Changes in somatosensory evoked responses by repetition of the median nerve stimulation

OBJECTIVE: We investigate the synaptic factor for the recovery function of evoked responses using a repetitive stimulation technique. METHODS: Somatosensory evoked cortical magnetic field (SEF) was recorded following stimulation of the median nerve using single to 6-train stimulation in 8 healthy subjects. The SEF responses after each stimulus in the train stimulation were extracted by subtraction of the waveforms. RESULTS: An attenuation of the SEF components was recognized after the second of the stimuli, but there was no significant attenuation with the third or later stimulations. The root mean square (RMS) of the 1M (peak latency at 20 ms after stimulation) and 4M (70 ms) components were smaller than that of the single stimulation during the train stimulation, while the 2M (30 ms) and 3M (45 ms) components were not attenuated, but the 3M was facilitated at the fourth to sixth stimulation. CONCLUSION: The synaptic factor was not responsible for the attenuation of the SEF components during repetitive stimulation in healthy subjects. The SEF change disclosed a functional difference among the SEF components during the train stimulation, especially among the later components.     

Task-specific magnetic fields from the left human frontal cortex

Summary In this study we attempted to extend our previous results on regional specialization of frontal cortical function in humans, by means of magnetoencephalography (MEG). We used a verbal task and predicted that some part of the left frontal lobe would be active during engagement in that task, since the left hemisphere is known to be implicated in language. We did not require a motor response because in previous experiments we observed bilateral frontal magnetic activity, and we suspected that it was due to the addition of movement-related fields to our recordings. Six right handed subjects (three males and three females) participated in the study. The task consisted in silently counting the number of word pairs that matched with respect to semantic category. Experimental runs were composed by series of 120 trials or word pairs. All six subjects presented dipolar magnetic field distributions on the left fronto-temporal area of the scalp, but not on the right, during different portions of the trial duration. These fields were successfully modeled as equivalent current dipoles (ECDs). The spatial ECD coordinates were translated onto magnetic resonance image (MRI) coordinates for each subject. The dipole positions were typically near the cortical surface corresponding to areas 6 and 44 of Brodmann. No dipole-like sources were observed in the right frontal lobe.This research was supported by grant NS 29540-005A1 from the National Institutes of Health, Washington, D.C.     

Three hands: fragmentation of human bodily awareness

We describe patient E.P. who occasionally perceives a ‘ghost' hand which copies the previous positions of the left hand with a 0.5–1 min time lag, but follows the movement patterns of the right hand. The symptoms started after an operation of a ruptured aneurysm, followed by an infarction of the right frontal lobe; E.P. also has a previously lesioned corpus callosum. Neuromagnetic recordings revealed that activity of the left secondary somatosensory cortex was strongly suppressed during the ghost arm percept, thereby providing an objective correlate for E.P.'s sensations. We conclude that simultaneous mental contents about body scheme may be based on neural information extracted at considerably different times, resulting in fragmentation of bodily awareness.     

Activation of the human posterior parietal cortex by median nerve stimulation

We recorded somatosensory evoked magnetic fields from ten healthy, right-handed subjects with a 122-channel whole-scalp SQUID magnetometer. The stimuli, exceeding the motor threshold, were delivered alternately to the left and right median nerves at the wrists, with interstimulus intervals of 1, 3, and 5 s. The first responses, peaking around 20 and 35 ms, were explained by activation of the contralateral primary somatosensory cortex (SI) hand area. All subjects showed additional deflections which peaked after 85 ms; the source locations agreed with the sites of the secondary somatosensory cortices (SII) in both hemispheres. The SII responses were typically stronger in the left than the right hemisphere. All subjects had an additional source, not previously reported in human evoked response data, in the contralateral parietal cortex. This source was posterior and medial to the SI hand area, and evidently in the wall of the postcentral sulcus. It was most active at 70–110 ms.     

Lateralization of Cerebral Activation in Auditory Verbal and Non-Verbal Memory Tasks Using Magnetoencephalography

The magnetic flux normal to the scalp surface was measured with a whole-head neuromagnetometer while right-handed subjects (N = 15) were engaged in either an auditory word- or a tone-recognition task. Sources of the recorded magnetic fields were modeled as equivalent current dipoles at 4 ms intervals and the number of sources in the later portion of the magnetic response was used as an index of the degree of brain activation. Significantly more sources were found in the left as compared to the right hemisphere in the word but not the tone task on a group basis. On an individual basis, 13/15 subjects had more sources in the left as compared to the right hemisphere during the word task, while in the tone task 3/10 subjects showed this pattern. Sources of activity were found in the left superior and middle temporal gyri in all subjects with available MRI scans. Sources were also found in the supramarginal gyrus and in medial temporal areas, including the hippocampus, in the majority of cases. MEG appears to be a promising tool for detecting activity in cerebral areas specialized for language and memory function.     

Responses to Median and Tibial Nerve Stimulation in Patients with Chronic Neuropathic Pain

Somatosensory evoked magnetic fields and electrical potentials were measured in eight patients with unilateral neuropathic pain. After median nerve stimulation on the painful side, the amplitudes of the evoked responses were enhanced 2 to 3 times at a latency of about 100 ms compared to the responses of the contralateral, unaffected side. After posterior tibial nerve stimulation an enhancement was found at latencies around 110 ms and 150 ms. The scalp distribution of the magnetic field at the latencies of abnormal responses was dipolar and the responses could be ascribed to a current dipole. Three (of the eight) patients underwent spinal cord stimulation (SCS) for their pain. The enhancement of the evoked responses to stimulation of the painful side decreased after spinal cord stimulation. After a long period of spinal cord stimulation only (e.g., a year) during which the patient reported to be pain free, these abnormal responses were no longer observed.     

Responses of the human auditory cortex to changes in one versus two stimulus features

Neuromagnetic responses were recorded with a 24 SQUID magnetometer in two oddball experiments to determine whether mismatch responses to changes in single stimulus features are additive. In experiment 1, the one feature deviants differed from standards in interstimulus interval (ISI) or frequency, and the two feature deviants in both ISI and frequence. In experiment 2, deviants differed in duration, frequency, or both. All deviants evoked a mismatch field (MMF) with sources close to each other in the supratemporal auditory cortex. Except for the ISI deviants, the MMF sources were about 1 cm anterior to the source of the 100ms response, N100m, to the standards. In the two experiments, MMFs obtained in response to the two feature deviants resembled closely the sum of MMFs in response to one feature deviants. The results suggest that the standards leave a multiple neuronal representation in the human auditory cortex. The particular neuronal traces of the representation react independently to changes in different features of sound stimuli.     

Atypical organisation of the auditory cortex in dyslexia as revealed by MEG

Neuroanatomical and -radiological studies have converged to suggest an atypical organisation in the temporal bank of the left-hemispheric Sylvian fissure for dyslexia. Against the background of this finding, we applied high temporal resolution magnetoencephalography (MEG) to investigate functional aspects of the left-hemispheric auditory cortex in 11 right-handed dyslexic children (aged 8–13 years) and nine matched normal subjects (aged 8–14 years). Event-related field components during a passive oddball paradigm with pure tones and consonant–vowel syllables were evaluated. The first major peak of the auditory evoked response, the M80, showed identical topographical distributions in both groups. In contrast, the generating brain structures of the later M210 component were located more anterior to the earlier response in children with dyslexia only. Control children exhibited the expected activation of more posterior source locations of the component that appeared later in the processing stream. Since the group difference in the relative location of the M210 source seemed to be independent of stimulus category, it is concluded that dyslexics and normally literate children differ as to the organisation of their left-hemispheric auditory cortex.     

应用磁源性影像对健康青年人大脑听觉皮层功能区定位的初步研究

目的　探讨磁源性影像在大脑听觉皮层功能区定位的应用价值。方法　应用全头型30 6通道生物磁仪 ,对 30例健康青年人 (2 0～ 32岁 )男 16例 ,女 14例进行了大脑听皮层诱发磁场的测试 ,5例观察了 0 5、1、2、4、8kHz纯音诱发的脑磁反应波M10 0 ,另 2 5例均测试了 2kHz纯音诱发的脑磁反应波M10 0 ,并将脑磁图的电生理资料与磁共振成像的解剖结构资料叠加 ,获得大脑听觉皮层磁源性影像 ,以确定M10 0在大脑初级听觉皮层的位置。结果　 30例受试者均可诱发出M10 0 ,且重复性好 ,其位置均位于大脑的颞横回 ;不同频率的纯音诱发的M10 0在大脑颞横回有其各自的定位和分布 ;初级听觉皮层在两侧大脑半球的位置相对而言左侧靠后 ,右侧靠前 ,两侧明显不对称。M10 0潜伏期在左侧和右侧大脑半球对侧声刺激均较同侧声刺激为短 ,但M10 0等电流偶极的位置与刺激声的侧别无关。结论　由脑磁图的M10 0所获得的磁源性影像 ,具有良好的时间和空间分辨率 ,可对大脑初级听皮层功能区进行精确定位 ,将在耳科学的临床和基础研究中具有广阔的应用前景。     

Influence of unfused cranial bones on magnetoencephalography signals in human infants

ObjectiveTo clarify the effects of unfused cranial bones on magnetoencephalography (MEG) signals during early development.MethodsIn a simulation study, we compared the MEG signals over a spherical head model with a circular hole mimicking the anterior fontanel to those over the same head model without the fontanel for different head and fontanel sizes with varying skull thickness and conductivity.ResultsThe fontanel had small effects according to three indices. The sum of differences in signal over a sensor array due to a fontanel, for example, was < 6% of the sum without the fontanel. However, the fontanel effects were extensive for dipole sources deep in the brain or outside the fontanel for larger fontanels. The effects were comparable in magnitude for tangential and radial sources. Skull thickness significantly increased the effect, while skull conductivity had minor effects.ConclusionMEG signal is weakly affected by a fontanel. However, the effects can be extensive and significant for radial sources, thicker skull and large fontanels. The fontanel effects can be intuitively explained by the concept of secondary sources at the fontanel wall.SignificanceThe minor influence of unfused cranial bones simplifies MEG analysis, but it should be considered for quantitative analysis.