The first hypothesis predicts a relative shift in the topographical position of homologous areas to accommodate the presence of new areas in humans the second hypothesis predicts a good spatial registration of homologous areas in the two species. Alternatively, human and macaque PPC may share a similar architectural plan, but human areas may be relatively enlarged ( Van Essen et al., 2001). This difference may be related to a relative expansion of the human inferior parietal lobule and to development of new cortical fields (e.g., Brodmann 39, 40) ( Brodmann, 1905 Eidelberg and Galaburda, 1984).

However, there is indirect evidence that the PPC in humans is preferentially expanded compared with monkeys, given that neighboring occipital visual areas in the two species have different geographic locations V1 and V2 extend more laterally, and MT is positioned more dorsally in macaques compared with humans ( Van Essen et al., 2001). The parietal lobe, as defined by conventional geographic landmarks, occupies approximately one-fifth of the total neocortex in both humans and monkeys ( Van Essen and Drury, 1997). The functional organization of human posterior parietal cortex (PPC) and its relationship to the PPC in the macaque monkey are primarily unknown. In contrast, large interspecies differences were noted in the topography of frontal areas. Surface-based registration of macaque cortical areas onto the map of fMRI responses suggests a relatively good spatial correspondence between human and macaque parietal areas. These regions were lateralized to the left hemisphere, activated by preparation of movements of either hand, and included the inferior and superior parietal lobule, precuneus, and posterior superior temporal sulcus, plus the dorsal premotor and anterior cingulate cortex anteriorly. However, the preparation of a pointing movement selectively activated a different group of regions, suggesting a stronger role in motor planning.

The human frontal eye field and two separate regions in the intraparietal sulcus were similarly recruited in all conditions, suggesting an attentional role that generalizes across response effectors. We studied the functional organization of human posterior parietal and frontal cortex using functional magnetic resonance imaging (fMRI) to map preparatory signals for attending, looking, and pointing to a peripheral visual location.

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