Perceptual correlates of massive cortical reorganization

121

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...

Section: Discussionsupporting

confidence: 91%

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

Yang

Gallen²,

Schwartz³

et al. 1993

121

“…Following digital nerve block of the middle finger in the left hand, for example, the neurons of the left hemisphere-corresponding to the right middle finger-also showed an expansion of receptive fields. Such effects were observed in flying foxes as well as in pri- (10)(11)(12). In this essay, I will briefly summarize our findings for these two patients and will also describe some preliminary results from a third patient (FA).…”

Section: Plasticity In Adult Somatosensory Pathwaysmentioning

confidence: 81%

“…Using this procedure, we found that even stimuli applied to points remote from the amputation line were often systematically mislocalized to the phantom arm. Furthermore, the distribution of these points was not random (10). There appeared to be two clusters of points with one cluster being represented on the lower part of the face ipsilateral to the amputation.…”

Section: Plasticity In Adult Somatosensory Pathwaysmentioning

confidence: 92%

See 1 more Smart Citation

Behavioral and magnetoencephalographic correlates of plasticity in the adult human brain

Ramachandran

1993

Self Cite

272

112

“…However, on being touched in the face or on the upper arms, A.Z. has never experienced simultaneous sensations in her phantom upper extremities, and standard methods of stimulation (17,18) did not produce such referred sensations.…”

mentioning

confidence: 99%

Beyond re-membering: Phantom sensations of congenitally absent limbs

Brugger¹,

Kollias²,

Müri³

et al. 2000

199

Phantom limbs are traditionally conceptualized as the phenomenal persistence of a body part after deafferentation. Previous clinical observations of subjects with phantoms of congenitally absent limbs are not compatible with this view, but, in the absence of experimental work, the neural basis of such ''aplasic phantoms'' has remained enigmatic. In this paper, we report a series of behavioral, imaging, and neurophysiological experiments with a university-educated woman born without forearms and legs, who experiences vivid phantom sensations of all four limbs. Visuokinesthetic integration of tachistoscopically presented drawings of hands and feet indicated an intact somatic representation of these body parts. Functional magnetic resonance imaging of phantom hand movements showed no activation of primary sensorimotor areas, but of premotor and parietal cortex bilaterally. Movements of the existing upper arms produced activation expanding into the hand territories deprived of afferences and efferences. Transcranial magnetic stimulation of the sensorimotor cortex consistently elicited phantom sensations in the contralateral fingers and hand. In addition, premotor and parietal stimulation evoked similar phantom sensations, albeit in the absence of motor evoked potentials in the stump. These data indicate that body parts that have never been physically developed can be represented in sensory and motor cortical areas. Both genetic and epigenetic factors, such as the habitual observation of other people moving their limbs, may contribute to the conscious experience of aplasic phantoms.