Organization of efferent neurons in area 19: the projection to extrastriate area 21a

The organization of efferent neurons in area 19 of the cat was examined by bulk injections of retrograde tracers, WGA-HRP and CTX-Au, into extrastriat e area 21a. In one case, the cortex was cut coronally and retrogradely labe led cells in area 19 were present in columnar register throughout layers 2 to 6, with the majority of labeled cells in layers 2/3. The number of colum ns per tissue section ranged from 0 to 4 and had a centre-to-centre spacing ranging from 0.6 to 0.9 mm. A few lightly labeled cells were found between the columns. In six other cases, the visual cortex was flattened, and cut in the tangential plane to reveal a pattern of irregular, widely spaced ban ds that were elongated in the mediolateral direction with a mean centre-to- centre spacing of 2.6 mm. The density of labeled cells within these bands f luctuated such that dense aggregates of cells were found, on average, at 0. 9 mm intervals along the bands. This tangential heterogeneity in density, a long with the patchy columnar staining witnessed in the coronal plane, sugg ests that the widely spaced efferent projection bands may have a patchy sub structure with a spacing of approximately 1 mm. The pattern of efferent pro jection bands and its substructure in area 19 is reminiscent of the stripe- like organization of V2 found in primates. (C) 2000 Elsevier Science B.V. A ll rights reserved.