Abstract
Recent investigations of the rat posterior parietal cortex (PPC) suggest that this region plays a central role in action control together with the frontal cortical areas. Posterior parietal-frontal cortical connections have been described in rats, but little is known about whether these connections are topographically organized as in the primate. Here, we injected retrograde and anterograde tracers into subdivisions of PPC as well as the frontal midline and orbital cortical areas to explore possible topographies within their connections. We found that PPC projects to several frontal cortical areas, largely reciprocating the densest input received from the same areas. All PPC subdivisions are strongly connected with the secondary motor cortex (M2) in a topographically organized manner. The medial subdivision (medial posterior parietal cortex, mPPC) has a dense reciprocal connection with the most caudal portion of M2 (cM2), whereas the lateral subdivision (lateral posterior parietal cortex, lPPC) and the caudolateral subdivision (PtP) are reciprocally connected with the intermediate rostrocaudal portion of M2 (iM2). Sparser reciprocal connections were seen with anterior cingulate area 24b. mPPC connects with rostral, and lPPC and PtP connect with caudal parts of 24b, respectively. There are virtually no connections with area 24a, nor with prelimbic or infralimbic cortex. PPC and orbitofrontal cortices are also connected, showing a gradient such that mPPC entertains reciprocal connections mainly with the ventral orbitofrontal cortex (OFC), whereas lPPC and PtP are preferentially connected with medial and central portions of ventrolateral OFC, respectively. Our results thus indicate that the connections of PPC with frontal cortices are organized in a topographical fashion, supporting functional heterogeneity within PPC and frontal cortices.
Highlights
The posterior parietal cortex (PPC) is a multimodal association area, proposed to play a role in a variety of higher cognitive functions
At the level of the forceps minor of the corpus callosum, the anterior cingulate cortical (ACC) area 24b is wedged between prelimbic cortex (PL) and rostral secondary motor cortex (rM2), and infralimbic cortex (IL) is seen ventral to PL (Figure 2B)
On the basis of differences in connectivity, secondary motor cortex (M2) has been divided into three parts along its rostrocaudal axis, the rostral part being situated rostral to the genu of the corpus callosum, the intermediate part extending from the genu to the anterior commissure, and the caudal M2 that extends from the anterior commissure until it is replaced caudally by the medial secondary visual cortex (Reep et al, 1990; Olsen and Witter, 2016)
Summary
The posterior parietal cortex (PPC) is a multimodal association area, proposed to play a role in a variety of higher cognitive functions. Many functional studies of PPC have focused on its role in spatial navigation (Kolb and Walkey, 1987; Chen et al, 1994a,b; Save and Moghaddam, 1996; Save and Poucet, 2000; Save et al, 2005; Nitz, 2006, 2012). We defined PPC in the rat on the basis of a combination of cyto- and chemo-architectonic criteria and patterns of thalamic connectivity. This resulted in a reliable subdivision into three domains: a medial (mPPC), lateral (lPPC) and caudolateral (PtP) subdivision (Olsen and Witter, 2016). The position and definition of the main borders of PPC with its neighbors, the visual, somatosensory, and motor cortices and the subdivisions used here are comparable (Hovde et al, 2018), some authors differentiate nearby subareas in mice based on projections from primary visual cortex and their specific visual properties (areas RL, A, AM, Wang and Burkhalter, 2007; Wang et al, 2011)
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