Magnetic fields produced by eye blinking.

Magnetic signals produced by voluntary eye blinking were recorded. The maximal signals were found over the posterior parts of the orbits. The polarity of the signal reversed over each eye and in the frontal midline. The amplitude increases with light and amplitude decreases with darkness were similar in the electrical and magnetic blink and eye movement signals. Three different simulation models were used in the interpretation of the results. It is concluded that the primary current source of the blink signal is the transretinal current density. Lid movements change the geometry of the volume conductor resulting in changes in the measured electrical and magnetic field patterns.

Cerebral neuromagnetic responses evoked by short auditory stimuli.

We presented 30 msec sinusoidal tone bursts to the subject's left ear once every 1300 msec. The number of standard tones (1000 Hz) between deviants (1030 Hz) varied randomly from 3 to 15 (even distribution) so that the probability of the standards was 0.9 and that of the deviants 0.1. During stimulation the subject was reading a book. Magnetic responses to the standards and deviants were measured at the estimated ends of the sylvian fissure and electrical responses at Fpz, Fz and Cz. In addition, extensive field maps over the right hemisphere were made from 60 to 75 points in 3 subjects. A least-squares fit was performed to find out the parameters of the equivalent current dipole in a spherically symmetrical head model. Field maps suggested that the source of magnetic response at 100 msec (N100m) can be approximated by a current dipole at the supratemporal plane, possibly at the primary auditory cortex. In two subjects the location and/or the orientation of the equivalent dipole changed during N100m, possibly due to change in the size of the activated cortical area. The deviant stimuli elicited in addition to N100m a second deflection, MMNm, peaking at about 200 msec. This response was regarded as specific to stimulus change. On the basis of field maps it was also concluded that MMNm got a contribution from activity at the supratemporal plane.

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.

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.