Distribution in area 17 of neurons projecting to the pontine nuclei: a quantitative study in the cat with retrograde transport of HRP-WGA.

The number and distribution of corticopontine neurons within area 17 of the cat were studied quantitatively with the use of retrograde transport of horseradish peroxidase-wheat germ agglutinin. Eight cats received stereotactic injections in the pontine nuclei; in three of these complete staining of the parts of the pontine nuclei receiving fibers from the visual cortex was achieved. Labeled cells were counted in frontal sections through the hemisphere, spaced at 0.5 mm. The borders of area 17 were determined cyto- and myeloarchitectonically and a flat map was produced for each animal. A map of the representation of the visual field in 10 degrees X 10 degrees blocks in the first visual area (Tusa et al., '78, '81) was transferred to our maps of area 17. The density and number of labeled corticopontine cells could then be determined within blocks of the cortex representing 10 degrees X 10 degrees of the visual field. The cell density (number of labeled cells per mm2 cortex) was found in general to be highest in parts of the cortex representing peripheral parts of the visual field. The cell density is low in cortex representing the central visual field, but the lowest density was found in the representation of a para-central region in the upper visual field. Furthermore, cortical regions representing the lower part of the visual field have a higher cell density than those representing the upper part; in four cases, 68-86% of all labeled cells were found in parts of area 17 representing the visual field below the horizontal meridian. Since there is an enlarged cortical representation of central vision, the much lower cell densities in "central" parts of area 17 than in "peripheral" parts may mean that all parts of the visual field are represented with equal numbers of corticopontine neurons ("linear" representation). This is not the case, however, since the number of labeled cells per 10 degrees X 10 degrees is considerably higher in the cortex representing the central 10 degrees and medial parts of the lower visual field than in the rest of area 17. Assuming that the corticopontine cells in the visual cortex transmit spatially relevant information, we conclude that there is an overrepresentation of central vision and the medial parts of the lower visual field in the corticopontine projection from area 17.