The claustrum is a thin, irregular, sheet-like neuronal structure hidden beneath the inner surface of the neocortex in the general region of the insula. Its function is enigmatic. Its anatomy is quite remarkable in that it receives input from almost all regions of cortex and projects back to almost all regions of cortex. We here briefly summarize what is known about the claustrum, speculate on its possible relationship to the processes that give rise to integrated conscious percepts, propose mechanisms that enable information to travel widely within the claustrum and discuss experiments to address these questions.
Keywords: neuroanatomy, connectivity, primate, electrophysiology, gap junctions, cortex
Most people working on the brain have heard of the claustrum—it was known to Ramón y Cajal—but very few have any idea what it does. It is thin and fairly small—in humans, its volume is a quarter of one percentage of that of the cerebral cortex (Kowianski et al. 1999 )—and so it is easily overlooked. Crick (1994) described the claustrum briefly, but since then we have left it to one side. So what prompted this article?
A key property of conscious sensations is their integrated nature. You are not aware of isolated percepts, but of a single, unifying experience. When holding a rose, you smell its fragrance and see its red petals while feeling its textured stem with your fingers. The philosopher Searle (2004) refers to the ‘conscious field’ in this context. 1
There is an approximate consensus among scholars who speculate on the neuronal basis of consciousness that its correlate must involve some form of cooperative activity, mediated by electrical and chemical synapses between forebrain neurons which are responding to different aspects of the same conscious experience. In vision, to which we have paid special attention, this would mean correlated activity among the different neuronal representations that encode the different visual aspects of the same object or event. We have suggested that this takes the form of coalitions of active neurons in cortex, thalamus and closely associated structures whose spiking activity (in some form) has reached a special threshold, and whose interactions tend to support each other (Crick & Koch 2003 ; Koch 2004 ).
These neurons are distributed over large distances (spanning, in humans, many centimetres) that include (for vision) visual cortex at the back of the brain, the frontal eye field and other frontal regions, posterior parietal and inferior temporal cortices, the hippocampus, and the associated thalamic and basal ganglia nuclei. Many of the neurons in these areas code for local aspects of any one scene, such as the orientation of an edge, or the colour and depth of a surface patch. Much of this information is ambiguous and is compatible with many different interpretations of the overall scene. In mathematical terms, the visual input is ill-posed (Poggio et al. 1984), if only because the brain is trying to reconstruct a three-dimensional representation of object and events occurring in the outside world from two-dimensional and noisy retinal inputs. Resolving these ambiguities must involve interactions among large groups of cells, most likely cortical pyramidal neurons, since their axons carry the bulk of long-distance communications within, and to structures outside, the cortex. These synaptically mediated interactions among assemblies of neurons involve both cooperation as well as winner-takes-all style competition (Hebb 1949 ; Palm 1990 ). This competition can be biased by bottom-up, saliency-driven or top-down volitionally controlled attention
(Desimone & Duncan 1995 ), as well as by the expected reward (Maunsell 2004 ).
A key feature of almost all neuronal theories of consciousness is the need for continuous interactions among groups of widely dispersed pyramidal neurons that express themselves in the ongoing stream of conscious percepts, images and thoughts.
This is apparent in Edelman and Tononi's concept of the dynamical core. This is a shifting assembly of spiking neurons throughout the forebrain that is stabilized using massive re-entrant feedback connections (Tononi & Edelman 1998 ; Edelman & Tononi 2000 ). Its representational content, integrated, yet at the same time highly differentiated, corresponds to the unitary yet amazingly particular content of phenomenal consciousness.
Dehaene & Changeux (2004 ; see also Dehaene et al. 2003 ) postulate a widely dispersed set of reciprocally connected pyramidal neurons with long-distance axons linking most, if not all, the cortical and thalamic regions. This network implements a neuronal workspace. a term borrowed from Baars's (1997. 2002 ) cognitive theory of consciousness. It distinguishes a large array of unconscious specialized processors running in parallel from a unified, limited-capacity ‘workspace’ that allows the local processors to exchange information.
Although using different terminologies, the basic ideas of these neuroscientists are surprisingly similar to ours: the need to rapidly integrate and bind information in neurons that are situated across distinct cortical and thalamic regions (see also Bachmann 2000 ; Llinas 2001 ).
It is in the light of this consensus and the existing, albeit limited, knowledge of the anatomical and functional organization of the claustrum, that the structure attracted us again. It appears to be in an ideal position to integrate the most diverse kinds of information that underlie conscious perception, cognition and action. What follows is an attempt to illustrate this and advocate the need for research examining its role.
2. The neuroanatomy of the claustrum
The claustrum is present in all mammalian species so far examined (Kowianski et al. 1999 ), from insectivore to man, though its precise shape and some of its connections appear to vary from species to species. 2 We shall consider mainly the cat and primate claustrum (for comprehensive reviews, consult Sherk 1986 ; Tanné-Gariépy et al. 2002 ; Edelstein & Denaro 2004 ).
So what and where is the claustrum? The word claustrum means ‘hidden away’, and indeed, the claustrum is a thin, irregular sheet of grey matter, one sheet on each side of the head, concealed between the inner surface of the neocortex. It lies below the general region of the insula, and above the outer surface of the putamen, with a fibre tract on each side of it (the extreme and external capsulae). Viewed face on, it has an irregular outline, for primates not unlike that of the contiguous United States ( figure 1 ). The latero-medial thickness varies from a fraction of a millimetre to several millimetres. The sheet is not flat but curved in a curious way. The claustrum is not located everywhere beneath the cerebral cortex. It is mainly, but not entirely, in the general region of the insular cortex. The exact shape varies from species to species. Figure 2 shows the general shape of the human claustrum and how it is tucked away underneath the cortex. There is a general tendency for any cortical area to connect to the area of the claustrum nearest to it ( figure 3 ).