Single Modality Studies
Since the results of the single modality studies have already been reported in detail [7
], only a short summary of the procedures is given.
After approval of the local ethics committee and in accordance to the Declaration of Helsinki, 15 healthy, right handed volunteers had been recruited for each arm of the study.
FMRI data were acquired on a 1.5T MR scanner (Magnetom Symphony, Siemens, Germany).
Blood Oxygen Level Dependent (BOLD)-contrast was measured with an EPI sequence with 28 axial slices of 5mm thickness, 10% gap, and field of view 230mm. TR was 2600ms, TE 60 ms, Flip angle 90°. Resolution was 64 x 64. We used Cartesian read-out and a band-width of 2442 Hz/ Px.
Anatomical data were acquired using a sagitally oriented T1 weighted MP RAGE 3D sequence (magnetization prepared rapid acquisition of gradient echo [27
] equivalent to a fast SPGR-sequence) with isotropic 1mm³ voxels and a T2 turbo spin echo sequence (TR 2530ms, TE 99ms, FOV 230mm, matrix 256) with the same slice orientation as the BOLD Sequence.
The subjects were stimulated on their index finger tips on both sides subsequently; the order was assigned randomly.
Because of high interindividual differences of skin thickness and susceptibility to pain, individual thresholds established directly preceding the fMRI-experiment were used. The stimulus intensities were tested in a ramp style pattern with slowly rising stimulation levels as described in detail [7
] with the experimenter and the subject facing each other outside the scanner room. The level was readjusted again in position in the scanner to take into account the change of pain threshold during distraction [28
Rest: no stimulation / indifferent temperature.
Pain: the strongest painful stimulation the subject is able to endure for up to 52s.
For fMRI the protocol was presented on a PC, running ERTS (BeriSoft AG, Frankfurt, Germany) synchronized with the MR-scanner.
Electrical stimulation was administered to the index finger via 2 MRI-compatible adhesive ECG skin electrodes, placed 5 cm apart, by an electroneurograph (Myograph DA1 Tönnies, Freiburg, Germany) positioned outside the scanner room. The active electrode was placed at the volar tip of the index finger.
The pulse duration was set to 0.2 ms, 10 pulses per second.
Thermal stimulation was presented by a MRI-compatible Peltier thermode with a stimulation area of 30x30mm, on which the volar tip of the index finger was placed. The generator (Thermal Sensory Analyser II, Medoc Advanced Medical Systems, Rimat Yishai, Israel) was positioned outside the scanner room. The stimulation was applied with rising temperatures, until the desired maximal tolerable pain was reached.
The experiment was set to investigate cortical correlates of suppression of tonic painful stimuli.
The stimulation levels were chosen after another experiment with different stimulation levels. The maximally tolerated pain, however was the level for which both thermal and electrical stimulation aimed in this study. The tonic painful stimulation used stimulus duration of 52 s of maximum intensity with the task to suppress the feeling of pain. It started with a rest phase (26 s) before each stimulus phase. The whole set consisted of 6 repetitions of rest and stimulation. (see Fig. ).
Stimulation paradigm during the trials: Alternating phases of rest and painful stimulation. The tonic stimulation was divided into an early and late phase for purposes of analysis; the stimulation, however, was the same.
For the purpose of analysis, the tonic-painful stimulation paradigm was divided into the first 13s and the following 39s (tonic pain early, abbreviated tpe and tonic pain late, tpl). The duration of the early phase was deducted from experience in pre-trials, where the time to achieve suppression of pain was between 10 to 15 seconds which is similar to the early phase reported by others [29
Suppression of Pain
The subjects had to suppress the feeling of pain every time it arose. They were free to choose the technique for suppression. Some examples were given: Mental imagery (“Think of your last holiday”), depersonalisation techniques (“imagine extending your finger and shoving the pain away from you”) or distracting by other means.
Rating the Pain Experience
Although there have been recommendations made for simultaneous rating [9
], own experience [30
] showed the possibility of import of motor task related activation into an otherwise sensory paradigm. Therefore we decided to rate at the end of the session after the experiment. Subjects were familiar with a four level rating scale from a previous experiment using graded stimuli: The levels were modelled after established “anchors” [31
] and were explained as follows:
Level 1: No stimulation (This corresponds to the rest condition),
Level 2: A slight sensation, securely above the perception threshold,
Level 3: A strong sensation, but not painful,
Level 4: Pain.
The levels of subjective experience (pain level) after suppression of pain under constant stimulation were reported verbally immediately after termination of the experiment.
Differences regarding psychophysical or demographic parameters were computed using the t-test and Mann-Whitney test with p< 0.05 as a threshold for significance.
For the fMRI analysis of the trials and the group comparison, SPM 5 (Wellcome Department of Imaging Neuroscience, London, UK) [32
] was used. We used the scanner-inherent motion correction together with the motion correction of SPM. Afterwards the scans were normalized to the MNI-Template [34
] and smoothed with an isotropic Gaussian kernel of 8mm full-width at half maximum. Standard high- and low-pass filtering of SPM was used. For high-pass filtering: Session cut-off period was set to 156s. The low-pass filter was set to the option hrf (hemodynamic response function).
A group analysis using the random effects model was conducted to avoid problems with group comparisons [35
]. The cluster size had to exceed 10 voxels, a voxel threshold of p<0.001 uncorrected was applied to the resulting datasets; only activation appearing during both right and left hand stimulation (for lateralized functions the contralateral activation also counted) on the maps exceeding this threshold were taken to be significant.
Areas of activation were identified with the help of the Talairach Daemon [36
The contrasts are defined as follows:
Primary contrasts, they exist for right and left side stimulation:
Tonic pain early versus rest: Tpe-r (this refers to the first 13 seconds).
Tonic pain late versus rest: Tpl-r (including the 14th to 52nd second).
Secondary contrasts, they refer to the comparison of thermal and electrical stimulation:
Areas activated more in thermal than electrical stimulation: T-E.
Areas activated more in electrical than thermal stimulation: E-T.
The presented interaction analyses uses a primary and a secondary contrast in conjunction: it takes data from a primary contrast and explores which areas are more active during thermostimulation or electrostimulation.