Claustrum

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Contents 1 Overview 2 History 3 Anatomy 4 Function 4.1 Integration of modalities 4.2 Role in Functional timing 5 Additional images 6 References 7 External links

[edit] Overview

The claustrum, which is suspected to be present in all mammals, is fairly thin (fraction of 1 mm to multiple mms) and is essentially a vertical curved sheet oriented sagittally inside of the insula, a portion of the cortex located in the lateral sulcus, and above the outer surface of the putamen, which is part of the basal ganglia. There are lateral and medial tracts connecting to many cortices as well as the hippocampus, the amygdala, and the caudate nucleus.

One of the interesting features of the claustrum is the uniformity in the types and numbers of cells. Since there is no function segregation or structural inhomogeneity ^[1], when waves of information come through the claustrum, the neurons will be especially sensitive to the timing of the inputs. Experiments to discover the physiological role of the claustrum are performed much the same with other structures, primarily utilizing lesion experiments, single cell recordings, and stimulation studies in animals, as well as clinical observations and imaging. The analogy Crick and Koch used was that the claustrum is somewhat like a conductor of an orchestra. The different parts (cortical subregions) must play in harmony or else the result is a cacophony of sounds. The neurons must take in different types of data (color, motion) across various modalities (visual, sound) and coalesce and bound them. An interesting question to ask though is why the claustrum is located there, when all this information is brought from all over the brain. It might be much faster to have it located under the cortex, decreasing the length of many of the loops. More studies on latency would be useful since timing appears to be a primary function. Also, more work looking at how claustral neurons respond during a task of moderate complexity would be highly practical.

[edit] History

     The claustrum has been studied for at least 191 years,

according to Dr. Edelstein. ^[2]. Many researchers have looked at it through the ages as an interesting structure that could hold the key to a number of neuro-based questions. The claustrum has a phylogenetic background appearing predominantly in insectivores, Prosimians, and Marsupials. It is difficult to trace where the evolution of the structure originated, but various parts are suspected to come from a number of animals, including the alligator, chameleon, turtle and others.

One big point of discussion with regards to the claustrum has been on the ontogeny. One side, which Dr. Edelstein deems the pallial group, believes the claustrum is a derivative of the insular cortex. While some designated this as a certain distinguished area not to be confused with the rest of the cortex, they still ascertained that the ontogeny was based on the development of the cortex. The other side states that the claustrum is derived from basal ganglia. Ramon y Cajal supported this view as did many other researchers ^[3]. Based on more recent methods of looking at the development of both the human and animal mind, including fMRI among others, there has been increasing evidence against the claustrum being a part of the cortex. They seem to have distinct developmental periods and are therefore deemed separate structures. The third view, which is where the majority of support is coming from presently, says that the claustrum is neither distinctly part of the cortex or from the basal ganglia complex. Based on a number of studies, including those that show projections on both sides of the claustrum, many believe the claustrum should be considered a seventh layer of the cortex, in the insular region ^[4]. This hypothesis which is growing support, including the aforementioned Dr. Larry Edelstein, is labeled as Filiminoff's hypothesis since he was estimated to be the first to come to this conclusion ^[5]

As far as historical cases shedding light on the anatomy and function of the claustrum, there haven't been many to date that survived the struggle of time. There was one documented case on an enlarged claustrum in a patient with epilepsy, as well as another case where the insula was malformed but the claustrum remained . These cases haven't lent too much towards the pursuit of truth regarding the claustrum. As far as various disorders, the claustrum isn't involved very much, pathologically speaking. Also, due to the location and complex integration of the claustrum with other parts of the brain, it is difficult to do lesion studies without affecting other regions. Likewise, ablation isn't an effective option either. Most studies involve either the removal and preservation of the brain, or in vivo scanning of the area of interest during different studies. This could involve fMRI, the newer Tractography technology, or others.

Below is worthwhile quote about Francis Crick documenting the importance of the claustrum as well as other research being performed at the Salk's Crick-Jacobs Center for Computational and Theoretical Biology:

" …Even in his last few moments, Francis was thinking about the new center. Several months ago he wrote a position paper suggesting the Center focus its work on the claustrum, a part of the brain amenable to both molecular and cellular analysis. Because the claustrum is not well understood, he set out to write a review paper on it, and he was working on the document literally hours before his death. Francis wrote that paper for us and for our research, not for himself…" ^[6]

[edit] Anatomy

The claustrum is a telencephalic subcortical structure. It is a thin sheet of grey matter underneath the inner part of the neocortex. It is on both sides of the brain, and can be found below the insular cortex, which is in the temporal lobe, and above the outside of the putamen, which is a sub-structure of the basal ganglia. The sheet is approximately one to several millimeters thick, and can cover up to a couple centimeters length wise depending on the animal.

One interesting aspect about the claustrum is the lack of cell types. In most part so the brain, especially in the cortical regions, there is considerable differentiation of cell types, giving way to a number of functions. In the claustrum, as Dr. Crick and others pointed out, there are three main types of cells ^[7] . The first which is deemed Type 1 is large with spine-covered dendritic processes. These cells receive input as well as project back towards various regions, both laterally and medially. The other two types of cells do not have spines, but can be told apart based on the cell body size. Both however are restricted to the claustrum and thus are labeled interneurons.

It is clear that the claustrum projects to the a number of cortices, including the primary motor, premotor, prefrontal, auditory, and visual, among others. In one study conducted in France by Judith Tanne-Gariepy et. al. (9), these projections were traced back to segregated areas, including differentiated areas along the dorsoventral axis for the pre-supplementary motor area and supplementary motor area – proper. Projections from the claustrum to various sub-regions of the motor cortex were shown to overlap somewhat, but did show a degree of local segregation ^{[8] [9]}. The truly interesting thing about the claustrum though is how it can take in multiple information modalities, including motor, visual, and auditory. It has even been shown that the same cells can process information across all these types, even though there is some semblance of segregation across a single type of information.

There have also been a number of interesting studies looking at the proteins inside the claustrum. In one experiment performed at the University of South Carolina by J.R. Augustine et. al. ^[10], they looked at calcium-binding proteins in the rhesus monkey claustrum, including calbindin D28K, parvalbumin, and carretinin. After removing the brains and properly preserving them, the group used various antibodies and antiserums to detect the presence of the proteins. The calbindin proteins were shown as likely elements in the inhibitory circuitry of the claustrum while the calretinin most likely served as calcium buffer to maintain homeostasis (. Another study looked at the serotonergic innervation in the claustrum ^[11]. The clear conclusion here was that in the ventral claustrum where the visual projections are, the stained axons were short and arranged randomly. However, in the dorsal, non-visual section, of the claustrum, the fibers ran consistently in long lengths along the dorsal-ventral direction. Like many of the other studies, this is a good first step towards determining the true functions of the claustrum, although there is still much room for work.

[edit] Function

As mentioned previously, there is still very little known about the actual function of the claustrum. While it has been studied for a considerable amount of time, it is still a minor structure in the brain. Anatomical hindrances combined with a lack of case studies have left many holes in the research of the claustrum. Also, until recently there hasn't been much of a reason to study the claustrum. With the paper written by Francis Crick and his Colleague Christof Koch, in which they discussed the claustrum as it could pertain to consciousness, many researchers have recently jumped on the proverbial bandwagon.

[edit] Integration of modalities

Through a number of different studies, summed up succinctly by Crick and Koch, it has been found that the claustrum is seemingly crucial in modality integration. Objects in real life have many different simultaneous characteristics such as: sound, shape, color, speed, weight, smell, etc... It is necessary for us to take in all of this information and integrate it together to potentially one object. In doing so, processing can take place, and the brain can determine the necessary actions to take. If this integration didn't take place, one would not be able to converge all the information into a single percept, and would thus be perpetually confused. The claustrum is crucial in this.

[edit] Role in Functional timing

Perhaps just as important as the ability to take in multiple modalities, is the claustrum's capacity for functional timing. Few latency experiments have been performed (more would definitely be useful), but it is clear that the claustrum in one way or another is a big part of taking all the different sources of information and integrating them together so they are processed at the same time. Without this ability, the inherent differences in processing timing for vision vs. hearing and other types would take over and one would never be able to combine information and have a single percept.

Suffice it to say however, that studies are still lacking, and more research will be needed in order to attain the necessary detail and confidence regarding the claustrum.

[edit] Additional images

Coronal section	Coronal section of brain immediately in front of pons.	Coronal section of brain through intermediate mass of third ventricle.	Section of brain showing upper surface of temporal lobe.
Coronal section of brain through anterior commissure.

[edit] References

[edit] External links

• BrainMaps at UCDavis Claustrum

• Claustrum at eMedicine Dictionary

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