The present study characterized the anatomical and functional organization of human lateral parietal cortex using non-invasive MRI techniques and compared it to the organization of the macaque parietal cortex. First, we used diffusion-tractography to parcellate the human parietal cortex into ten subregions. Then resting state fMRI was used to examine the functional connectivity patterns of these subregions. Finally, comparing the resting state fMRI data in humans and macaques revealed fundamental similarities in lateral parietal organization and demonstrated that the resting state functional connectivity of lateral parietal cortex reflected major known anatomical connections. There are fundamental similarities in lateral parietal organization in humans and macaques but two features, however, distinguished human parietal cortex. An anterior IPL region active during tool observation was identified (red cluster in ) and a central IPL region was characterized by strong interactions withaPFC.
Diffusion-tractography approaches have identified component areas within the pre-SMA/SMA region (
Johansen-Berg et al., 2004), lateral premotor and ventrolateral prefrontal cortex (
Tomassini et al., 2007;
Klein et al., 2007;
Anwander et al., 2007;
Schubotz et al., 2010), and cingulate cortex (
Beckmann et al., 2009), but this is the first time such a large expanse of cortex has been parcellated. Previous attempts to establish relationships between human and macaque parietal cortex have focused on retinotopic mapping, identification of similarities in functional activation profiles, and post-mortem cytoarchitectonic analysis (). Because it has different strengths and weaknesses to these methods the diffusion-tractography parcellation and functional connectivity approach provides independent and convergent evidence about the key component subdivisions of human parietal cortex.
| Table 1Correspondences in parietal terminology between the current study and previously published nomenclature. DWI-defined clusters are from the current study and the colors listed refer to those shown in figs. and . Cytoarchitectonic (more ...) |
The IPL parcellation yielded five clusters and in most cases it was possible to identify their relationships with cytoartchitectonically defined areas PFop, PFm, PGa, and PGp (
Caspers et al., 2008). One cluster (blue in ), however, corresponded to several cytoarchitectonically defined areas, PF, PFt, and hIP2 (
Caspers et al., 2006;
Choi et al., 2006), which were presumably grouped together because of similarities in their connections. The differences in anatomical connections that gave rise to parcellation should mean that each IPL cluster differs in the access it has to the information represented in other cortical areas. Activity related to grasping, effecting strategy and response changes, attentional shifts, and memory retrieval were preferentially associated with different clusters (). Notably the location of the most anterior IPL cluster was similar to that of activity recorded during tool use observation that has only been observed in human IPL (
Peeters et al., 2009).
It has been claimed that human IPL may contain unique regions not found in other primates (
Husain and Nachev, 2007;
Peeters et al., 2009). Resting state functional connectivity, however, in the posterior and anterior IPL, with PH, dlPFC, and PMv respectively, was similar in both humans and macaques () and probably reflects similar anatomical connections in both species. By contrast the human central IPL region was notable in that, unlike macaque central IPL, its activity was correlated with that of aPFC. It is in this region of the IPL that calculation and language related activity is found (
Simon et al., 2002;
Simon et al., 2004) and activity changes occur as people weigh up and change response strategy (
Daw et al., 2006;
Boorman et al., 2009). The aPFC region is close to the frontal pole where activity has been related to the highest levels of cognitive control when “branching” between different sub-tasks is required (
Koechlin et al., 1999). The two areas are often co-active (
Daw et al., 2006;
Boorman et al., 2009). There is evidence that the frontal pole is especially large in humans (
Semendeferi et al., 2001) and that its increase in size is due to its lateral expansion, in hominoids, into the approximate region we found to be connected to mid-IPL. Consistent with our results,
Petrides and Pandya (2007) have reported no connections between frontal polar area 10 and parietal cortex in the macaque although
Rozzi et al. (2006) note some between mid-IPL and anterior lateral prefrontal cortex. Brain differences emerge during speciation in a number of ways including as a result of new connections invading an area or as specialized divisions of an area becoming spatially separate (
Krubitzer, 2007;
Krubitzer, 1995) and such changes may underlie the present findings. The central IPL region is an important one to be considered in any attempt to account for the distinctive expansion of parietal cortex seen in humans even in comparison to other hominids (
Bruner, 2010).
Parcellation of SPL and posterior IPS resulted in five clusters. Once again, it was possible to identify correspondences between the clusters and cytoarchitectonically defined regions () including 7PC, 5L, hIP3, and 7A (
Scheperjans et al., 2008a). Retinotopic and other mapping procedures have been used to identify sensorimotor regions in IPS and SPL; the diffusion-tractography approach provides convergent evidence for the anatomical separation of many of these areas including VIP (
Sereno and Huang, 2006) (
Bremmer et al., 2001), also referred to as IPS5 (
Konen and Kastner, 2008), IPS3, and IPS4 (
Sereno et al., 2001;
Swisher et al., 2007). One ventromedial posterior IPS cluster (yellow, ), however, covered a region associated with two retinotopic maps, IPS1 and IPS2 (
Silver et al., 2005), which have activity related to visuospatial attention and oculomotor intention (
Schluppeck et al., 2006;
Schluppeck et al., 2005). IPS1, IPS2, and IPS3 have all been proposed as possible homologues of macaque LIP. The assignment of IPS1 and IPS2 to one cluster and the assignment of IPS3 to another cluster suggests that IPS1 and IPS2 share a number of connections and predicts that their functions may be especially similar. There is some evidence that both IPS1 and IPS2 are distinguished by the lateralization of their activity (
Szczepanski et al., 2010). A modern, detailed study of the cytoartchitecture of this region has yet to be published but again the diffusion-tractography results suggest that it may differ from the adjacent 7A region that encompasses much of IPS3.
The patterns of functional connectivity associated with the different SPL/IPS parietal clusters clarify the relationship between the parietal cortex in the human and other primates and are suggestive of function. Macaque VIP is known to be connected with V5, PMd, PMv, and FEF (
Luppino et al., 1999;
Lewis and Van Essen, 2000) and the anterior lateral SPL/IPS putative 7PC/VIP/IPS5 region (red, ) was found to be functionally connected with the same regions in the present study (). The anterior medial, putative 5L, region (blue, ) interacted strongly with PMd. Connections between area 5 and PMd have also been documented in the macaque (
Johnson et al., 1996;
Caminiti et al., 1996;
Matelli et al., 1998). The mid lateral SPL/IPS region near the IPS4 map (green, ) was found to be functionally connected to PMd, particularly inferior PMd, in a manner reminiscent of the connections known to exist between MIP and inferior PMd in macaque (
Matelli et al., 1998). As in macaque MIP such a pattern would be expected if the region were to be involved in the directional control of movement and complements evidence that a similar region is active during visual guided hand movements even in the absence of eye movements (
Grefkes et al., 2004;
Rushworth et al., 2001;
Hinkley et al., 2009). The functional connectivity between the most posterior IPS region (yellow, ), near IPS1 and IPS2 maps, and V5 suggested it resembles the ventral division of macaque LIP, LIPv (
Lewis and Van Essen, 2000). There was evidence of functional connectivity between the fourth, posterior lateral SPL/IPS cluster (magenta, IPS3 location) and FEF suggesting it also bore resemblances to LIP. Human SPL/IPS organization differed from macaque parietal organization in that the areas engaged in functional interactions with both FEF and V5, important interactions for any area concerned with visuospatial attention, were most prominent on the medial rather than the lateral bank of the IPS (
Grefkes and Fink, 2005).
In summary, we show the feasibility of using non-invasive MRI methods to establish the anatomical and functional subregions of the human parietal cortex and to compare these with the macaque. Although only tracer injection studies in animal models provide definitive evidence for existence of synaptic connections between brain regions comparative MRI-based studies are important because they facilitate translation to the human brain. Similarly, while studies such as the present one cannot replace detailed cytoarchitectonic descriptions of brain areas, they indicate ways of exploiting such descriptions in
in vivo investigations of human brain anatomy and function both in health and in illness. The possibility that functional connectivity is mediated by polysynaptic pathways cannot be ruled out. Nevertheless, similar, previous functional connectivity studies have recently identified correspondences between medial parietal and adjacent posterior cingulate areas in humans and macaques (
Margulies et al., 2009). The qualitative similarity in human and macaque IPL is notable, suggesting the neurophysiology and anatomy of macaque IPL is likely to be an important guide for understanding human IPL (
Caminiti et al., 2010). Nevertheless within this framework, human specializations, especially interactions between mid-IPL and aPFC, can be discerned.
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