Rat posterior parietal cortex: topography of corticocortical and thalamic connections

Abstract

Anatomical and functional findings support the contention that there is a distinct posterior parietal cortical area (PPC) in the rat, situated between the rostrally adjacent hindlimb sensorimotor area and the caudally adjacent secondary visual areas. The PPC is distinguished from these areas by receiving thalamic afferents from the lateral dorsal (LD), lateral posterior (LP), and posterior (Po) nuclei, in the absence of input from the ventrobasal complex (VB) or dorsal lateral geniculate (DLG) nuclei. Behavioral studies have demonstrated that PPC is involved in spatial orientation and directed attention. In the present study we used fluorescent retrograde axonal tracers primarily to investigate the cortical connections of PPC, in order to determine the organization of the circuitry by which PPC is likely to participate in these functions, and also to determine how the topography of its thalamic connections differs from that of neighboring cortical areas. The cortical connections of PPC involve the ventrolateral (VLO) and medial (MO) orbital areas, medial agranular cortex (area Fr2), portions of somatic sensory areas Par1 and Par2, secondary visual areas Oc2M and Oc2L, auditory area Te1, and retrosplenial cortex. The secondary visual areas Oc2L and Oc2M have cortical connections which are similar to those of PPC, but are restricted within orbital cortex to area VLO, and within area Fr2 to its caudal portion, and do not involve auditory area Te1. The cortical connections of hindlimb cortex are largely restricted to somatic sensory and motor areas. Retrosplenial cortex, which is medially adjacent to PPC, has cortical connections that are prominent with visual cortex, do not involve somatic sensory or auditory cortex, and include the presubiculum. We conclude that PPC is distinguished by its pattern of cortical connections with the somatic sensory, auditory and visual areas, and with areas Fr2, and VLO/MO, in addition to its exclusive thalamic connectivity with LD, LP and Po. Because recent behavioral studies indicate that PPC, Fr2 and VLO are involved in directed attention and spatial learning, we suggest that the interconnections among these three cortical areas represent a major component of the circuitry for these functions in rats.

Navigating from hippocampus to parietal cortex

“…This combination of thalamic inputs thus provides useful borders between PPC, somatosensory cortex, and visual cortex, respectively. These borders coincide with both cyto-and myeloarchitectonic criteria (76,100,108), as well as with changes in a number of other histochemical markers (108).…”

“…Projections target the orbitofrontal cortex, the prelimbic/infralimbic domain, and the dorsally adjacent part of the anterior cingulate cortex (68,(72)(73)(74)(75). These regions have strong connections with the PPC in both rats and primates (33,57,(76)(77)(78). Since the initial report showing impaired initial acquisition of place navigation in a water-maze after aspiration lesions of the medial prefrontal cortex (79), the results of a large number of experiments have supported the conclusion that hippocampal-prefrontal connections are important for certain spatial behaviors.…”

“…The PPC is also strongly and reciprocally connected to almost all of the other multimodal association cortices, such as (pre)frontal, temporal, and limbic association cortex (see main text for details). It further is connected with motor and premotor areas of the cortex (33,34,76,96). A final feature which helps to differentiate the PPC from the adjacent somatosensory cortex is that only the former exhibits strong callosal connections (96,113).…”

“…76 and 89). In any case, signals from retrosplenial cortex are likely to reach PPC indirectly via strong connections to the medial frontal cortex, in particular, the anterior dorsal cingulate cortex (90), which projects in turn to the PPC (57,76). The potential significance of these pathways is underscored by the observation that, unlike postrhinal and prefrontal lesions, damage to the retrosplenial cortex causes strong and lasting disruption of spatial navigation, both in the water maze (81,(91)(92)(93) and in a path-integration task testing the ability to return directly to a home base in darkness (28).…”

“…Thus, a number of hippocampal-entorhinal output pathways could be involved in transferring information about the animal's current location to the PPC and further on to the motor-planning system (94,95), using connections from PPC to premotor and motor domains (33,34,76,96). The relative contributions of the various pathways remain uncertain.…”