Purpose

The studies of the mechanisms of plasticity and of the processes of re-organization occurring after brain lesions can benefit enormously from the improvements in spatial resolution and sensitivity afforded by functional MRI at 7T. Previous fMRI studies at 1.5T and 3T on sensorimotor reorganization showed difficulties in single subject analysis to separate sensory and motor activation due to limited spatial resolution and types of sensory stimulus1-3. Recent studies at 7T described finger somatotopy of all fingers4-8. However, these studies employed some form of mechanical and/or electrical stimulation, difficult to apply in clinical setting. Human touch was also used as stimulus in a 7T fMRI study to investigate cortical representation of individual fingers9, with the limitation of reproducibility. Recently, we have presented preliminary results at 7T on the possibility of investigating the primary somatosensory area by using a non-invasive, safe and reproducible tactile stimulus, suitable also in patients10. Here we report further data on the usability of this tool at different magnetic fields.

Methods

Five healthy right-handed volunteers (mean ± SD =29±7y) participated in the study. Three different motor and/or sensory tasks were used. The first task consisted of a gentle tactile stimulation (tactile task) obtained by applying a tactile stimulator developed ad hoc. The stimulator was realized by an MRI-compatible pneumatic system connected through flexible plastic tubes to little vesicles, able to be inflated and deflated at 1 Hz rate (Linari Engineering). The gentle stimulation was delivered throughout the application of each vesicle to the distal phalanx of a finger. In this study we investigated the thumb (D1) and the index (D2) of the right hand. The second task consisted of a simple finger tapping of two fingers (D1 and D2) at 1 Hz rate (motor task). In the third task, the two hand fingers were passively brushed by an external operator by means of a toothbrush, at the same frequency of 1 Hz (brush task). Data were acquired on a 1.5T HDxt, a 3T MR750 and a 7T MR950 scanners (GE Healthcare). Three functional series were acquired, according to a block design (“D1-rest-D2-rest” scheme, 4 repetitions) by using a GRE-EPI sequence (voxel = 1.5×1.5 ×1.5 mm3 at 7T; 2x2x2.5 mm3 at 3T; 3x3x3 mm3 at 1.5T). Analysis of fMRI data was performed using a combination of custom-written software (Matlab) and BrainVoyager. Statistical analysis was performed according to a GLM using a canonical hemodynamic function and one regressor for each condition, plus motion parameters as nuisance regressors. Data analysis was conducted in both Talairach’s and native space, depending on the type of analysis: multi-subject [q (FDR) < 0.05] and single-subject [p< 0.005] analyses respectively. On statistical maps, an imaginary line was traced connecting the activation foci at 7T of motor and tactile tasks (M1-S1 line). The t-values of each voxel along this line were extracted and plotted on a graph for each stimulus and magnetic fields.

Results

Figure 1 shows the activated areas for the group analysis at 7T. All the stimuli succeeded to activate their respective network in all subjects, as well as at 3T. In comparison to the other two tasks, tactile stimulus is more selective, eliciting a specific portion in the post-central gyrus (BA 3b-1-2), well reproducible among all subjects (single subject analysis). Among different magnetic field strengths, stimuli present similar patterns (Figure 2), except for the tactile stimulus that cannot elicit any response at 1.5T in the group analysis as well in four out five subjects. Profiles of activity calculated along the M1-S1 line are reported in Figure 3. Curves representing motor (blue colors) and brush (red colors) tasks present two peaks, one in front of and one behind the central sulcus. On the contrary, the tactile task shows, at 7T, a single narrow peak in the post-central region, that becomes wider and less significant decreasing the magnetic field strength.

Discussion and Conclusion

In motor and brush tasks of right hand, we found in the contralateral hemisphere a very extended activated area around the central sulcus, including both post – and pre-central cortices. No significant changes were observed varying the magnetic field strength. On the other side, tactile task was able to activate selectively a specific portion of S1 area in the post-central gyrus. These results suggest the joined use of motor and tactile tasks, in order to separate different contributions in primary sensorimotor cortex, distinguishing between sensory and motor components. Moreover, handiness and safety of tactile stimulator make it a useful tool to study somatosensory reorganization also in patients with brain lesion.

Acknowledgements

No acknowledgement found.

References

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