The Connected Hippocampus: Preface
The hippocampus is an intriguing and anatomically remarkable structure: it is possessed of a remarkable curvilinear appearance in coronal section, and it is easy to spot in anatomical section with the naked eye in just about any mammalian species. A special and important function has been ascribed to it as a result of the pioneering work of John O’Keefe (Nobel Laureate, 2014), who described the remarkable ‘place cells’, which fire as a function of the location of the rat in the environment. Two other important discoveries also give it great importance: long-term potentiation, and amnesia. Long-term potentiation, the demonstration that synapses are plastic, was first described in the hippocampus by Tim Bliss and Terje Lomo. The famous amnestic patient, HM, had a more-or-less complete surgical ablation of the hippocampus. Correspondingly, the hippocampus has been implicated in many important neurocognitive functions, with a particular latter-day emphasis on its role in spatial and cognitive mapping, and in declarative (or explicit) memory. A substantial body of data suggests that the hippocampal formation plays a critical role in the biological processes underlying at least some forms of memory. Sometimes, however, it feels when reading the many, many papers published annually on the hippocampus that it sits apart from the brain, with its functions analysed in a narrow hippocampo-centric framework – as if the purpose of the rest of the brain is to serve the information processing needs of the hippocampus! This point is made a little facetiously and exaggeratedly, of course. Nonetheless, we felt the need to assuage these feelings by assembling this volume to encourage researchers to situate the hippocampus as part of a network connected to the rest of the brain and not to consider it in isolation.
We therefore present a selection of chapters that concentrate on understanding the functions of the hippocampus in terms of the connectivity of the hippocampus itself: in other words, in terms of its cortical and subcortical inputs and outputs. To take just one important illustrative example: the anterior thalamic and rostral thalamic nuclei are abundantly connected with the hippocampal formation, and have the capacity to profoundly shape hippocampal spatial and mnemonic information processing, a key point sometimes be overlooked in analyses favouring of hippocampally-directed cortical processing. We also know that damage to the anterior thalamus results in episodic memory impairment more-or-less similarly severe as that resulting from hippocampal lesions; this may be a function of lost thalamo-hippocampal information transfer. However, the textbooks and the primary literature often heavily emphasise the lessons from patients with hippocampal damage, while neglecting the similarly instructive patients with thalamic damage who also suffer amnesia. The complexity of thalamic signals and their contribution to the encoding of experience-dependent memory traces in hippocampal formation needs further investigation, as signal processing in the hippocampal formation does not always follow a corticofugal route, but is also affected profoundly by thalamofugal signals. We should conclude that memory is not a specialized property of a limited set of cortical areas; rather, all areas of the cortex as well as several subcortical structures are capable of experience-dependent change over a wide range of time scales. We therefore hope that we will correct the common misconception that the hippocampus is a closed system, self-sufficiently responsible for the declarative memory formation.
We here would like to thank all the authors of the chapters presented in this volume – there is a considerable body of work to savour here, and the pleasant feeling of having one’s pet prejudices tested and changed a little to be enjoyed.
Shane O’Mara and Marian Tsanov, Institute of Neuroscience, Trinity College, Dublin
Table of Contents
- Michael E. Hasselmo: If I had a Million Neurons: Potential Tests of Cortico-Hippocampal Theories
- Edmund T. Rolls: Diluted Connectivity in Pattern Association Networks Facilitates the Recall of Information from the Hippocampus to the Neocortex
- Maureen Ritchey, Laura A. Libby and Charan Ranganath: Cortico-hippocampal Systems Involved in Memory and Cognition: The PMAT Framework
- John P. Aggleton and Kat Christiansen: The Subiculum: the Heart of the Extended Hippocampal System
- Julie R. Dumont and Jeffrey S. Taube: The Neural Correlates of Navigation Beyond the Hippocampus
- Marian Tsanov: Septo-hippocampal Signal Processing: Breaking the Code
- Robert P. Vertes: Major Diencephalic Inputs to the Hippocampus: Supramammillary Nucleus and Nucleus Reuniens. Circuitry and Function
- Jean-Christophe Cassel and Anne Pereira de Vasconcelos: Importance of the Ventral Midline Thalamus in Driving Hippocampal Functions
- Seralynne D. Vann and Andrew J. D. Nelson: The Mammillary Bodies and Memory: More than a Hippocampal Relay
- Emilie Werlen and Matthew W. Jones: Modulating the Map: Dopaminergic Tuning of Hippocampal Spatial Coding and Interactions
- Sheri J. Y. Mizumori and Valerie L. Tryon: Integrative Hippocampal and Decision-making Neurocircuitry During Goal-relevant Predictions and Encoding
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