Organization of Posterior Parietal–Frontal Connections in the Rat

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.

Reconsidering the Border between the Visual and Posterior Parietal Cortex of Mice

“…In the rat, mPPC is the only PPC subarea that receives connections from all orbitofrontal areas, MO, VO and VLO. The more lateral areas lPPC and PtP only receive projections from VLO (Olsen et al, 2019). We observed a similar topography for the new PPC subdivisions in the mouse, with the most medial area (MMA), being the only one receiving projections from MO.…”

“…This is suggestive of a circuit where area 2/ALI projects directly to MMA, or indirectly via AM, and subsequently to the Cg and frontal areas. In rat, it was illustrated that the brain areas, M2, Cg, OFC and PPC, show reciprocal connections with each other in a topographic manner (Bedwell et al, 2014;Olsen et al, 2019). This topographic organization has been extensively described for mPPC, lPPC and PtP in the rat brain, showing a medial to lateral gradient in connectivity.…”

“…We observed a similar topography for the new PPC subdivisions in the mouse, with the most medial area (MMA), being the only one receiving projections from MO. Based on primate research, it is hypothesized that the lateral half of the OFC is involved in identifying the outcome of an individual choice, while the medial OFC compares these different choices to one another, based on a common universal value, leading to influencing what action should be executed (Olsen et al, 2019;Murray, 2014, 2011;Rushworth et al, 2011). This is further confirmed in rats where MO inactivation showed an increase in choosing the riskier option (Stopper et al, 2014), a shift to reliability on the value of observable outcomes, not predicted ones (Bradfield et al, 2015), and a decrease in probabilistic discrimination (Dalton et al, 2016;Izquierdo, 2017).…”

Action representation in the mouse parieto-frontal network

“…www.nature.com/scientificreports www.nature.com/scientificreports/ Although it was long thought that rodent brains lacked the prerequisite complexity to subserve higher cognitive functions, a growing body of work shows that both rats and mice exhibit accomplished performance in sensory-motor tasks such as virtual navigation 46 and evidence-based decision making 21,50 , and they show stimulus history effects [51][52][53] . Though PPC and M2 are considerably less elaborate in rodents than primates, both in terms of relative size and the number of subfields 54 , there are several anatomical features common to both species which could support action recognition, including strong input from higher visual areas 55,56 and topographically organized reciprocal connections linking PPC with frontal motor cortices [57][58][59][60][61] . Given the anatomical and functional similarities, we reasoned that neurons in the rodent parieto-frontal circuit might exhibit mirror-like responses to the observation and execution of the same actions and were surprised by the effective absence of observational tuning in both areas.…”

Reconsidering the border between the visual and posterior parietal cortex of mice

“…leading to influencing what action should be executed (Olsen et al, 2019;Murray, 2014, 2011;Rushworth et al, 2011). This is further confirmed in rats where MO inactivation showed an increase in choosing the riskier option (Stopper et al, 2014), a shift to reliability on the value of observable outcomes, not predicted ones (Bradfield et al, 2015), and a decrease in probabilistic discrimination (Dalton et al, 2016;Izquierdo, 2017).…”