Third Somatomotor Map Distinguished in Human Cerebellum

Seminal neurophysiological studies in the 1940s, done in cats and monkeys, discovered two somatomotor "body maps" in the cerebellum. In the anterior lobe, body parts are organized in an inverted fashion: hindlimb representation is anterior, the face posterior, and the forelimb in between. In the posterior lobe, the organization is the opposite (upright).

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These two somatomotor maps have been observed repeatedly in humans using noninvasive neuroimaging techniques evoked by movements and tactile stimulation, and through functional MRI (fMRI) scanning that has shown connections between cerebral motor regions and the cerebellum.

Now, Massachusetts General Hospital researchers have presented the first direct evidence of a third somatomotor map, a distinct representation of foot movements within the vermis of the human cerebellum. Randy L. Buckner, PhD, director of the Psychiatric Neuroimaging Division at the Martinos Center for Biomedical Imaging at Mass General, and colleagues describe their findings in the Journal of Neurophysiology.

Background

Hints of a third somatomotor representation have been reported in several previous papers, but definitive demonstration faced multiple challenges:

• The hypothesized map was thought to be located in a small region within the already small cerebellum
• The inferior portions of the cerebellum near the vermis have a low signal-to-noise ratio in fMRI compared with the cerebral cortex
• The third map was expected to be in close spatial proximity to the second map, and distinguishing between the two posterior lobe representations was expected to be difficult

Methods

The Mass General team recruited eight healthy participants, ages 19 to 25, and took several steps to overcome those challenges:

• The experimental fMRI paradigm targeted six types of movements along the anterior-posterior body axis (tongue, left and right hands, glutes, left and right feet)
• To boost the signal-to-noise ratio, the researchers acquired a substantial amount of data: each participant was scanned on four days with six motor runs per day, for a total of 576 separate blocks of motor movements (96 blocks for each of the six separate movements)
• To avoid between-individual spatial blurring, a custom analysis pipeline was created for individualized data processing

The team expected that given the orientation of the second somatomotor map, one but not both of the hand and foot representations would juxtapose one another, allowing the other body part to demonstrate separation of the second and hypothesized third maps.

Key Results

As hoped, when left and right foot movements were contrasted, a response was detected in the cerebellar vermis that was spatially discontinuous from the known second map. Furthermore, the region of the posterior cerebellum between the two foot representations was associated selectively with the hand, showing the two foot representations are parts of a functionally distinct map.

Additional Scrutiny Planned

The experiments couldn't map responses related to complex movements or provide information about the spatial orientation of the third map. The specific functional role of the third map is, therefore, still unknown, and examination at higher spatial resolution is planned.

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