Two areas located in the anterior bank of parieto-occipital sulcus (APOs), in the caudalmost part of the superior parietal lobule, have been recently defined on a functional basis: a ventral one, area V6 and a dorsal one, area V6A. Area V6 is a retinotopically organized purely visual area, whereas V6A contains non-topographically organized visual neurones and neurones sensitive to eye/arm movements, even in darkness.

The aim of this work was to define the cytoarchitectonic organization of APOs to look for possible anatomical evidence to support its functional subdivision in V6 and V6A. Experimental protocols complied with National law on the care and use of laboratory animals. The animals were killed with an overdose of Penthotal I.V. Architectonic analysis of Nissl material showed two main structural patterns: the ‘occipital’ one, located on the fundus of parieto-occipital sulcus and on the most ventral third of APOs, as rostral extension of extrastriate areas; the ‘parietal’ pattern, located on the dorsal two-thirds of APOs, as caudal part of superior parietal lobule. The ‘occipital’ cortex is characterized by a thin cortical width and a large homogeneous layer IV with densely packed granular cells; the layer III is thin, uniform and densely packed, the layer V is poorly developed and the layer VI is well defined. The ‘parietal’ cortex is characterized by an increase in thickness and a decrease in cellular density; the layer IV is subdiveded in two sublayers, a lower one with higher and an upper one with lower cellular density; layer III presents an increasing gradient in density and cellular size, layer V is thick and layer VI poorly defined. Two different cytoarchitectonic sectors can be further identified within the ‘parietal’ cortex of APOs. The ventral sector is characterized by a dense layer III and a darkly stained layer V, populated by large pyramidal cells. In contrast, the dorsal sector shows a less dense layer III and a layer V in which only isolated groups of clear pyramids are present.

The architectonic limits of all the above-described cortical subdivisions are in agreement with the electrophysiological data recorded in this region. Thus the ‘occipital’ cortex corresponds to the functionally defined area V6, whereas the ‘parietal’ cortex of the APOs corresponds to the functionally defined area V6A. The two architectonic sectors observed in area V6A indicate the presence of two subregions within this area (‘V6Av’ and ‘V6Ad’), in agreement with the observed different distribution of functional properties and anatomical connections.

This work was supported by grants from MIUR.

All procedures accord with current National guidelines.