The Claustrum: Puddles are Plumbs

Abstract

The claustrum is so named for its appearance as a thin layer of cells narrowly enclosed between the corpus striatum and the overlying insular cortex. However, in comparative study of a diversity of mammalian species we found a good portion of the claustrum forming “puddles” of tissue that appear to be spill out from the enclosed claustral lamina in various locations. In the domestic pig, a huge egg‐shaped mass projects back from the posterior edge of the lamina. In carnivores, a large pyramidal shaped mass of claustrum pours out the top of the lamina. In primates (humans included), a large globular puddle protrudes anterior and inferior from the lamina. The posterior portion of the carnivore pyramid mass has been shown in cats to be entirely an accessory of visual cortical function; and can be considered a visual lobe. All input to, and output from, this visual lobe comes from, and goes to, visual cortex. The other puddles may harbor other specialized connections and functions, placed around the laminar stem rather like fruits on a tree. Comprehensive morphological and physiological studies, modeled on those done in cats, are needed to resolve the question of the function of the claustrum. It may well be an accessory to all of mammalian cerebral cortices, with local elaborations related to behavioral specializations. Such studies may offer a major advance in understanding of function of the cerebral cortex.