Toshiaki Imada

Somatic evoked high-frequency magnetic oscillations reflect activity of inhibitory interneurons in the human somatosensory cortex

High-frequency potential oscillations in the range of 300-900 Hz have recently been shown to concur with the primary response (N20) of the somatosensory cortex in awake humans. However, the physiological mechanisms of the high-frequency oscillations remained undetermined. We addressed the issue by analyzing magnetic fields during wakefulness and sleep over the left hemisphere to right median nerve stimulation with a wide bandpass (0.1-2000 Hz) recording with subsequent high-pass (> 300 Hz) and low-pass (< 300 Hz) filtering. With wide bandpass recordings, high-frequency magnetic oscillations with the main signal energy at 580-780 Hz were superimposed on the N20m during wakefulness. Isofield mapping at each peak of the high-pass filtered and isolated high-frequency oscillations showed a dipolar pattern and the estimated source for these peaks was the primary somatosensory cortex (area 3b) very close to that for the N20m peak. During sleep, the high-frequency oscillations showed dramatic diminution in amplitude while the N20m amplitude exhibited a moderate increment. This reciprocal relation between the high-frequency oscillations and the N20m during a wake-sleep cycle suggests that they represent different generator substrates. We speculate that the high-frequency oscillations represent a localized activity of the GABAergic inhibitory interneurons of layer 4, which have been shown in animal experiments to respond monosynaptically to thalamo-cortical input with a high-frequency (600-900 Hz) burst of short duration spikes. On the other hand, the underlying N20m represents activity of pyramidal neurons which receive monosynaptic excitatory input from the thalamus as well as a feed-forward inhibition from the interneurons.

Human somatosensory evoked magnetic fields to vibratory stimulation of the index finger : is there frequency organization in SI?

OBJECTIVEnFrequency organization in the human somatosensory cortex was studied.nnnDESIGN AND METHODSnSomatosensory evoked magnetic fields (SEFs) from 12 subjects were measured following vibratory stimulation of the index finger by using a 122 channel whole head SQUID system. Sensory stimuli comprising a 40 ms vibration at frequencies of 50, 100, 200 and 400 Hz were delivered to the volar surface of the tip of the right index finger. Using a single dipole model, the sources of the magnetic fields were estimated and mapped onto magnetic resonance images of each subject. The analysis of variance test (ANOVA) was used for statistics.nnnRESULTSnSource localization was determined on the main two peaks (M60 and M110) of the SEFs. All of the sources were located in the area 3b of somatosensory cortex (SI). There were no statistically significant differences between the locations of the dipoles evoked by different frequency stimulations.nnnCONCLUSIONSnThese results demonstrate the absence of systematic frequency organization at the hand representation area of the SI cortex. We speculate that high frequency vibration above 100 Hz are coded by the fast-spiking interneurons which synapse with Pacinian pyramidal neurons in SI.

Are there discrete distal-proximal representations of the index finger and palm in the human somatosensory cortex? A neuromagnetic study.

OBJECTIVEnThe distal-proximal representations of the finger and palm in the first somatosensory cortex (SI) were studied in humans.nnnMETHODSnSomatosensory evoked magnetic fields (SEFs) from 11 subjects were measured, following mechanical stimulation of the skin by using a 122 channel whole head SQUID system. Sensory stimulus comprising of a 10 ms vibration at the frequency of 200 Hz was delivered to 6 successive sites in 3 cm increments, along the distal-proximal direction over the volar surface of the right index finger and palm. Using a single dipole model, the sources of the magnetic fields were estimated and mapped onto magnetic resonance images of each subject. ANOVA was used for statistics.nnnRESULTSnSource localization was determined on the main peak (M50) of the SEFs. All of the sources were located in the area 3b of SI. Contrary to the well-defined distal-proximal representations in the hand area of simian SI cortex, there was no statistically significant differences between the locations of the dipoles in human SI cortex evoked by stimulation of different sites.nnnCONCLUSIONnThe result, however, should be interpreted with caution, because it cannot be denied that the spatial separation of sources in the distal-proximal somatotopy is beyond the resolving capacity of magnetoencephalography (MEG). In addition, at variance with the discrete distal-proximal gradient in the mechanoreceptor density, there was no statistically significant differences between the signal strengths of the dipoles for stimulation of the different locations.

Neuromagnetic measurements of the human primary auditory response

The spatio-temporal organization of the human primary auditory response (N19 and P30) was determined by analyzing magnetic fields from the right hemisphere evoked by monaural clicks to the left ear. The magnetic response consisted of peaks corresponding to electrical N19 (N19m) and P30 (P30m) within 40 msec after stimulation. However, the onset of electrical N19 occurred 5-9 msec earlier than that of the magnetic counterpart. Furthermore, the relative amplitude of N19 and P30 (N19/P30) was larger (more than 1) than that (about 0.5) of N19m and P30m (N19m/P30m). The findings suggest an additional contribution from a subcortical source for electrical N19. The estimated equivalent sources for N19m and P30m were localized at the postero-medial part of Heschls gyrus (the primary auditory cortex). Furthermore, the dipole orientation of N19m was directed postero-ventrally and that of P30m was in the opposite direction, which is compatible with the anatomy of the supratemporal primary auditory cortex. These results resolve a long standing controversy of cortical vs. subcortical structures as the generators of the N19.

Multichannel detection of magnetic compound action fields with stimulation of the index and little fingers

Magnetic compound action fields (CAFs) over the right arm were measured from 63 sensor positions with two 7-channel SQUID gradiometer systems following electrical stimulation of the index and little fingers as well as the ring finger separately. The wave forms of the CAFs were primarily biphasic, corresponding to the depolarization and repolarization currents of the stimulated nerves. Maximum amplitudes of the CAFs were 60-140 fT for the index finger stimulation and 40-90 fT for the little finger stimulation. The field mapping of the CAFs revealed a propagating quadrupolar pattern with different distributions for the index and little fingers. The results agree with the anatomical location of the median and ulnar nerves for the index and little finger stimulation respectively. The isofield maps, due to ring finger stimulation, showed complex patterns as a result of simultaneous activation of the median and ulnar nerves. By comparing the amplitudes of the maxima of the CAFs due to index finger stimulation with those after median nerve stimulation at the wrist, the numerical ratios of the constituent digital nerve fibers for the index finger within the median nerve at the wrist were estimated. The ratios of 0.14-0.41 (mean 0.27), determined with measurement of the CAFs, are fairly consistent with those calculated from the reported histological data.

Discrepancy between brain magnetic fields elicited by pattern and luminance stimulations in the fovea: Adequate stimulus positions and a measure of discrepancy

SummaryA conventional equivalent current dipole estimation provides one of the quantitative measures to evaluate the discrepancy between two single-dipole-like magnetic field patterns, though there is one problem; all stimulus positions in the visual field do not necessarily contribute to the generation of a single-dipole-like magnetic field. Another important problem occurs when the field pattern is complex and cannot be approximated by a dipole. This makes it difficult to evaluate the discrepancy between two magnetic field patterns by the dipole parameters. In this paper, we determined the stimulus positions adequate for generating single-dipole-like magnetic field patterns by evaluating the magnetic fields goodness-of-fit to the field generated by a single dipole. We propose to use a similarity (SIM) as a quantitative measure of the discrepancy between two complex magnetic field patterns. The SIM is defined as an angle between two magnetic field vectors. We evaluated the discrepancy between the 100 ms post-stimulus responses to pattern-reversal (Rv) stimulus, pattern-onset (Pat) stimulus, and luminance-onset (Lumi) stimulus. The following results were obtained: (1) Stimulation of some of the octants in the fovea, far from the vertical meridian, elicited a single-dipole-like magnetic field pattern at a latency of 100 ms, though stimulation of the central part of the fovea, and stimulation of the octants along the vertical meridian, did not elicit a single-dipole-like magnetic field pattern; (2) The discrepancy between responses was quantitatively evaluated by the SIM even if the field patterns were complex; (3) The SIM analysis showed that the discrepancy between the responses to the Rv and the Lumi stimuli, as well as that between the responses to the Pat and the Lumi stimuli, were greater than that between the responses to the Rv and the Pat stimuli.

Magnetic Detection of Activity of Presumed Inhibitory Interneurons in the Human Somatosensory Cortex

Previous studies of somatosensory evoked potentials (SEPs) and fields (SEFs) have shown that high-frequency signals in the range of 300–900 Hz concur with the primary response (N20 and N20m) of the somatosensory cortex [1],[2]. However, the physiological mechanisms of the high-frequency oscillations remained undetermined. We addressed the issue by analyzing magnetic fields during wakefulness and sleep over the left hemisphere to right median nerve stimulation. Results obtained from this study have been described in detail elsewhere [3].