Sensory Thalamic Nuclei Size
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The thalamus is a neural structure found in all vertebrates, located at the dorsal end (top) of the brain stem. It consists of 30 to 40 “nuclei”, or interconnected groups of neurons. “Association” nuclei include the pulvinar nuclei, the lateral dorsal nucleus (LD; also known as the “visual pulvinar”), and the dorsomedial nucleus. The pulvinar is itself a conglomerate of 6-8 nuclei, and is associated with connections to higher-order visual and auditory cortex.
The total volume of the pulvinar and LD together shows positive allometry; the combined volume fits within expectations for an ape with a large brain. Individually, the pulvinar is slightly larger, and the LD slightly smaller, than expected. The pulvinar has about twice as many neurons as the great ape pulivinar, while the number of neurons in the human LD is not significantly different than that of gorillas (the only great ape measured) (Armstrong, 1981). The dorsomedial nucleus, which has heavy connectivity to frontal cortex, also shows more neurons than expected.
Besides these differences, the pulvinar (along with other dorsal thalamic nuclei), attracts migrating GABA-ergic interneurons during development (Letinic & Racic, 2001). This developmental pattern has not been observed in any other primates, including the great apes (Sherwood et al, 2008). The migratory pattern of GABA-ergic interneurons to the dorsal thalamus is believed to be unique to humans, and has also been suggested to contribute to higher-order functions as well (Sherwood et al, 2008). This is consistent with the view that most of the thalamus plays a critical role in cortico-cortico communication (Sherman, 2007), and as a hub for attentional modulation and other global brain states, such as sleep (McCormick, 1991; Steriade, 2001).
It has been suggested that sub-nuclei of the pulvinar may have differentially expanded/contracted during human evolution, due to specializations in higher-order functions of humans (Armstrong, 1981). This is supported by the fact that the “pars inferior” aspect of the pulvinar, which has well-described visual functions shared with other great apes, does not show the increased allometry that the rest of the pulvinar does (Armstrong, 1981). In addition, left (and not right) pulvinar has been associated with some language-related functions, such as naming and verbal memory (Johnson and Ojemann, 2000).
The pulvinar and LP are considerably different between prosimians and anthropoid primates (Wong et al, 2009), suggesting an early origin of the basic structure of the human pulvinar.
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