Inferior parietal lobule projections to the presubiculum and neighboring ventromedial temporal cortical areas

The entorhinal and perirhinal cortices have long been accorded a special role in the communications between neocortical areas and the hippocampal formation. Less attention has been paid to the presubiculum, which, however, is also a component of the parahippocampal gyrus, receives dense inputs from several cortical areas, and itself is a major source of connections to the entorhinal cortex (EC). In part of a closer investigation of corticohippocampal systems, the authors applied single-axon analysis to the connections from the inferior parietal lobule (IPL) to the presubiculum. One major result from this approach was the finding that many of these axons (at least 10 of 14) branch beyond the presubiculum. For 4 axons, branches were followed to area TF and to the border between the perirhinal and entorhinal cortices, raising the suggestion that these areas, which sometimes are viewed as serial stages, are tightly interconnected. In addition, the current data identify several features of presubicular organization that may be relevant to its functional role in visuospatial or memory processes: 1) Terminations from the IPL, as previously reported for prefrontal connections (Goldman-Rakic et al. [1984] Neuroscience 12:719-743), form two to four patches in the superficial layers. These align in stripes, but only for short distances ( approximately 1.5 mm). This pattern suggests a strong compartmentalization in layers I and II that is also indicated by cytochrome oxidase and other markers. 2) Connections tend to be bistratified, terminating in layers I-II and deeper in layer III. 3) Single axons terminate in layer I alone or in different combinations of layers. This may imply some heterogeneity of subtypes. 4) Individual axons, both ipsilateral projecting (n = 14 axons) and contralateral projecting (n = 6 axons), tend to have large arbors (0.3-0.8 mm across). Finally, the authors observe that projections from the IPL, except for its anteriormost portion, converge at the perirhinal-entorhinal border around the posterior tip of the rhinal sulcus. These projections partially overlap with projections from ventromedial areas TE and TF, and this convergence may contribute to the severe deficits in visual recognition memory resulting from ablations of rhinal cortex.