[Physical basis of the generation of neuromagnetic fields]

Electrical processes at the neuronal level are considered, which cause the neuromagnetic fields. Those fields presently measured are due to the synchronous activity of about 10(5) neurons. Short magnetic signals can be caused by afferent and efferent strains of action potentials, the longer ones being due to the synaptic activity at the ends of apical dendrites in the fissural cortex.

Neuromagnetic studies of somatosensory system: principles and examples

A review is given about the basic principles of neuromagnetic recordings and somatically evoked magnetic fields with examples from the authors' own work. MEG provides good spatial resolution for activity in the fissural cortex. It has, for example, allowed differentiation of current sources in the primary and secondary somatosensory cortices. Some cortical areas activated by painful stimuli have also been localized by means of MEG.

A four-channel SQUID magnetometer for brain research

A 4-channel differential SQUID magnetometer has been built. Its design principles and construction are described. Proper matching of the detection coil to the SQUID input is discussed. Examples of auditory, dental pain and visual evoked response data are presented.

Responses of the primary auditory cortex to pitch changes in a sequence of tone pips: neuromagnetic recordings in man

Auditory evoked magnetic fields of the human brain were recorded with a four-channel 1st order gradiometer. Pitch deviance in a sequence of repetitive tone pips elicited magnetic evoked-response changes with a topography suggesting that a neuronal mismatch process to the deviant tones activates the primary auditory cortex.

The activation of unmyelinated or myelinated afferent fibers by brief infrared laser pulses varies with skin type

Brief infrared laser pulses were applied to 3 different skin areas in man. Reaction times indicated that A delta-fibers were activated in the dorsal and volar skin of the finger whereas only C-fibers were activated in the hairy skin of the forearm. The qualitative sensations were in line with this interpretation. We conclude that the activation of unmyelinated or myelinated afferent fiber populations with brief laser pulses varies with skin type.

Somatosensory evoked cerebral magnetic fields from SI and SII in man

We have recorded cerebral magnetic fields elicited by electrical stimulation of median and peroneal nerves. Field mapping indicates that the deflections at 30-80 and 150-180 msec are due to activity at SI. Additional activity at 90-125 msec is generated at SII, on the superior bank of the sylvian fissure. At SI, the source locations are in agreement with the known somatotopy. Only contralateral stimuli evoke responses at SI, whereas both ipsi- and contralateral stimuli elicit responses at SII.

Neuromagnetic localization of cortical activity evoked by painful dental stimulation in man

We have recorded cerebral magnetic fields evoked by painful dental stimulation. The field pattern indicates a current source at the upper bank of the anterior Sylvian Fissure, corresponding to the anterior end of the secondary somatosensory cortex. This finding suggests cortical representation of tooth pulp in man. The neuromagnetic technique, allowing the investigation of this cortical area, thus provides a new non-invasive tool for pain research.

Cerebral magnetic fields preceding self-paced plantar flexions of the foot

Cerebral magnetic fields preceding self-paced plantar flexions of the feet were studied with a SQUID gradiometer in 4 subjects. A slow magnetoencephalographic (MEG) shift was observed to begin as early as 1 sec before the movement. The shift changed its polarity between frontal and parietal areas. The MEG shifts preceding right and left foot movements were similar in shape, but their polarities differed at many recording locations. Simultaneous movements of both feet were preceded by shifts approximately equal to the sum of the shifts preceding the unilateral foot movements at the same recording location. The results suggest that the EEG and MEG shifts preceding foot movements are largely generated by tangential current sources on the mesial surface of the contralateral hemisphere around the motor representation area of the foot.

Neuromagnetic responses from the second somatosensory cortex in man

Cerebral magnetic fields elicited by electric stimulation of peripheral nerves were studied in man. Responses were found over the Sylvian fissure at latencies of 95-125 ms for both contra- and ipsilateral stimuli. The field distribution indicated that the responses are generated in the second somatosensory cortex SII at the upper bank of the Sylvian fissure. These responses seem to provide the first non-invasive tool to study the function of SII in man.

Slow EEG potentials preceding self-paced plantar flexions of hand and foot

Slow EEG shifts preceding voluntary self-paced plantar flexions of hand and foot were studied in five healthy right handed subjects. The EEG was recorded from a coronal electrode chain at the central areas. The movements were preceded by slow negative shifts beginning even as early as one second before the movement and culminating in fast slopes during the early EMG activity at the onset of the movements. The EEG shifts preceding hand and foot movements were differently distributed over the scalp: hand movements were preceded by contralaterally maximal shifts a few hundred milliseconds before the movement, whereas the potential distribution preceding foot movements were symmetrical or ipsilaterally dominant. It is suggested that the differences in the scalp distributions are due to the different orientation of the current dipoles at the cortical motor areas of hand and foot.

Brainstem auditory-evoked potentials in progressive myoclonus epilepsy without Lafora bodies

We studied brainstem auditory-evoked potentials of 11 patients suffering from the Finnish type of progressive myoclonus epilepsy (without Lafora bodies). All patients were ambulatory and had regular antiepileptic medication. The responses of 5 patients were interpreted as totally normal. In 6 cases there were slight deformations of the wave forms or of the amplitudes of the responses. The central conduction time of all patients was within the +/- 2 S.D. limits of the normal values. Postauricular muscular responses were not more prominent in patients than in controls. The results indicate that there is not any significant dysfunction in the cholinergic auditory pathways of the brainstem of these patients.

Interstimulus interval dependence of the auditory vertex response and its magnetic counterpart: implications for their neural generation

Auditory vertex responses elicited by short tone bursts were compared with their magnetic counter parts. Special attention was paid to the behaviour of the N100 deflection of the response. Electrical responses were recorded from scalp locations Fp2, Fz, Cz, Pz, C4 and T4 and the magnetic responses half way between P4 and T6, at a point where the response has one of its amplitude extrema. Different ISIs (from 1 to 16 sec) were applied in order to differentiate specific and nonspecific evoked potential components from each other. The main results were as follows: (1) The scalp distsribution of the electical vertex response depends on the ISI used: with frequent stimulation there are no marked differences in the amplitudes of N100 between frontal and central areas but with long ISIs the amplitude maxima move to the vertex. (2) The magnetic responses also show a clear ISI dependence. The magnetic counterpart of N100 saturates at shorter ISIs than N100 recorded from the vertex. Independent of the ISI the magnetic counterpart of P200 is constantly very small. On the basis of the different sensitivities of the EEG and MEG to current sources of different orientations it is concluded that the auditory vertex response contains both modality specific and non-specific components. Experimental conditions, especially the ISI used, determine the relative contributions of these components to the potential recorded on the scalp.

Auditory evoked transient and sustained magnetic fields of the human brain. Localization of neural generators

A long auditory stimulus elicits a magnetic evoked response in the human brain, consisting of transient deflections followed by a sustained response. The distributions of the magnetic fields indicate that the auditory evoked transient response at a latency of 100 ms as well as the auditory sustained response are generated at and around the primary auditory cortex.

Evoked potentials elicited by long vibrotactile stimuli in the human EEG

Evoked potentials elicited by long vibrotactile stimuli were studied in six subjects. Stimuli were vibratory bursts (60 HZ, 600 ms, 680 micrometers) delivered to the back of the hand trains of four stimuli with an interstimulus-interval of 2 s. Stimulus trains were repeated once every minute. EEG was recorded from vertex and from ipsi- and contralateral parietal areas. First stimuli of the trains elicited a transient response followed by a large negative sustained potential which reached its peak amplitude about 700 ms after the stimulus onset and which was not restricted to the stimulus duration. The negative component of the transient on-response as well as the sustained potential was at the 1st stimulus of the train maximal in amplitude at the vertex, and no significant difference between the ipsi- and contralateral parietal areas was detected. Stimulus repetition in the train resulted in a marked decrease of the transient response, and in an almost complete disappearance of the sustained potential, which was only occasionally observed during repeated stimuli.

Effects of small ethanol doses on the auditory evoked transient and sustained potentials in the human EEG

Effects of small doses of ethanol on transient and sustained potentials evoked by short tones were examined. Transient and sustained potentials showed an opposite effect of ethanol. The results indicate that ethanol has different effects on neural systems underlying transient and sustained potentials evoked in the human EEG.

Auditory evoked transient and sustained potentials in the human EEG: II. Effects of small doses of ethanol

The effect of small doses of ethanol (0.4 g/kg) on auditory evoked transient and sustained potentials was studied. Tones of 1-second duration were presented in trains of four stimuli (interstimulus interval = 1 second; intertrain interval = 1 minute). The electroencephalogram was recorded from derivation Cz-Al. Ethanol depressed the transient responses both at the first stimulus of the train and during repeated stimuli. The sustained potentials elicited by the first stimuli of the train were not affected by ethanol, whereas the sustained potentials elicited by repeated stimuli were larger in amplitude under the influence of ethanol than during control experiments. It is suggested that the decrease of the transient responses under the influence of ethanol is mainly due to depression of the reticular formation, whereas the increase of sustained potentials reflects ethanol-induced release of intracortical inhibition.

Auditory evoked transient and sustained potentials in the human EEG: I. Effects of expectation of stimuli

The characteristics of auditory evoked transient and sustained potentials were recorded using trains of four-tone stimuli of 1-second duration (interstimulus interval = 1 second) presented once every minute. The subject either attentively expected the stimuli or ignored them while reading. The electroencephalogram was recorded from derivations Cz-Al and Fz-Al. Expectation of the stimuli was associated with increased amplitudes of the transient responses both at the first stimulus of the train and during stimulus repetition. In contrast, the sustained potential at the first stimulus of the train was unchanged or smaller when the subject expected the stimuli. During stimulus repetition, however, the amplitude of the potential was enhanced by expectation of the stimuli. The results support the hypothesis of two sustained potential components and stress the importance of stimulus repetition rate when sustained potentials are studied.

Effect of stimulus repetition on negative sustained potentials elicited by auditory and visual stimuli in the human EEG

Effect of repetition of auditory and visual stimuli on sensory-evoked sustained potentials was studied. The stimuli were tones and flashes of 1 sec duration presented in trains of six stimuli with an inter-stimulus-interval of 1 sec. The repetition rate of the trains was 1 train/min. The EEG was recorded from electrodes located at Cz, Pz and Oz. Both auditory and visual stimuli elicited negative sustained potentials which during the first stimuli of the trains were maximal in amplitude and shortest in latency at Cz. Repetition of the stimulus resulted in almost complete disappearance of the sustained potentials; for auditory stimulation a small negative shift was recorded only at Cz, whereas for visual stimulation a small sustained potential was seen only at Pz and Oz. The results indicate that the slow potentials evoked by short tones or lights are composed of at least two components. One of these components may reflect neural processing associated with orienting reflex. The other component seems to be related to specific neural processing of the stimulus.