Size of Sensory Thalamic Nuclei

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. “Sensory” thalamic nuclei connect sensory input to the cerebral cortex. These nuclei include the medial geniculate nucleus (MGN; auditory), lateral geniculate nucleus (LGN; visual), and the ventrobasal complex (VB, also known as the ventral posterior nucleus; somatosensory and vestibular). Activity in these nuclei may be modulated by feedback signals from the reticular nucleus of the thalamus, which itself receives widespread information from the cerebral cortex (e.g. Bal et al, 2000, McAlonan et al, 2000).

In a comparison across hominid species, Armstrong & Armstrong (1979) found that, while the absolute volumes of these sensory nuclei are larger in the larger brains, they showed negative allometry (did not in size as much as expected, given the increase in size of other parts of the brain). In fact, given decreases in density, the total number of neurons did not show significant differences between humans and great apes in the LGN (visual) and MGN (auditory) nuclei. The human VB (somatosensory/vestibular) have slightly greater number of neurons than the great ape VB.

The increase in number of neurons in the human VB could be related to the importance of touching (as opposed to grooming and/or the increased development of tactile sensation necessary to support fine motor skills and the making of tools (Armstrong & Armstrong, 1979). VB size could also be affected by differences in vestibular input due to differences in ape and human locomotion.