Positional firing properties of perirhinal cortex neurons

Abstract

The neuroanatomical connections of the postrhinal cortex (POR) are consistent with a role in processing information about space. The POR receives input from visual associ- ation and visuospatial cortex, and is reciprocally connected to the medial entorhinal area (Burwell and Amaral, 1998b). The POR also projects strongly to the perirhinal cortex (PER), but the return projection is relatively weak (Burwell and Amaral, 1998a). Both the POR and the PER are reciprocally connected with the hippocampus and subicu- lum (Naber et al., 1999). The PER has been implicated in object identification or stimulus-stimulus association (Buckley and Gaffan, 1997; Bussey et al., 2001; Eacott et al., 2001; Gaffan et al., 2000; Liu and Bilkey, 2001; Mumby and Glenn, 2000; Mumby and Pinel, 1994; Otto and Gar- ruto, 1997), but less is known about POR function. Al- though mixed, there is evidence that the POR is involved in the processing of spatial information in some paradigms (Bussey et al., 1998, 2000a,b; Liu and Bilkey, 2002; Vann et al., 2000). Both the PER and POR are necessary for encoding context in a contextual fear conditioning task (Bucci et al., 2000, 2002). Thus, it may be that the object identification functions of the PER and the visuospatial functions of the POR are necessary for processing infor- mation about spatial environments. Place cells in the hippocampus were first described by O'Keefe and Dostrovsky (1971), and many subsequent studies have replicated those findings. Neuronal correlates of space have since been noted in neighboring areas such as the subiculum (Phillips and Eichenbaum, 1998; Sharp, 1999; Sharp and Green, 1994) and the medial entorhinal area (Quirk et al., 1992). While these place fields are larger than those found in hippocampus and behave differently under certain experimental conditions, they predictably fol- low spatial cues (but see Sharp, 1999). Cells in PER, in contrast, do not display stable spatial selectivity; only one- third of these cells display spatial correlates, and the firing patterns do not respond predictably to experimental ma- nipulation of cues (Burwell et al., 1998). PER lesions, however, result in decreased stability of hippocampal place fields (Muir and Bilkey, 2001). It is reasonable to hypothesize that the POR, which is connected to visuo- spatial regions and projects to the PER, medial entorhinal area, and hippocampus proper, may display neuronal ac- tivity that reflects the pre-processing of information neces- sary for the formation of stable place fields in the hip- pocampus.