculus. Physiological studies provided information regarding conduction velocities and multiple destinations of efferent axons. Both the callosal projection and the projection to V-II were shown to originate primarily in layer II-III. However, approximately 10-20% of the callosal projection and 20-40% of the projection to V-II originated in layers IV and V. In contrast, the projection to the dorsal lateral geniculate nucleus originated nearly exclusively in layer VI, while corticotectal neurons occurred primarily in layer V. A significant number of corticotectal neurons were, however, found in layer IV. Thus, the above efferent systems were found to differ in their principal laminar origin and in the diffuseness of that origin. The origins of corticocortical projections were considerably more diffuse than those of corticofugal projections. In addition to differences in laminar origin, efferent systems also differed significantly in the conduction velocities of their axons. The projection to visual area II and to the lateral geniculate nucleus consisted primarily of very slowly conducting axons, while the projection to the superior colliculus was fast conduction. The callosal projection consisted of both slow and fact conduction axons. Finally, the question of branching of V-I efferent axons was addressed. Although the laminar origin of the projections to the contralateral hemisphere and to visual area II overlapped considerably, none of these corticocortical axons could be shown to project to both locations or to a subcortical destination. In contrast, approximately one-third of corticotectal axons were shown to project a collateral into the thalamus. Although the destination of this collateral is unclear, it is medial to the lateral geniculate nucleus and may be the pulvinar.