Association between somatosensory sensitivity and regional gray matter volume in healthy young volunteers: a voxel-based morphometry study

Abstract

Two-point discrimination (2PD) test reflects somatosensory spatial discrimination ability, but evidence on the relationship between 2PD and cortical gray matter (GM) volume is limited. This study aimed to analyze the relationship between cortical GM volume and 2PD threshold in young healthy individuals and to clarify the characteristics of brain structure reflecting the individual differences in somatosensory function. 2PD was measured in 42 healthy (20 females) volunteers aged 20-32 years using a custom-made test system that can be controlled by a personal computer. The 2PD of the right index finger measured with this device has been confirmed to show good reproducibility. T1-weighted images were acquired using a 3-T magnetic resonance imaging scanner for voxel-based morphometry analysis. The mean 2PD threshold was 2.58 ± 0.54 mm. Whole-brain multiple regression analysis of the relationship between 2PD and GM volume showed that a lower 2PD threshold (i.e. better somatosensory function) significantly correlated with decreased GM volume from the middle temporal gyrus to the inferior parietal lobule (IPL) in the contralateral hemisphere. In conclusion, a lower GM volume in the middle temporal gyrus and IPL correlates with better somatosensory function. Thus, cortical GM volume may be a biomarker of somatosensory function.

Keywords: 2-point discrimination; VBM; gray matter; inferior parietal lobe; middle temporal gyrus.

Fig. 1

Results of 2PD measurements. (a) Mean psychophysical curve in 2PD measurement for all participants. (b) Individual 2PD threshold for all participants.

Fig. 2

Association between 2PD threshold and GM volume based on VBM analysis. Clusters in green indicate a significant positive association with 2PD threshold; that is, the lower the GM volume, the lower the 2PD threshold (higher somatosensory function). Clusters are focused in the left middle temporal gyrus and IPL (P < 0.05, correcting at cluster level using FWE correction). Clusters in red indicate a negative association; that is, the higher the GM volume, the lower the 2PD threshold (higher somatosensory function). Clusters are focused in the left cerebellum crus I (P < 0.001, uncorrected voxel level, with expected voxels per cluster, k, > 89 contiguous voxels).

Fig. 3

Relationship between 2PD threshold and GM volume at each brain region that showed significant correlations with the 2PD threshold. Each region is calculated using the mask image created with the AAL atlas. A total of 6 regions positively correlate with the 2PD threshold (P < 0.01); that is, the lower the 2PD threshold (better sensory function), the lower the GM volume. Meanwhile, 3 regions negatively correlate with the 2PD threshold (P < 0.01); that is, the lower the 2PD threshold, the higher the GM volume. All these regions are found in the cerebellum. (a) Lt. middle temporal gyrus, (b) Lt. cerebellum VIII, (c) Rt. middle frontal gyrus, (d) Lt. superior frontal gyrus (dorsolateral), (e) Rt. cerebellum crus II, (f) Rt. middle temporal gyrus, (g) Lt. IPL, (h) Rt. cerebellum VII, and (i) Lt. primary motor cortex.