Reduced functional connectivity of somatosensory network in writer's cramp patients

Abstract

Background: The involvement of motor cortex and sensorimotor integration in patients with writer's cramp (WC) has been well documented. However, the exact neurophysiological profile within the somatosensory system, including primary somatosensory cortex (SI), contralateral (SIIc), and ipsilateral (SIIi) secondary somatosensory areas remains less understood.

Methods: This study investigated the neuromagnetic cortical activities of median nerve stimulation in 10 patients with WC and 10 healthy controls (HC). To comprehensively explore all the aspects of somatosensory functioning, we analyzed our data with the minimum norm estimate (MNE), the time-frequency approach with evoked and induced activities, and functional connectivity between SI and SIIc (SI-SIIc), SI and SIIi (SI-SIIi), and SIIc and SIIi (SIIc-SIIi) from theta to gamma oscillations.

Results: No significant between-group differences were found in the MNE cortical amplitudes of SI, SIIc, and SIIi. Power strengths of evoked gamma oscillation and induced beta synchronization were also equivalent between WC and HC groups. However, we found significantly reduced theta coherence of SI-SIIi, alpha coherence of SI-SIIi and SIIc-SIIi, as well as beta coherence of SIIc-SIIi in patients with WC.

Conclusion: Our results suggest the involvement of somatosensory abnormalities, primarily with the form of functional connectivity, in patients with WC.

Keywords: Coherence; dystonia; magnetoencephalography (MEG); median nerve stimulation; secondary somatosensory cortex (SII).

Figure 1

(A) Spatiotemporal dynamics of minimum norm estimate (MNE) of somatosensory‐evoked fields (SEFs) following median nerve stimulation in healthy controls (HC, n = 10) and patients with writer's cramp (WC, n = 10). The grand‐averaged MNE activation is mapped onto the Montreal Neurological Institute Colin27 template. M20 and M35 of primary somatosensory (SI) activities are located in the anterior parietal cortex. Later‐latency responses of contralateral and ipsilateral secondary somatosensory areas (SIIc and SIIi, respectively) occur in the bilateral parietal operculum with stronger activation in the SIIc than the SIIi. (B) The statistical results show no significant differences between HC and WC in terms of M20, M35, SIIc, and SIIi activation amplitudes.

Figure 2

(A) Time‐frequency maps of evoked gamma oscillations in SI region. There is no difference in mean evoked gamma power (frequency range of 30 to 50 Hz and time range of 15 to 45 msec after stimulus onset). (B) Time‐frequency maps of induced beta event‐related synchronization (ERS) in SI region. Although the grand‐averaged plot shows a greater strength in the healthy controls, the statistical results exhibit no significant between‐group differences in terms of beta ERS peak amplitude.

Figure 3

Cortical coherence of theta (5–7 Hz), alpha (8–12 Hz), beta (13–30 Hz), and gamma (31–50 Hz) oscillations in HC and WC groups. For each frequency band, the functional connectivity between SI and SIIc (SI–SIIc) is stronger than that between SI and SIIi (SI–SIIi), as well as that between SIIc and SIIi (SIIc–SIIi). Patients with WC show specific reduction in SI–SIIi coherence in the theta and alpha frequency bands, and SIIc–SIIi coherence in the alpha and beta frequency band (*p < 0.05, **p < 0.01).