Original contributionEvent-related fMRI with painful electrical stimulation of the trigeminal nerve
Introduction
Anatomic and physiological studies in animals, as well as functional imaging studies in humans, have shown that besides the thalamus, multiple cortical areas are activated by painful stimuli: the primary somatosensory cortex, the secondary somatosensory cortex, the parietal operculum, the insula, the anterior cingulate cortex, and the prefrontal cortex [1], [2], [3]. These areas process most likely different aspects of pain [4]. The cerebral network activated by painful stimuli has been investigated with positron emission tomography (PET) as well as with functional magnetic resonance tomography (fMRI) by using block designs (for review, see [4]). For inducing pain, thermal, chemical, and electrical stimulations were applied [5]. One disadvantage of PET and fMRI studies with block design is the inferior temporal resolution. Higher temporal resolution can be achieved with magnetoencephalography (MEG) or electroencephalogramm (EEG). However, these techniques suffer from restricted spatial resolution [6], [7]. Event-related fMRI, on the other hand, may overcome these limitations due to the superior spatial resolution and its ability to use designs with fast adaptation (as expected in pain), which makes it possible to observe the propagation of excitation [8], [9]. Recent fMRI studies investigating somatosensory activation by using an event-related design were able to disentangle neuronal subprocessing within the nociceptive system [1], [9], [10].
The aim of the present study was to implement an event-related fMRI pain paradigm by applying electrical stimulation of the cranial nerve. One further aim of special interest was to investigate whether brain stem activations could be detected with this technique.
Section snippets
Subjects
The investigation was approved by the local ethics committee, and informed consent was obtained from all volunteers. Twelve healthy, right handed volunteers (8 men and 4 women, age range 26-42 years) participated in the study. The subjective individual pain threshold was tested by using an analog scale immediately before fMRI.
Imaging
Imaging was performed on a 1.5 T clinical scanner with a standard head coil (Magnetom Vision plus, Siemens Medical Systems, Erlangen). Functional images were acquired
Results
The electrical pulses caused painful stimulation of the first branch of the trigeminal nerve, which was associated with bilateral eye blinking for each stimulation due to an evoked brain stem blink reflex. The volunteers showed no adaptation to the randomized stimulation. The stimulation was perceived equally painful during the whole course of the experiment.
In all volunteers, electrically induced pain stimulation in the face and simultaneous fMRI examination was feasible. Only minor signal
Discussion
The present study demonstrates that event-related fMRI with electrical stimulation of cranial nerves is feasible. Painful stimulation of the trigeminal nerve can be used to visualize pain-related cortical activations in ipsi- and contralateral SII, contralateral insula, and thalamus. These results are consistent with activations observed with other pain paradigms (for a review, see [2]). Activation of the contralateral thalamus is known from experimental animal studies and PET and fMRI studies
Conclusion
These findings demonstrate that it is possible to perform event-related fMRI studies using short, painful electrical stimulation of cranial nerves, which appears promising for further investigations of cranial nerve pathologies, central post-stroke pain after brain stem or thalamic lesions, using adapted paradigm and postprocessing techniques.
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