The claustrum in review

Abstract

The claustrum is among the most enigmatic of all prominent mammalian brain structures. Since the 19th century, a wealth of data has amassed on this forebrain nucleus. However, much of this data is disparate and contentious; conflicting views regarding the claustrum's structural definitions and possible functions abound. This review synthesizes historical and recent claustrum studies with the purpose of formulating an acceptable description of its structural properties. Integrating extant anatomical and functional literature with theorized functions of the claustrum, new visions of how this structure may be contributing to cognition and action are discussed.

Keywords: attention; cerebral cortex; claustrum; connections; function.

Figure 1

(A) The structural boundaries of the rat claustrum (shown in red) and its proximity to the white matter (shown in blue) of the forceps minor (fm) and EC as defined by Paxinos and Watson (2007) (used with permission from Elsevier). The dotted lines in (A) indicate the regions depicted in (B), which shows immunohistochemical staining for parvalbumin (PV) (red) and crystallin mu (Crym), a marker of insular cortex (staining originally published in Mathur et al., 2009). At levels of the striatum (str), the body of the claustrum is labeled by PV-immunoreactivity (-ir) and surrounded by Crym-ir, indicating that the claustrum is not immediately juxtaposed to the white matter. At the level of the fm, however, PV-ir and Crym-ir does not reveal structural boundaries of the claustrum as defined by Paxinos and Watson (2007). (C) The structural boundaries of the claustrum redrawn to depict the definition based on PV-ir and Crym-ir, as well as G protein gamma 2 subunit (Gng2)-ir (Mathur et al., 2009). Scale bars: 200 μm for fm sections; 100 μm for str sections.

Figure 2

Retrograde neuronal tract tracer labeling in the region of the claustrum following tracer deposit into the thalamic mediodorsal nucleus of various species. Examples of species lacking an extreme capsule, (A) the hedgehog, (used by permission from John Wiley and Sons; Dinopoulos et al., 1992), and (B) the rat, where PV-ir is depicted in green and retrogradely labeled cells in red (see original publication by Mathur et al., 2009). In both cases, the retrogradely labeled cells appear to reside in the insular cortex and surround the body of the claustrum (red asterisk), which in the case of the rat is defined by PV-ir. In species containing an extreme capsule, (C) the tree shrew (used by permission from Elsevier; Carey and Neal, 1986), and (D) the cynomolgus monkey (used by permission from Elsevier; Erickson et al., 2004) both exhibit a similar pattern of retrograde labeling. The rat data (B) suggests that the labeled cells in (C) and (D) are insular cortex cells that have been separated from the rest of the more superficial insular cortex cell layers through time by the development of the extreme capsule. Scale bar: 100 μm.

Figure 3

(A) Proposed claustrum circuitry involved in stimulus encoding. Neural activity encoding a novel/salient sensory stimulus in sensorimotor and/or association cortices activates the corresponding subdivision of the contralateral claustrum (Step 1). If the sensory stimulus is salient enough to pass the claustral filter, the ipsilateral cingulate cortex receives and processes the incoming claustral signal (Step 2). (B) Proposed circuitry involved in an action response to a salient stimulus. The cingulate cortex signals to the appropriate subdivision of the contralateral claustrum (Step 3) that, in turn, provides attentional allocation to the original sensorimotor/association cortex encoding the salient stimulus (Step 4). The activated sensorimotor/association cortex finally signals to the striatal complex for selection of an appropriate action (Step 5).