Ryosuke Takino

Dynamic activation of distinct cytoarchitectonic areas of the human SI cortex after median nerve stimulation.

MEG recordings visualized non-invasively a serial mediolateral activation of the human somatosensory 3b area followed by a stationary activation of area 1 after median nerve stimulation. Somatosensory evoked fields (SEFs) were recorded over the hand area contralateral to the right median nerve stimulation at the wrist in six normal subjects. A newly developed MEG vector beamformer technique applied to the SEFs revealed two distinct sources (areas 3b and 1) in the primary somatosensory cortex (SI) during the primary N20m-P22m response in all subjects. The first source was located in area 3b, which started to move sequentially toward mediolateral direction 0.7 ms prior to the peak of N20m and ended its movement 1.4 ms after the peak with a total distance of 11.2 mm. We speculate that the movement reflects a sequential mediolateral activation of the pyramidal cells in area 3b, which is mediated by horizontal connections running parallel to the cortical surface. The second source in area 1, located 5.6 mm medial and 4.2 mm posterior to the first source, was active 1.0 ms after the N20m peak. Then, the first source became inactive and the second source was dominant. In sharp contrast with the first source, the second source was stationary. The different behavior of these two components (moving vs stationary) indicates independent parallel inputs to area 3b and area 1 from the thalamus.

Movement interference attenuates somatosensory high-frequency oscillations: contribution of local axon collaterals of 3b pyramidal neurons

OBJECTIVES We examined the effects of movement interference on high-frequency oscillations (HFOs) and N20m in 10 healthy subjects. METHODS For the movement interference condition, somatosensory evoked magnetic fields (SEFs) following electric median nerve stimulation were recorded during voluntary movement of the digits. For the control condition, the SEFs were recorded without interference. The N20m and HFOs were separated by 3-300Hz and 300-900Hz bandpass filtering. Then, the peak-to-peak amplitudes were measured. RESULTS Both interference/control amplitude ratios for the N20m and HFOs were smaller than 100%. In contrast, the HFO/N20m amplitude index, which was calculated by dividing the interference/control amplitude ratio for the HFOs with that for the N20m, was significantly greater in the movement interference condition than in the control condition. CONCLUSIONS Although the overall amplitude of the HFOs was decreased by movement, enhancement of the HFOs by the movement was revealed by the HFO/N20m amplitude index. Thus, we suggest that the HFOs represent activity of the inhibitory interneurons excited by both thalamocortical afferent impulses and excitatory synaptic inputs from pyramidal neurons in area 3b through their local axon collaterals, thereby reflecting both feed-forward and feedback inhibitory effects onto the post-synaptic pyramidal neurons.

Is there training-dependent reorganization of digit representations in area 3b of string players?

OBJECTIVE The digit representations in area 3b were studied to examine whether there is training-dependent reorganization in string players. METHODS Somatosensory evoked magnetic fields were recorded following electrical stimulation of digits 1 (D1), 2 (D2) and 5 (D5) of both hands in 8 string players and of the left hand in 12 control subjects. The N20m and P30m responses, and high-frequency oscillations (HFOs) were separated by 3-300 Hz and 300-900 Hz bandpass filtering. RESULTS The dipole locations on the coronal plane and strengths of D1, D2 and D5, and D1-D5 cortical distance estimated at the peak of N20m or P30m did not differ between left and right hand in string players or between left hand in string players and controls. On the other hand, the dipole locations of D2 estimated from N20m and P30m and of D1 from N20m were significantly anterior, the D2-D5 distance from P30m longer, and the number of HFO peaks larger for D5 in string players than controls. CONCLUSIONS/SIGNIFICANCE With strong mutual competition among the fingering digits, the scale of reorganization should be much smaller as compared with the competition-free denervation-induced reorganizations. Taken together, the training-dependent reorganization of somatosensory cortex in string players is manifest not only in the enlarged cortical representation but also in the enhanced HFOs presumably representing activity of the fast-spiking interneurons.

Neural mechanisms for generation of tactile interference effects on somatosensory evoked magnetic fields in humans.

OBJECTIVES We examined modification of somatosensory evoked fields following electric middle finger stimulation with interference to the same and surrounding digits in 13 subjects. METHODS During electric middle finger stimulation, concurrent tactile stimulation was applied to the middle finger, to the index and ring fingers, and to the thumb and the little finger, individually. RESULTS The amplitudes of the N20m and the P30m were significantly reduced by the interference to the middle finger, and to the index and ring fingers. The former interference induced more prominent attenuation than the latter. The amplitudes of the P60m did not show significant changes by any kind of the interference. CONCLUSIONS The N20m and the P30m were attenuated according to the cortical distance between electrically and mechanically activated 3b areas. Pyramidal neurons are interconnected by intrinsic horizontal collaterals, even if their representations are segregated. The activation of the intrinsic collaterals induces direct excitation and indirect inhibition (via inhibitory interneurons) to the target pyramidal neurons. The result indicates that the activation of the intrinsic collaterals inhibits, on balance, the postsynaptic pyramidal targets, thereby generating the attenuation of the N20m and P30m.

Contribution of primary somatosensory area 3b to somatic cognition: a neuromagnetic study.

Interference effects on somatic cognition were compared with those on primary magnetic N20m responses. During tactile interference to various sets of digits, sensory thresholds for electric middle finger stimulation were measured, and then N20m was elicited with the intensity 4 mA above the sensory threshold measured without interference. After the recording, subjective magnitudes for the test stimuli were reported. Modifications of N20m and above psychophysical measures were dependent on the distance between electrically and mechanically activated areas. The differential N20m attenuation is considered to be generated within the neural circuitry in area 3b consisting of mechanically and electrically activated pyramidal neurons. The result indicates that such circuitry plays a fundamental role in magnitude estimation of somatic stimuli.

Specific somatosensory processing in somatosensory area 3b for human thumb: a neuromagnetic study

OBJECTIVES We examined the relation between somatosensory N20m primary responses and high-frequency oscillations (HFOs) after thumb and middle finger stimulation. METHODS Somatosensory evoked fields (SEFs) from 12 subjects were measured following electric stimulation of the thumb and middle finger. SEFs were recorded with a wide bandpass (3-2000 Hz) and then N20m and HFOs were separated by subsequent 3-300 and 300-900 Hz bandpass filtering. RESULTS The N20m peak-to-peak amplitude did not differ significantly between thumb and middle finger SEFs. In contrast, HFOs had a significantly larger number of peaks and were higher in the maximum amplitude and the total amplitude after thumb stimulation than after middle finger stimulation. CONCLUSIONS Our present data demonstrate a different relation between N20m and HFOs after thumb and middle finger stimulation. In view of the fact that the human thumb has uniquely evolved functionally and morphologically, the somatosensory information from the thumb will be processed differently for a fine motor control. We speculate that HFOs are generated by inhibitory interneurons in layer 4 in area 3b. Thus, enhanced activity of interneurons reflected by high amplitude HFOs exerts stronger inhibition on downstream pyramidal cells in area 3b for thumb stimulation.

High-frequency transcutaneous electrical nerve stimulation (TENS) differentially modulates sensorimotor cortices: An MEG study

OBJECTIVE Transcutaneous electrical nerve stimulation (TENS) affects excitability of the central motor system as well as the somatosensory system. To determine whether TENS has influence on excitability in the sensorimotor cortices of TENS-treated finger muscle, we investigated magnetoencephalogram associated with voluntary, self-paced finger movement before and after TENS. METHODS High-frequency TENS was applied on the extensor digitorum muscle for 15 min. Subjects underwent alternate middle finger and thumb extension movements before and after the TENS. We recorded movement-related cortical magnetic field (MRCF) associated with TENS-treated middle finger movement and that from untreated thumb movement. RESULTS The current source for motor field (MF) was located in the pre-central motor cortex and anteriorly-oriented, and that for motor evoked field one (MEF1) was found in the post-central somatosensory cortex and posteriorly-oriented. The amplitude of MF for TENS-treated middle finger movement decreased but unchanged for untreated thumb movement after TENS. The amplitude of MEF1 decreased for either finger movement after TENS. CONCLUSION High-frequency TENS to the forearm muscle modulates excitability of the limited area of motor cortex but wider area of primary somatosensory cortex. SIGNIFICANCE High-frequency TENS to the forearm muscle modulates excitability of the primary somatosensory cortex and motor cortex in a different manner.

Publication criteria for evoked magnetic fields of the human brain: A proposal

Magnetoencephalography (MEG) is a record of the magnetic fields produced by the electrical activities of the brain using MEG systems. There are three types of sensors for MEG systems: magnetometer and two types of gradiometer. Among them, two types of gradiometer, axial and planar, have been used worldwide. Unfortunately, the waveforms recorded by the two types of gradiometer are often different from each other. This poses a serious problem in comparing and evaluating the data from the two gradiometers. We consider that the MEG study should be published in a way that allows other workers using different types of gradiometer to evaluate and replicate the results of MEG studies. There have been, however, no publication criteria for reports of studies on stimulus-evoked or event-related magnetic fields in human subjects. In this article, we propose publication criteria for evoked or event-related magnetic fields of the human brain: original waveforms of selected channels covering a region of interest, a root mean-squared (RMS) waveform and a contour map at an appropriate time.

Functional connectivity between forearm and digits representations in human somatosensory area 3b.

OBJECTIVE We compared the effects of tactile interference to the forearm on magnetic responses evoked by electric stimulation of the little finger (D5) and the thumb (D1). METHODS Electric stimulation was delivered to D5 or D1 individually. In each stimulus session, magnetic recordings were conducted with or without concurrent tactile interference to the radial side of the anterior forearm. RESULTS With forearm interference, the amplitude of the primary response (N20m) following D5 stimulation was reduced to 90.7% of the control value without interference, while that following D1 stimulation was not affected (100.7%). CONCLUSIONS In human somatosensory area 3b, the representation of the forearm is immediately adjacent to that of the D5, and distant from that of the D1. Thus, the result suggests that the tactile interference effect on N20m depends on the cortical distance between electrically and mechanically activated 3b areas. SIGNIFICANCE Intrinsic synaptic connections between the 3b hand representation and its surroundings have been hypothesized as a neural basis for plastic changes of the human brain, such as a phantom hand phenomenon. The present finding implies that these connections may play some physiological roles even in normal adult humans.

P33-6 Subjective distinctness of tactile sensation is represented in the activity of human secondary somatosensory cortex (SII)

Y. Iguchi1, Y. Hoshi1, R. Takino2, T. Kimura3, I. Ozaki4, I. Kida1, T. Tsubokawa5, I. Hashimoto6 1Integrated Neuroscience Team, Tokyo Institute of Psychiatry, Japan, 2Dept. of Developmental and Clinical Psychology, Shiraume Gakuen Univ., Japan, 3Dept. of Acupuncture and Moxibustion, Tokyo Ariake Univ. of Medical and Health Sciences, Japan, 4Dept. of Physical Therapy, Faculty of Health Sciences, Aomori Univ. of Health and Welfare, Japan, 5Dept. of Anesthesiology, Graduate School of Medical Science, Kanazawa Univ., Japan, 6Kanazawa Institute of Technology, Japan