Divergence of single axons in afferent projections to the cat's visual cortical areas 17, 18, and 19: A parametric study

The proportions of neurons projecting via axon collaterals to two areas in the cat's occipital cortex (diverging neurons) were determined quantitatively in subcortical and cortical afferents by making use of the retrograde axonal transport of two different tracers. The proportions of diverging neurons were determined for that part of the afferent sites in which neurons filled with tracers from both injected areas occurred (overlap zone). A number of experimental variables were tested for their role in possibly influencing the results of quantitative double‐label experiments, among them the types and the combinations of retrograde tracers, the position of the injections, the survival time, and the histological procedure. The most important variable was the position of the cortical injection, which had to be restricted clearly to the cortical grey matter and to one cortical area in order to avoid false‐positive double labeling. Other experimental variables affected the total number of retrogradely labeled neurons and/or the ratio between neurons labeled with the two different tracers rather than the proportions of double‐labeled neurons. In particular DL proportions were largely independent of the number and density of labeled neurons. They only deviated significantly from mean values in those sections in which the number of labeled neurons amounted to less than 20% of the maximal number of labeled neurons found in one section throughout the overlap zone. Our results show that divergence is common in afferents to the cat visual cortex. The amount of divergence, however, varies considerably according to the origin of the afferent projection. The proportion of diverging neurons expressed as the percentage of the total number of neurons projecting to areas 17 and 18 was 3% in the A‐laminae of the dorsal part of the lateral geniculate nucleus, about 8% in the posteromedial lateral suprasylvian area, and about 15% in the C‐laminae of the dorsal part of the lateral geniculate nucleus, in the medial interlaminar nucleus, in the lateral part of the lateral posterior nucleus, and in the claustrum. The proportions of diverging neurons in the afferent projections to areas 17 and 19, and to areas 18 and 19 were about 10%. Diverging neurons were also found in the projections of the intralaminar thalamic nuclei to the visual cortex. The findings demonstrate that the primary thalamocortical projection in the cat's central visual system, from the A‐laminae of the dorsal part of the lateral geniculate nucleus to area 17, contains a negligible proportion of diverging neurons, whereas other afferent subcortical and cortical projections may distribute up to 15% of their total information to at least two target areas in the occipital cortex.