Uniformity and Diversity of Cortical Projections to Precuneate Areas in the Macaque Monkey: What Defines Area PGm?

We report on the corticocortical connections of areas on the mesial surface of the macaque posterior parietal cortex, based on 10 retrograde tracer injections targeting different parts of the precuneate gyrus. Analysis of afferent connections supported the existence of two areas: PGm (also known as 7m) and area 31. Both areas received major afferents from the V6A complex and from the external subdivision of area 23, but they differed in most other aspects. Area 31 showed greater emphasis on connections with premotor and parietal sensorimotor areas, whereas PGm received a greater proportion of its afferents from visuomotor structures involved in spatial cognition (including the lateral intraparietal cortex, inferior parietal lobule, and the putative visual areas in the ventral part of the precuneus). Medially, the anterior cingulate cortex (area 24) preferentially targeted area 31, whereas retrosplenial areas preferentially targeted PGm. These results indicate that earlier views on the connections of PGm were based on tracer injections that included parts of adjacent areas (including area 31), and prompt a reassessment of the limits of PGm. Our findings are compatible with a primary role of PGm in visuospatial cognition (including navigation), while supporting a role for area 31 in sensorimotor planning and coordination.