Equivalence of parieto-preoccipital subareas for visuospatial ability in monkeys

Parieto-preoccipital lesions in monkeys produce a variety of behavioral deficits, many of which can be classified as either visuospatial or tactual. Since the lesions typically invade a number of cytoarchitectonic areas, the diverse behavioral effects could be a consequence of damage of different functional systems. To determine whether visuospatial ability depends critically on particular parieto-preoccipital -- or on a particular combination of them, monkeys with lesions of one, two, or all three of these sectors were tested both on the "landmark' task, a visual distance discrimination, and, for comparison, on a visual pattern discrimination. Impairment was found only on the landmark task, and the severity of the impairment depended on the number of sectors included in the removal, completely independent of their locus. In an attempt to integrate these results with current neurobiological data, we propose that the parieto-preoccipital region consists of two subdivisions that are organized hierarchically for the mediation of spatial perception. The lower-order subdivision, composed largely of the modality-specific preoccipital area, dorsal OA, is postulated to serve visuospatial processes selectively. The higher-order subdivision, composed mainly of the poly-sensory parietal area, PG, is postulated to serve a supramodal spatial ability to which both the visual and tactual modalities contribute.