Somatotopy in the Human Somatosensory System

Area of Science:

• Neuroimaging
• Somatosensory Neuroscience
• Human Brain Mapping

Background:

• Previous functional magnetic resonance imaging (fMRI) studies have established digit somatotopy in primary somatosensory cortex (SI).
• High-resolution fMRI has resolved within-digit somatotopy in SI but failed to resolve spatial organization in secondary somatosensory cortex (SII).
• Long acquisition times for high-resolution fMRI spanning SI and SII have been a limitation.

Purpose of the Study:

• To investigate the topographic organization of somatosensory cortex (SI and SII) using ultra-high-field (7 Tesla) fMRI with multiband imaging.
• To achieve high spatial resolution for mapping body part representations at the individual subject level.
• To overcome limitations of previous fMRI studies regarding acquisition time and spatial resolution in SII.

Main Methods:

• Utilized 7 Tesla fMRI with multiband imaging to enhance blood oxygenation level dependent (BOLD) contrast and reduce acquisition time.
• Subjects (n=6 for body and digit mapping, n=2 for body imaging only) received vibrotactile stimulation to the face, hand digits, and foot.
• High spatial resolution (1.5 mm) was employed to map representations in SI and SII for both left and right body sides.

Main Results:

• Demonstrated an orderly representation of the face, hand digits, and foot in the contralateral SI for each individual subject.
• Observed a clear separation of face, hand, and foot representations in SII, although the spatial organization varied across individuals.
• Failed to resolve separate representations for individual hand digits within SII, even at 1.5 mm resolution, due to overlapping representations.

Conclusions:

• Ultra-high-field fMRI with multiband acceleration enables high-resolution mapping of somatosensory cortex (SI and SII) in individual subjects.
• While SI exhibits precise somatotopy, SII displays broader, more variable representations of body regions, with individual digit topography not discernible.
• Further advancements in imaging techniques or analysis may be needed to resolve fine-grained somatotopic organization within SII.