Dodecapus: An MR-compatible system for somatosensory stimulation
Introduction
Topographic mapping is a fundamental organizing principle of sensory systems in the brain. In primary visual cortex (V1), adjacent locations receive inputs from adjacent photoreceptors on the retina, which is known as retinotopy. Similarly, different frequencies of sounds are represented in a tonotopic map in auditory cortex. Functional magnetic resonance imaging (fMRI) has been used to non-invasively reveal topographic maps in visual, auditory, somatosensory, and parietal cortices (Overduin and Servos, 2004, Sereno et al., 1995, Sereno et al., 2001, Sereno and Tootell, 2005, Servos et al., 1998, Servos et al., 1999, Talavage et al., 2004). Somatotopic mapping is more difficult than retinotopy and tonotopy, however. One major limitation for an fMRI experiment is that the physical stimulus device must be compatible with the scanner environment. Visual stimuli can be projected onto a plastic screen inside the scanner bore, and auditory stimuli can be delivered through MR-compatible headphones. In somatosensory experiments, physical touch or vibration on the body surface is required to elicit sensorimotor activations, and multiple stimulators are usually needed in order to stimulate different body parts. Active finger tapping and self-paced movements are commonly used for localizing sensorimotor sites in clinical scans. The timing, intensity, and coverage of manual stimulations generated by the subject or delivered by experimenters are not as consistent and precise as those driven by mechanical devices. However, most vibrotactile devices contain metals or electrical circuits and may not be compatible with the MR environment.
Several MR-compatible devices for somatosensory stimulation have been proposed. Flexible shafts made of carbon fiber have been successfully used to deliver vibrotactile stimuli mechanically in the scanner (Golaszewski et al., 2002a). That device does not use any metallic component inside the scanner, and produces precise vibrating frequencies and amplitudes. Magnetomechanical vibrotactile devices (MVDs) are made of MR-compatible coils, but are sensitive to placement and orientation inside the scanner (Graham et al., 2001). A piezoceramic vibrotactile stimulator generates 1–300 Hz vibrations, but requires high voltage to produce relatively small displacements (Harrington et al., 2000, Harrington and Downs, 2001, Francis et al., 2000, Gizewski et al., 2005, McGlone et al., 2002). Both magnetomechanical and piezoceramic vibrotactile devices lead electrical wires into the scanner, which may interfere with MR signal acquisition; furthermore, they may be heated by RF pulses if not shielded properly. Similar concerns may exist for experiments that apply electrical stimulation on the skin (Blankenburg et al., 2003, McGlone et al., 2002). Pneumatically driven vibrators and air-puff devices are devoid of electromagnetic interference because they use plastic tubes and MR-compatible materials inside the scanner (Briggs et al., 2004, Golaszewski et al., 2002b, Overduin and Servos, 2004, Servos et al., 1998, Servos et al., 1999, Stippich et al., 1999, Stippich et al., 2004, Zappe et al., 2004). Pneumatically driven devices usually generate tactile stimulation at lower frequencies (< 150 Hz), which are sufficient for eliciting somatosensory responses. Each of the aforementioned approaches for somatosensory stimulation has advantages and limitations. The selection of devices depends on their applications in various scientific and clinical contexts.
Accurate and detailed somatotopic mapping of the human body surface will improve basic understanding of the somatosensory system, guide neurosurgical planning, and assess plasticity and recovery after brain damage or body injuries (Borsook et al., 1998, Corbetta et al., 2002, Cramer et al., 2000, Cramer et al., 2003, Cramer and Bastings, 2000, Cramer and Crafton, 2006, Lee et al., 1998, Lee et al., 1999, Moore et al., 2000b, Ramachandran, 2005, Ramachandran and Rogers-Ramachandran, 2000, Rijntjes et al., 1997). Studies using fMRI have revealed somatotopic representations of the hand, fingers, wrist, elbow, shoulder, foot, toes, lips, and tongue in human brains (Alkadhi et al., 2002, Beisteiner et al., 2001, Blankenburg et al., 2003, Dechent and Frahm, 2003, Francis et al., 2000, Gelnar et al., 1998, Golaszewski et al., 2006, Hanakawa et al., 2005, Hlustik et al., 2001, Kurth et al., 2000, Lotze et al., 2000, McGlone et al., 2002, Miyamoto et al., 2006, Moore et al., 2000a, Overduin and Servos, 2004, Ruben et al., 2001, Servos et al., 1998, Stippich et al., 1999, Stippich et al., 2004, van Westen et al., 2004; also see reviews in Burton, 2002). The human face contains important sensory organs and is essential for verbal and nonverbal communications in daily life. However, only a few studies investigated somatotopy of the human face (including the lips and ears) using fMRI (Corbetta et al., 2002, DaSilva et al., 2002, Disbrow et al., 2000, Hodge et al., 1998, Iannetti et al., 2003, Miyamoto et al., 2006, Nihashi et al., 2002, Servos et al., 1999, Stippich et al., 1999) and other non-invasive and invasive techniques (Nakamura et al., 1998, Nevalainen et al., 2006, Nguyen et al., 2004, Nguyen et al., 2005, Sato et al., 2002, Sato et al., 2005, Schwartz et al., 2004, Yang et al., 1993). In most studies, only two or three locations on the face were stimulated manually or automatically. Somatotopic mapping of the whole face using fMRI is challenging because of the difficulty in delivering tactile stimuli to the face surrounded by a head coil. Most of the aforementioned MR-compatible devices have been used mainly for stimulation on the fingers. For instance, MVDs (Graham et al., 2001) were not tested for face stimulation inside the head coil. In addition, multiple stimulators are required to cover the whole face during the same scanning session. The arrangement and fixation of multiple stimulators with respect to the face inside the head coil remain a challenging task.
In this study, a computer-controlled MR-compatible pneumatic system (Dodecapus) was constructed and used to automatically and systematically deliver somatosensory stimuli around the whole face inside the head coil. Block-design and phase-encoded paradigms were used to map the locations and internal organization of face representation in motor, parietal, and primary and secondary somatosensory cortices. The phase-encoded technique has been successfully employed in retinotopic, tonotopic, spatiotopic, and somatotopic mapping experiments (Engel et al., 1994, Overduin and Servos, 2004, Sereno et al., 1995, Sereno et al., 2001, Servos et al., 1998, Servos et al., 1999, Talavage et al., 2004). In a different session, the same system and paradigms as in the face somatotopy experiment were used to map the lip and finger representations.
Section snippets
Participants
Six healthy right-handed subjects (2 males, 4 females; aged 20–30) participated in this study. All subjects participated in one fMRI session for face mapping, and four of them participated in one additional session for lip and finger mapping. Subjects gave informed consent, according to protocols approved by the Human Research Protections Program of the University of California, San Diego.
System design and setup
The Dodecapus system is composed of the following components (Fig. 1): a portable stimulus computer (XPC,
Results
Results of functional scans were rendered on inflated cortical surfaces using FreeSurfer (Dale et al., 1999, Fischl et al., 1999, Sereno et al., 1995, Sereno et al., 2001). Fig. 3, Fig. 4 illustrate somatotopic representations of the face, lips, and fingers of the same subject (JG). Fig. 4C shows a summary of somatotopic areas in this subject with outlines derived from the phase-encoded mapping scans (Fig. 3, Fig. 4). Table 1 summarizes the Talairach coordinates of multiple somatotopic areas
Discussion
Functional magnetic resonance imaging has become a routine tool in cognitive neuroscience but is currently less used for clinical studies. Experimental setup in the MR environment remains a challenging task, especially for somatosensory experiments. In this study, a computer-controlled, MR-compatible system was constructed and demonstrated that can deliver air puff tactile stimuli automatically inside or near the RF head coil in the scanner bore.
Fully automatic tactile stimulation could
Conclusions
Dodecapus, a computer-controlled MR-compatible system was constructed to automatically and independently deliver light air puffs to 12 locations on the body surface through a manifold inside the magnet. While we focused primarily on face stimulation, this flexible system can also deliver air puffs to lips, fingers and other body parts during the same scan. Two-condition block design paradigms were employed to localize the representations of the face, lips and fingers in the primary and
Acknowledgments
We thank Rick Buxton, Eric Wong, Tom Liu, and Larry Frank at the UCSD fMRI Center for scan time, pulse sequences, and advice, Larry May for hardware support, Ronald Kurz for machine shop help, and Laura Kemmer for pilot experiments and comments. Supported by NSF BCS 0224321 (Sereno), NIH R01 NS41925 (E. Wong), NIH R01 NS36722 (R. Buxton), NIH R01 HD041581 (J. Stiles), and The Swartz Foundation (S. Makeig and T.-P. Jung).
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