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Introduction

The somatosensory modality presents unique challenges for human functional
neuroimaging studies. Unlike in the auditory and visual modalities, there are few (if any)
commercially available somatosensory stimulators designed for functional magnetic
resonance imaging (fMRI). While auditory and visual stimuli are routinely delivered to
subjects in the MR scanner, delivering somatosensory stimulation directly to the body
surface is more challenging due to the high magnetic fields near the scanner bore and
the sensitivity to radio-frequency interference of MR image acquisition.

Because of the difficulty of constructing suitable devices, many somatosensory
fMRI studies use manual stimulation, for instance stroking the skin of the subject with a
brush (Kell, et al. 2005) or sponge (Disbrow, et al. 2000), or by pressing plastic gratings
(Sathian, et al. 1997; Zhang, et al. 2005) or metal shapes (Weisser, et al. 2005) against
the skin. While effective, manual stimulation methods are less precise and reproducible
than automated stimulation. Direct electrical stimulation is another popular choice in
somatosensory fMRI studies (Kurth, et al. 1998; Ruben, et al. 2001) although electrical
stimulation has several disadvantages. Electrical stimulation activates a poorly-defined,
diffuse population of afferents that are unlike those activated by everyday sensory
stimuli, and electrical currents can be painful for the subject. In high-field strength MR
scanners, induced currents in the electrodes could cause heating and injury to the
subject.

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