RAT POSTERIOR PARIETAL CORTEX - TOPOGRAPHY OF CORTICOCORTICAL AND THALAMIC CONNECTIONS

Anatomical and functional findings support the contention that there i s a distinct posterior parietal cortical area (PPC) in the rat, situat ed between the rostrally adjacent hindlimb sensorimotor area and the c audally 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 genicula te (DLG) nuclei. Behavioral studies have demonstrated that PPC is invo lved in spatial orientation and directed attention. In the present stu dy we used fluorescent retrograde axonal tracers primarily to investig ate the cortical connections of PPC, in order to determine the organiz ation of the circuitry by which PPC is likely to participate in these functions, and also to determine how the topography of its thalamic co nnections differs from that of neighboring cortical areas. The cortica l connections of PPC involve the ventrolateral (VLO) and medial (MO) o rbital areas, medial agranular cortex (area Fr2), portions of somatic sensory areas Par1 and Par2, secondary visual areas Oc2M and Oc2L, aud itory area Te1, and retrosplenial cortex. The secondary visual areas O c2L and Oc2M have cortical connections which are similar to those of P PC, but are restricted within orbital cortex to area VLO, and within a rea Fr2 to its caudal portion, and do not involve auditory area Te1. T he cortical connections of hindlimb cortex are largely restricted to s omatic sensory and motor areas. Retrosplenial cortex, which is mediall y adjacent to PPC, has cortical connections that are prominent with vi sual cortex, do not involve somatic sensory or auditory cortex, and in clude the presubiculum. We conclude that PPC is distinguished by its p attern of cortical connections with the somatic sensory, auditory and visual areas, and with areas Fr2, and VLO/MO, in addition to its exclu sive thalamic connectivity with LD, LP and Po. Because recent behavior al studies indicate that PPC, Fr2 and VLO are involved in directed att ention and spatial learning, we suggest that the interconnections amon g these three cortical areas represent a major component of the circui try for these functions in rats.