Atlases of the rat brain are widely used as reference for orientation, planning of experiments, and as tools for assigning location to experimental data. Improved quality and use of magnetic resonance imaging (MRI) and other tomographical imaging techniques in rats have allowed the development of new three-dimensional (3-D) volumetric brain atlas templates. The rat hippocampal region is a commonly used model for basic research on memory and learning, and for preclinical investigations of brain disease. The region features a complex anatomical organization with multiple subdivisions that can be identified on the basis of specific cytoarchitectonic or chemoarchitectonic criteria. We here investigate the extent to which it is possible to identify boundaries of divisions of the hippocampal region on the basis of high-resolution MRI contrast. We present the boundaries of 13 divisions, identified and delineated based on multiple types of image contrast observed in the recently published Waxholm Space MRI/DTI template for the Sprague Dawley rat brain (Papp et al., Neuroimage 97:374-386, 2014). The new detailed delineations of the hippocampal formation and parahippocampal region (Waxholm Space atlas of the Sprague Dawley rat brain, v2.0) are shared via the INCF Software Center (http://software.incf.org/), where also the MRI/DTI reference template is available. The present update of the Waxholm Space atlas of the rat brain is intended to facilitate interpretation, analysis, and integration of experimental data from this anatomically complex region.

The hippocampal region, comprising the hippocampal formation and the parahippocampal region, has been one of the most intensively studied parts of the brain for decades. Better understanding of its functional diversity and complexity has led to an increased demand for specificity in experimental procedures and manipulations. In view of the complex 3D structure of the hippocampal region, precisely positioned experimental approaches require a fine-grained architectural description that is available and readable to experimentalists lacking detailed anatomical experience. In this paper, we provide the first cyto- and chemoarchitectural description of the hippocampal formation and parahippocampal region in the rat at high resolution and in the three standard sectional planes: coronal, horizontal and sagittal. The atlas uses a series of adjacent sections stained for neurons and for a number of chemical marker substances, particularly parvalbumin and calbindin. All the borders defined in one plane have been cross-checked against their counterparts in the other two planes. The entire dataset will be made available as a web-based interactive application through the Rodent Brain WorkBench (http://www.rbwb.org) which, together with this paper, provides a unique atlas resource.

The rat hippocampal region is frequently studied in relation to learning and memory processes and brain diseases. The region is complex, consisting of multiple subdivisions that are challenging to delineate anatomically. Published atlases of the rat brain typically lack the underlying histological criteria necessary to identify boundaries, and textbooks descriptions of the region are often inadequately illustrated and thus difficult to relate to experimental data. An overview of both anatomical features and criteria used to delineate boundaries is required to assign location to experimental material from the hippocampal region. To address this issue, we have developed a web-based atlas application in which images of histological sections are integrated with new and up-to-date criteria for subdividing the rat hippocampus formation, fasciola, and associated parahippocampal regions. The atlas application consists of an interactive image viewer with high-resolution images of an extensive series of sections stained for NeuN, calbindin, and parvalbumin, and an index of structures with detailed descriptions of the criteria used to define the boundaries. Images can be inspected with a graphical overlay of selected subregions. Bi-directional links between images and the index of structures are provided. In summary, we provide a novel content-rich digital atlas resource facilitating identification of morphological features relevant for delineating the anatomical subdivisions of the rat hippocampal region. The atlas application is available at .

The aim of this study was to investigate the role of the human hippocampus in episodic retrieval of odors, in comparison with episodic retrieval of visual objects. Subjects encoded a set of unique odors and objects, and retrieval was tested the next day during functional magnetic resonance imaging (fMRI). Subjects were shown the names of old (studied) and new (unstudied) odors and objects, and asked to indicate which of these stimuli had been presented the previous day. The results showed that brain activation was weaker and more restricted during retrieval of odors than during retrieval of objects, which possibly reflects a general visual dominance effect. Yet, retrieval of odors and objects yielded overlapping clusters of activation the bilateral hippocampi, and the left-sided activation was specifically increased during successful retrieval (hits > correct rejections) in both modalities. Moreover, retrieval of odors uniquely activated olfactory cortical regions, likely to reflect cortical reinstatement of sensory details. Our fMRI study is the first to make a direct comparison between olfactory and visual episodic memory, and the results provide clear evidence for modality-independent functions of the hippocampus.

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