Human auditory cortex is activated by omissions of auditory stimuli

Cortical signals associated with infrequent tone omissions were recorded from 9 healthy adults with a whole-head 122-channel neuromagnetometer. The stimulus sequence consisted of monaural (left or right) 50-ms 1-kHz tones repeated every 0.2 or 0.5 s, with 7% of the tones randomly omitted. Tones elicited typical responses in the supratemporal auditory cortices. Omissions evoked strong responses over temporal and frontal areas, independently of the side of stimulation, with peak amplitudes at 145–195 ms. Response amplitudes were 60% weaker when the subject was not attending to the stimuli. Omission responses originated in supratemporal auditory cortices bilaterally, indicating that auditory cortex plays an important role in the brain's modelling of temporal characteristics of the auditory environment. Additional activity was observed in the posterolateral frontal cortex and in the superior temporal sulcus, more often in the right than in the left hemisphere.     

Auditory evoked M100 reflects onset acoustics of speech sounds

Magnetoencephalography (MEG) was used to investigate the response to speech sounds that differ in onset dynamics, parameterized as words that have initial stop consonants (e.g., /b/, /t/) or do not (e.g., /m/, /f/). Latency and amplitude of the M100 auditory evoked neuromagnetic field, recorded over right and left auditory cortices, varied as a function of onset: stops had shorter latencies and higher amplitudes than no-stops in both hemispheres, consistent with the hypothesis that M100 is a sensitive indicator of spectral properties of acoustic stimuli. Further, activation patterns in response to stops/no-stops differed in the two hemispheres, possibly reflecting differential perceptual processing for the acoustic–phonetic cues at the onset of spoken words.     

Magnetoencephalographic Localization of Subdural Dipoles in a Patient with Temporal Lobe Epilepsy

We report magnetoencephalographic localization of subdural electrode dipoles placed at the basal and mesial surfaces of the temporal lobe in a patient with temporal lobe epilepsy. The locations of the three dipoles were predicted from their magnetic fields with a computer model of the head as a conducting sphere. The predicted locations were within 1, 3, and 4 mm of the actual locations. These results, obtained in an area of the brain from which epileptiform discharges are frequently recorded, strongly support the capability of magnetoencephalography to accurately localize electrical events in this brain region.     

Evidence for multiple generators in evoked responses using finite difference field mapping: Auditory evoked fields

Summary Electric potential maps and magnetic field maps have been used to study brain electrical activity. During the temporal course of an evoked cortical response, the electrical activity of specific neuronal subpopulations change in a sequential manner giving rise to measurable electrical potentials and magnetic fields. For these potentials and fields, both the amplitude and rate of amplitude change have characteristic, time-dependent waveforms. Presently, amplitude waveforms from multiple locations are used to generate magnetic field and electric potential maps which have been found to be useful in understanding the activity of the neurons which give rise to these maps (Romani 1990). This paper introduces a data transformation technique which results in a derived map that we have termed a "finite difference field map" (FDFM). This mapping technique provides information associated with the rate at which the amplitude of the neuronal electric activity changes. In this paper, some advantages of FDFM analysis are illustrated by application of this technique to the study of the auditory evoked cortical field (AECF) N1m waveform. Using data obtained from normal subjects it will be demonstrated that application of the FDFM technique allows the localization of the primary N1m source at an earlier latency than is possible using the conventional waveform data. The source location determined at an early latency by FDFM analysis was identical to that obtained at later latency from the conventional field data. These data suggest that the primary N1m source is stationary. In addition, analysis of the time sequence of FDFM field maps contains evidence of a second spatially separate source which is co-active with primary N1m source.     

Motor cortical epilepsia partialis continua in a patient with a localized sensory cortical lesion

We describe a 33-year-old man with cyclosporine encephalopathy who showed continuous jerking in the left upper limb due to epilepsia partialis continua. Jerk-locked back averaging (JLA) of magnetoencephalogram disclosed a spike preceding the jerk localized at the hand motor area, whereas JLA of electroencephalogram revealed no premyoclonus spikes. The paired-pulse motor cortical transcranial magnetic stimulation revealed motor cortical hyperexcitability, while the paired-pulse somatosensory evoked potential showed no sensory cortical hyperexcitability. The brain MRI showed a high intensity lesion localized at the hand sensory area. These results suggest that the jerks were produced by discharges at the motor cortex probably disinhibited by the sensory cortical lesion.     

Magnetoencephalographic responses to painful impact stimulation

Magnetoencephalographic (MEG) field recordings are unique to detect current dipoles in SI and SII. Few devices are available for painful mechanical stimulation in magnetically shielded MEG rooms. The aim of the present MEG (dual 37-channel biomagnetometer) study was to investigate the location of the cortical generators evoked by painful impact stimuli of different intensities. An airgun was placed outside the shielded MEG room, and small plastic bullets were fired at the arm and trunk of the subjects in the room. The velocity of the bullet was measured and related to the evoked pain intensity. Stimuli were delivered for each of the following three conditions: strong pain intensity elicited from the upper arm and upper trunk; weak pain intensity elicited from the upper trunk. The evoked MEG responses had a major component with the characteristically polarity-reversal deflections indicating a dipole located beneath the coils. The response could be estimated by a single current dipole. When the estimated locations of the dipoles were superimposed on the individual magnetic resonance images (MRIs), consistent bilateral activation of areas corresponding to the secondary sensory cortices (SII) was found.     

An evaluation of dipole reconstruction accuracy with spherical and realistic head models in MEG.

MEG forward problem has been solved for about 2000 dipoles placed on the brain surface using a very fine 3-layer realistic model of the head and the boundary element method (BEM). For each dipole, spherical models, one-layer realistic BEM models and coarser 3-layer realistic BEM models, were used to reconstruct the dipole. It was found that the localization bias induced by using a spherical model of the head increased from 2.5 mm in the upper part of the head to 12 mm in the lower part, on average. It was also found that, for the same computing time, a 3-layer model of the head gave on average 2 mm better localization errors than a one-layer model of the head. Orientation errors of less than 20 degrees could only be retrieved with a 3-layer realistic model. Localization and orientation errors highly depended on the dipole position in the brain.     

脑磁源的定位计算的最优化

脑磁源的定位是脑磁图（Ｍａｇｎｅｔｏｅｎｃｅｐｈａｌｏｇｒａｐｈｙ，ＭＥＧ）研究的一个基本问题。该问题属不定问题，即根据探头测量的微小磁场所建立的方程组求得的未知脑磁源有无穷组解。因此需要通过合理的数学模型补充适当信息。目前的两种主要的数字模型为偶极子源搜索法和Ｌｐ最小模法。对前者若采用全局搜索费时太多，若采用梯度类搜索法又容易收敛到局部极值点。故一般得用其他方法引导，定出初值，再使用梯度类搜索法