Abstract
A central feature of theories of spatial navigation involves the representation of spatial relationships between objects in complex environments. The parietal cortex has long been linked to the processing of spatial visual information and recent evidence from single unit recording in rodents suggests a role for this region in encoding egocentric and world-centered frames. The rat parietal cortex can be subdivided into four distinct rostral-caudal and medial-lateral regions, which includes a zone previously characterized as secondary visual cortex. At present, very little is known regarding the relative connectivity of these parietal subdivisions. Thus, we set out to map the connectivity of the entire anterior-posterior and medial-lateral span of this region. To do this we used anterograde and retrograde tracers in conjunction with open source neuronal segmentation and tracer detection tools to generate whole brain connectivity maps of parietal inputs and outputs. Our present results show that inputs to the parietal cortex varied significantly along the medial-lateral, but not the rostral-caudal axis. Specifically, retrosplenial connectivity is greater medially, but connectivity with visual cortex, though generally sparse, is more significant laterally. Finally, based on connection density, the connectivity between parietal cortex and hippocampus is indirect and likely achieved largely via dysgranular retrosplenial cortex. Thus, similar to primates, the parietal cortex of rats exhibits a difference in connectivity along the medial-lateral axis, which may represent functionally distinct areas.
Highlights
The ability to find our way through complex environments and interact with them is generally thought to involve the use of multiple stimulus sources and frames of reference (O’Keefe and Nadel, 1978; Gallistel, 1990; McNaughton et al, 1991)
LOW-DENSITY PRIMARY VISUAL CORTEX PROJECTIONS TO PARIETAL CORTEX Whole brain cortical flat maps showing retrograde tracer filled cells were generated for each brain that was sectioned in the coronal plane (n = 10)
We found that parietal cortex (PC) outputs were similar to cortical inputs with a majority of the projections terminating in dorsal retrosplenial cortex and lower density projections to other PC subregions (e.g., LPtA to V2ML), cingulate cortex (Cg1), temporal cortex (TE), lateral secondary visual cortex (V2L) and primary visual cortex (V1)
Summary
The ability to find our way through complex environments and interact with them is generally thought to involve the use of multiple stimulus sources and frames of reference (O’Keefe and Nadel, 1978; Gallistel, 1990; McNaughton et al, 1991). Movements that are based on environmental cues have been characterized as involving an initial egocentric mapping of the perceived location and orientation of objects relative to oneself, and a subsequent remapping to a world-centered (i.e., allocentric) framework, allowing a subject to act upon objects, or move to particular goals in relation to them This allocentric frame of reference is a central feature of “cognitive mapping” theories (O’Keefe and Nadel, 1978) based in large part on the finding that hippocampal neurons form a population code of spatial location in an environment (McNaughton et al, 2006; Moser et al, 2008). Other studies have identified a role for the rodent PC region in processing route-centric information such as the progress made along a path in a complex maze (Nitz, 2006, 2012), and the anticipation of movements (Whitlock et al, 2012), a prominent feature in cells that have conjunctive responses for egocentric and allocentric information (Wilber et al, 2014)
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