T. Varpula

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.

Magnetic field fluctuations arising from thermal motion of electric charge in conductors

A model is developed for the source of magnetic field fluctuations emanating from thermal agitation of electric charge in conductors. The calculation of the thermal magnetic noise with the model involves the solution of a general volume conductor forward problem. Frequency‐dependent equations for this problem are derived from Maxwell’s equations. The model is applied to calculate the magnetic noise generated by infinite conducting slabs. A good agreement between theoretical predictions and experimental results were found. Thermal magnetic noise fields inside magnetically shielded rooms are estimated and noise in biomagnetic measurements is discussed.

Studies of auditory evoked magnetic and electric responses: Modality specificity and modelling

SummaryThe sources of auditory evoked electric and magnetic responses were modelled with multiple current dipoles. The activity at the auditory cortices, 2.6 cm beneath the skull, explains the observed % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrVeFv0de9GqpGe9Ff% ea0dXdd91qFHe9FjuP0-is0dXdbba9pGe9xq-Jbba9suk9fr-xfr-x% frpeWZqaaeaabiGaciaacaqabeaadaqaaqaaaOqaamaanaaabaGaam% OtaiaaigdacaaIWaGaaGimamXvP5wqonvsaeHbfv3ySLgzaGqbciab% -1gaTbaaaaa!4065!

Magnetic fields produced by the human eye (invited)

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Measurement of biomagnetic fields using multichannel superconducting-magnetometer techniques

field distribution. An additional current source near the vertex is needed to explain the corresponding potential distributions. The locations of the current sources of % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrVeFv0de9GqpGe9Ff% ea0dXdd91qFHe9FjuP0-is0dXdbba9pGe9xq-Jbba9suk9fr-xfr-x% frpeWZqaaeaabiGaciaacaqabeaadaqaaqaaaOqaamaanaaabaGaam% OtaiaaigdacaaIWaGaaGimamXvP5wqonvsaeHbfv3ySLgzaGqbciab% -1gaTbaaaaa!4065!

Magnetic measurements of cardiac mechanical activity

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