Introduction to the JCN Special Issue on the Claustrum

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Hypotheses surrounding the function of the claustrum have flourished over the past decade, spurred on by tantalizing suggestions that this forebrain structure may be necessary, if not sufficient, for the emergence of consciousness (Crick and Koch, 2005; Stiefel et al., 2013; Koubessi, et al., 2014). In this special Issue of the Journal of Comparative Neurology, we have assembled a set of papers that address key questions of the phylogeny, architecture, and connectivity of the claustrum.
Development of new techniques for examining deep brain structures and functional networks, including viral tracer methods, optogenetic manipulation, and improvements in existing methods for studying structural connectivity and functional properties of neural circuits has pushed the research on the claustrum from a niche interest to a major research area. This maturation has reinforced our awareness that we have far more unanswered questions than we have answers. For the first time, however, these questions are amenable to experimental inquiry, and alongside other recent publications, the studies presented in this volume identify specific and testable hypotheses surrounding claustrum function.
Classical studies of the organization and connections and response properties of neurons of the claustrum employed cat and monkey models. However, more recent work, especially genetic analysis, has almost exclusively used rodents. In order to relate the data from rodents to other species it is critical to consider species differences in the organization, neurochemistry and connections of the claustrum. This volume includes studies from mice, rats, bats, tree shrews, marmosets, and macaques, which will serve as a foundation for exploration of comparative organization the future.
The problem of the relationship between claustrum and endopiriform nucleus is addressed in several ways. Watson and Puelles review work on the developmental origins of both the claustrum and the ventral endopiriform nucleus and conclude that these are separate structures. They proffer the avian mesopallium as the homologue of the mammalian claustrum complex, potentially answering one of the most persistent questions of claustrum anatomy. The implications of their work for the main competing models of telencephalic developmental origins are explored in the commentary provided by Wullimann. Watson et al. reach a similar conclusion regarding parcellation of the mature claustrum complex, based on the connectivity of the endopiriform nucleus with infralimbic and entorhinal cortex of the rat. Watakabe examined the claustrum‐endopiriform nucleus question in his study of gene expression in the marmoset claustrum, and found a more complicated picture, with some evidence of compartmentalization in the marmoset claustrum, but no critical gene or expression marker which clearly resolves its components. Orman et al. show that the claustrum and endopiriform are resolvable by latexin immunohistochemistry in their study of the claustrum complex in the fruit bat, a species with a distinctively large and well‐defined claustrum. In addition, their electrophysiological recordings elucidate the degree of intraclaustrum connectivity, a critical issue for some theories of claustrum function.
While the question of anatomical parcellation of the claustrum is vital for comparative understanding of this structure, anatomical boundaries must also be evaluated in the context of the functional properties and connectivity of the claustrum. The papers by Atlan et al. and Gamberini et al. report the 3D topography of the claustrum in mice and Old World monkeys, respectively. White et al. quantitatively analyze the cortico‐claustrum connections in the rat, using strength as an indicator of functional importance. They show an intriguing pattern of increased connectivity in higher‐order association areas and regions of high convergence, including cingulate areas, compared to primary sensory‐motor cortex. Zeng et al. also demonstrate hierarchical claustrum projections to cortical areas in the mouse, using an elegant series of viral tracer injections.
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