We have divided the cortical regions surrounding the rat hippocampus into three cytoarchitectonically discrete cortical regions, the perirhinal, the postrhinal, and the entorhinal cortices. These regions appear to be homologous to the monkey perirhinal, parahippocampal, and entorhinal cortices, respectively. The origin of cortical afferents to these regions is well-documented in the monkey but less is known about them in the rat. The present study investigated the origins of cortical input to the rat perirhinal (areas 35 and 36) and postrhinal cortices and the lateral and medial subdivisions of the entorhinal cortex (LEA and MEA) by placing injections of retrograde tracers at several locations within each region. For each experiment, the total numbers of retrogradely labeled cells (and cell densities) were estimated for 34 cortical regions. We found that the complement of cortical inputs differs for each of the five regions. Area 35 receives its heaviest input from entorhinal, piriform, and insular areas. Area 36 receives its heaviest projections from other temporal cortical regions such as ventral temporal association cortex. Area 36 also receives substantial input from insular and entorhinal areas. Whereas area 36 receives similar magnitudes of input from cortices subserving all sensory modalities, the heaviest projections to the postrhinal cortex originate in visual associational cortex and visuospatial areas such as the posterior parietal cortex. The cortical projections to the LEA are heavier than to the MEA and differ in origin. The LEA is primarily innervated by the perirhinal, insular, piriform, and postrhinal cortices. The MEA is primarily innervated by the piriform and postrhinal cortices, but also receives minor projections from retrosplenial, posterior parietal, and visual association areas.

The Morris water maze task was originally designed to assess the rat's ability to learn to navigate to a specific location in a relatively large spatial environment. This article describes new measures that provide information about the spatial distribution of the rat's search during both training and probe trial performance. The basic new measure optimizes the use of computer tracking to identify the rat's position with respect to the target location. This proximity measure was found to be highly sensitive to age-related impairment in an assessment of young and aged male Long-Evans rats. Also described is the development of a learning index that provides a continuous, graded measure of the severity of age-related impairment in the task. An index of this type should be useful in correlational analyses with other neurobiological or behavioral measures for the study of individual differences in functional/biological decline in aging.A test of spatial learning introduced over a decade ago by Richard Morris (Morris, 1981;Morris, Garrud, Rawlins, & O'Keefe, 1982) has become widely used in neurobiological studies of limbic-cortical function and in the characterization of cognitive decline in aged rats. Originally devised as a test of "place" learning, the water maze task permits a number of variations that have made it useful for isolating cognitive deficits apart from nonspecific impairments in sensorimotor function. The purpose of the present article is to describe some new methods for behavioral analysis that were developed to supplement those measures commonly used for the water maze. As a framework for presenting the new methods, the following introduction provides a brief discussion of the traditional analysis of performance in this task. Measures Traditionally Used for Behavioral Analysis in the Water MazeThe Morris maze apparatus consists of a large, circular pool filled with water that has been made opaque through the addition of powdered milk or some other substance. In the typical "hidden-platform" version of the task, rats are trained to find a camouflaged escape platform Correspondence concerning this article should be addressed to Michela, Gallagher, Department of Psychology, CB# 3270, Davie Hall, Chapel Hill, North Carolina 27599-3270. HHS Public AccessAuthor manuscript Behav Neurosci. Author manuscript; available in PMC 2017 October 13. Author ManuscriptAuthor Manuscript Author ManuscriptAuthor Manuscript that is positioned just below the water surface. The location of this platform remains constant from trial to trial. Because there are no local cues that mark the position of the platform, the rat's ability to locate it efficiently depends on the rat's use of a configuration of extramaze cues surrounding the pool. Indeed, rats can learn to swim directly to the escape platform within relatively few training trials from any of a number of start locations at the perimeter of the pool. Learning is reflected in shorter latencies to escape and by decreases in the length of the path that the rat tra...

KEY WORDS: parahippocampal, entorhinal polysensory cortex, memory, hippocampal formation,This review is prompted by recent findings that the perirhinal and parahippocampal cortices in the monkey brain are important components of the medial temporal lobe memory system. Given the potential importance of the comparable rcgions 1 0 niemory function in rhe rat brain, it is surprising that so little is known about their neuroanatomy. In fact, there are no comprehensive studies of the borders, cytoarchitecture, or connections of the cortical regions surrounding the posterior portion of the rhinal sulciis in the rat. This review is meant to summarize the current state of our knowledge regarding these regions in the rat brain. Based on existing data and our own observations, a new terminology is introduced that retains the term perirhinal cortex for the rostral portion of the region and renames the caudal portion the posirhi!,ind cortex. Issues of continuing uncertainty are highlighted, and information gleaned from the monkry literature is used to predict what anatomical traits thc rat perirhinal region might demonstrate upon further examination. To the extent possible with available data, the similarities and differences of the rat and monkey perirhinal, postrhinal, and parahippocampal regions are evaluated. (Fig. 1B).The entire region was located more caudally than inRose. This is because the rostrally adjacent area 13 (insular cortex) extended Farther caudally, substantially beyond the caudal limit of the underlying claustrum. As

The cortical regions dorsally adjacent to the posterior rhinal sulcus in the rat can be divided into a rostral region, the perirhinal cortex, which shares features of the monkey perirhinal cortex, and a caudal region, the postrhinal cortex, which has connectional attributes similar to the monkey parahippocampal cortex. We examined the connectivity among the rat perirhinal (areas 35 and 36), postrhinal, and entorhinal cortices by placing anterograde and retrograde tracers in all three regions. There is a dorsal-to-ventral cascade of connections in the perirhinal and entorhinal cortices. Dorsal area 36 projects strongly to ventral area 36, and ventral area 36 projects strongly to area 35. The return projections are substantially weaker. The cascade continues with the perirhinal to entorhinal connections. Area 35 is more strongly interconnected with the entorhinal cortex, ventral area 36 somewhat less strongly, and dorsal area 36 projects only weakly to the entorhinal cortex. The postrhinal-to-perirhinal connections also follow this general pattern. The postrhinal cortex is more heavily connected with dorsal area 36 than with ventral area 36 and is more heavily connected with area 36 than with area 35. The rostral portion of the postrhinal cortex has the strongest connections with the perirhinal cortex. Like in the monkey, the perirhinal and postrhinal cortices have different patterns of projections to the entorhinal cortex. The perirhinal cortex is preferentially connected with the rostrolateral portion of the entorhinal cortex. The postrhinal cortex projects to a part of this same region but is also connected to caudal and redial portions of the entorhinal cortex. The perirhinal and postrhinal projections to the entorhinal cortex originate in layers III and V and terminate preferentially in layers II and III.