Amplitude and timing of somatosensory cortex activity in task-specific focal hand dystonia

Abstract

Objective: Task-specific focal hand dystonia (tspFHD) is a movement disorder diagnosed in individuals performing repetitive hand behaviors. The extent to which processing anomalies in primary sensory cortex extend to other regions or across the two hemispheres is presently unclear.

Methods: In response to low/high rate and novel tactile stimuli on the affected and unaffected hands, magnetoencephalography (MEG) was used to elaborate activity timing and amplitude in the primary somatosensory (S1) and secondary somatosensory/parietal ventral (S2/PV) cortices. MEG and clinical performance measures were collected from 13 patients and matched controls.

Results: Compared to controls, subjects with tspFHD had increased response amplitude in S2/PV bilaterally in response to high rate and novel stimuli. Subjects with tspFHD also showed increased response latency (low rate, novel) of the affected digits in contralateral S1. For high rate, subjects with tspFHD showed increased response latency in ipsilateral S1 and S2/PV bilaterally. Activation differences correlated with functional sensory deficits (predicting a latency shift in S1), motor speed and muscle strength.

Conclusions: There are objective differences in the amplitude and timing of activity for both hands across contralateral and ipsilateral somatosensory cortex in patients with tspFHD.

Significance: Knowledge of cortical processing abnormalities across S1 and S2/PV in dystonia should be applied towards the development of learning-based sensorimotor interventions.

Figure 1

Somatosensory Paradigm. 1. Low rate (mean ISI; 2s) digit 2 (D2) stimulation (black bars alone). 2. High rate (mean ISI; 0.33s) D2 stimulation alone. 3. Oddball D2 stimulation with ipsilateral D3 stimulation as the standard stimulus (white bars). RD3 and LD3 were used as standards with RD2 and LD2 as deviants respectively in the healthy control group. The most affected digit on the affected hand of the tspFHD group was used as the deviant with the adjacent unaffected or less affected digit used as the standard and the corresponding digits matched on the unaffected hand of the tspFHD group.

Figure 2

Summary of MEG sensor data during tactile stimulation of the second digit (RD2). Averaged MEG sensor data (A) produces two peaks in primary somatosensory (S1) and secondary somatosensory/parietal ventral cortex (S2/PV). Equivalent current dipole fits (B) and sensor topography maps (C) of these sensor peaks place these sources in the contralateral (left) hemisphere.

Figure 3

Average amplitude (RMS) over time: Oddball condition. Average of the peak sensor amplitude (RMS: root mean square) from the time of the somatosensory stimulus at 0 msecs to 150 msecs after the stimulus for the ipsilateral (A) and contralateral (B) hemispheres. (B) is represented for the affected hands of subjects with tspFHD, the unaffected hands of subjects with tspFHD, and the right and left hands of healthy volunteers. The approximate early (S1) and late (S2/PV) peaks are marked (in red lines). Similar waveforms occur for both the low rate and high rate conditions.

Figure 4

Dipole Moment (SQ) in S1 and S2/PV. Average of the peak sensor dipole moment (Q) for the ipsilateral and contralateral early S1 (30-70msecs) and late S2/PV (70-130msecs) responses under the three experimental conditions is compared between the affected hands of subjects with tspFHD, the unaffected hands of subjects with tspFHD, and the right and left hands of healthy volunteers. The three experimental conditions include the deviant stimuli at low rate (condition 1 in Figure 1, mean ISI: 2 secs), the deviant plus the standard stimuli or oddball condition (condition 3 in Figure 1, mean ISI: 0.33 secs), the deviant at high rate (condition 2 in Figure 1, mean ISI: 0.33 secs). Asterisks indicate the responses that are significantly different (p<0.05) from others that are connected by lines. A) Average dipole moment of contralateral S1 to the hand stimulated. B) Average dipole moment of ipsilateral S1 to the hand stimulated. C) Average dipole moment of contralateral S2/PV to the hand stimulated. D) Average dipole moment of ipsilateral S2/PV to the hand stimulated.

Figure 5

Latency of S1 and S2/PV responses. Average latency at the peak sensor amplitude (RMS) for the ipsilateral and contralateral early S1 (30-70msecs) and late S2/PV (70-130 msecs) responses under three experimental conditions is compared between the affected hands of subjects with tspFHD, the unaffected hands of subjects with tspFHD, and the right and left hands of healthy volunteers. The three experimental conditions include the deviant stimuli at low rate (condition 1 in Figure 1, mean ISI: 2 secs), the deviant plus the standard stimuli or oddball condition (condition 3 in Figure 1, mean ISI: 0.33 secs), the deviant at high rate (condition 2 in Figure 1, mean ISI: 0.33 secs). Asterisks indicate the responses that are significantly different (p<0.05) from others that are connected by lines. A) Average latency in contralateral S1 to the hand stimulated. B) Average latency in ipsilateral S1 to the hand stimulated. C) Average latency in contralateral S2/PV to the hand stimulated. D) Average latency in ipsilateral S2/PV to the hand stimulated.

Figure 6

Clinical performance measures. Average clinical measures for tspFHD affected hand, tspFHD unaffected hand, and normative data from healthy controls including (A) sensory measures of graphesthesia and stereognosis, (B) motor speed measures of tapping speed using a Tapper and digital reaction time measured with a stopwatch, (C) grip, 3 chuck pinch, lateral pinch, and lumbrical strength, and (D) functional independence measured by the Café 40 questionnaire.

Figure 7

Scatter plots of the significant correlations (p<0.05) between MEG measures and clinical performance measures. A) Correlation between the sensory score (sum of the percentage correct answers for the measures of graphesthesia and stereognosis) and the latency at the peak amplitude in ipsilateral S1 of the hand stimulated for the high rate condition in the affected hands of subjects with tspFHD. B) Correlation between the motor score (sum of the average scores of the finger tapper and stopwatch) and the latency at the peak amplitude in ipsilateral S2/PV of the hand stimulated for the low rate condition in the affected hands of subjects with tspFHD C) Correlation between the sum strength score (grip, 3 jaw chuck pinch, lateral pinch, and lumbricals) and the latency at the peak amplitude in ipsilateral S2/PV of the hand stimulated for the low rate condition in the affected hands of subjects with tspFHD.