Noninvasive somatosensory homunculus mapping in humans by using a large-array biomagnetometer.

Abstract

To validate the feasibility of precise noninvasive functional mapping in humans, a large-array biomagnetometer was used to map the somatosensory cortical locations corresponding to numerous distinct tactile sites on the fmgers, hand, arm, and face in different subjects. Source localizations were calculated by using a single equivalent current dipole (ECD) model. Dipole localizations were transposed upon the corresponding subject's magnetic resonance image (MRI) to resolve the anatomic locus of the individual dipoles within a given subject. Biomagnetic measurements demonstrated that (i) there were distinct separations between the ECD locations representing discrete sites on the face and hand; (u) the ECD localizations from facial sites clustered in a region inferior to ECD localizations from hand and digit sites; and (iii) there was clear spatial resolution of ECD locations representing closely spaced tactile sites on the hand and face. The ability of magnetoencephalography (MEG) to provide high-resolution spatial maps of the somatosensory system noninvasively in humans should make MEG a useful tool to defme the normal or pathological organization of the human somatosensory system and should provide an approach to the rapid detection of neuroplasticity.Functional mapping of the human somatosensory system has commonly used invasive surgical techniques which involve electrical stimulation of the brain (1), direct recordings of evoked potentials and electrical stimulation (2), somatosensory evoked responses (SERs) recorded on electrocorticography (ECoG) (3), or cortical surface recordings of somatosensory evoked potentials (SEP) during surgery (4, 5). The invasiveness of these approaches has limited the number of patients which may be studied and the types of questions which may be addressed. However, a variety of neuroimaging tools have been developed which may noninvasively study human mental functions. The human somatosensory cortex has been partially mapped using positron emission tomography (PET) (6), electroencephalography (EEG) (7-9), and magnetoencephalography (MEG) (10)(11)(12)(13)(14)(15)(16)(17)(18) METHODS Somatosensory stimulus-evoked magnetic brain activity generated by the left and right cortex in two neurologically normal undergraduate male subjects was recorded inside of a magnetically shielded room by using a Magnes 37-channel biomagnetometer (Biomagnetic Technologies, San Diego). The neuromagnetic field pattern was recorded over a 144-mm-diameter circular area above the parietotemporal cortex. Intrinsic noise in each channel was <10 ff/Hzl/2 in all but one channel.The biomagnetometer was placed over the contralateral hemisphere relative to the side being stimulated. Subjects were instructed to hold extremely still, and to count silently the number of stimuli.Tactile stimulators provided skin surface stimulation. The stimulators, which were circular rubber bladders of 1 cm diameter encased within a plastic outer shell, expanded with air during each time period corresponding to a single stim...

Functional magnetic resonance imaging in dermatology: The skin, the brain and the invisible

“…[34][35][36][37][38][39][40][41] Numerous EEG studies addressing neurosyphilis, neuro-Behcet's, xeroderma pigmentosum and lipoid proteinosis later followed. [42][43][44][45] In the early 1990s, MEG was used to map the somatosensory cortex, [46] investigate central itch/pain processing [47][48][49] and brain activity in epidermal nevus syndrome. [50] In 1994, the first dermatology-related study using positron emission tomography (PET) was published.…”

Dodecapus: An MR-compatible system for somatosensory stimulation

“…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, 2004Nguyen et al, , 2005Sato et al, 2002Sato et al, , 2005Schwartz et al, 2004;Yang et al, 1993). In most studies, only two or three locations on the face were stimulated manually or automatically.…”

The separation of overlapping neuromagnetic sources in first and second somatosensory cortices

“…As it has been shown, both in electrophysiologal studies in animals carried out since the late thirties (Marshall et al 1941) and in invasive investigations in human patients (Penfield and Rasmussen 1950), the SI and SII are somatotopically organized. (-..../ The measurement of magnetic fields resulting from thousands of cells and spanning several mm 2 of neuronal mass activity permits the exact localization of the cortical somatosensory map in a completely noninvasive manner (Hari et al 1993;Hari et al 1991;Elbert et al 1994;Yang et al 1993). The MEG is particularly sensitive to the activity in the fissural cortex in area 3b, since this activity generates current dipoles which are more or less tangentially oriented to the surface of the head.…”