Editorial: The Claustrum: charting a way forward for the brain's most mysterious nucleus

Citation: Deutch AY and Mathur BN (2015) Editorial: The Claustrum: charting a way forward for the brain's most mysterious nucleus. There are many reasons why investigators decide to expend their energy on studying a particular nucleus in the brain. Sometimes the choice of structures has little to do with new insights that may be gleaned but is more attributable to the tractability of the site to experimental manipulation. However, some sites remain relatively obscure because their structure almost seems designed to impede research. The claustrum belongs to the latter group. Entombed between tightly constricting white matter bundles, the claustrum both escaped and resisted significant attention for much of the twentieth century. However, over the past decade there has been a resurgence in interest in the claustrum. This renewed attention is in large part attributable to a paper by Crick and Koch (2005), which advanced the hypothesis that the claustrum plays a central role in binding and consciousness. Seductively written, and with Crick and Koch's imprimatur, a flurry of studies ensued. While it appeared that this opened the door to experimental studies of claustral function, over the past decade it has become apparent that the door is merely ajar. This collection of papers is an attempt to bring together many of the key recent findings on the claustrum. Perusing the volume quickly reveals that there is a striking emphasis on the anatomy of the claustrum, with most contributions emphasizing comparative anatomy of the claustrum, localization of different types of chemically-coded neurons within the claustrum, and new details on the morphology of claustral neurons. This selective attention to the anatomy of the claustrum has led some wags to comment that the anatomy of the claustrum should by now have been defined. However, striking differences in claustral organization and the anatomy of the region surrounding the claustrum across various species led to confusion regarding the boundaries of the claustrum. This resulted in some definitions being overly inclusive, adding to the claustrum several cortical regions that are now considered to be distinct. Other definitions were too restrictive and did not account for the ontogeny of the claustrum. This situation led Crick and Koch (2005) to decry the lack of objective criteria for the definition of the claustrum. The past decade has mainly been spent trying to respond to their plea. This collection of papers appears to have arrived at a consensus regarding the definition …