Sampling theory for neuromagnetic detector arrays.

The sampling theorem for wave-number-limited multivariable functions is applied to the problem of neuromagnetic field mapping. The wave-number spectrum and other relevant properties of these fields are estimated. A theory is derived for reconstructing neuromagnetic fields from measurements using sensor arrays which sample either the field component Bz perpendicular to the planar grid of measurement points, or the two components delta Bz/delta x and delta Bz/delta y of its gradient in the xy plane. The maximum sensor spacing consistent with a unique reconstruction is determined for both cases. It is shown that, when two orthogonal components of the gradient are measured at every site of the measurement grid, the density of these sensor-pair units can be reduced, without risk of aliasing, to half of what is necessary for single-channel sensors in an array sampling Bz alone. Thus the planar and axial gradiometer arrays are equivalent in the sampling sense provided that the number of independent measurements per unit area is equal.

Functional organization of the human first and second somatosensory cortices: a neuromagnetic study.

Multichannel neuromagnetic recordings were used to differentiate signals from the human first (SI) and second (SII) somatosensory cortices and to define representations of body surface in them. The responses from contralateral SI, peaking at 20-40 ms, arose mainly from area 3b, where representations of the leg, hand, fingers, lips and tongue agreed with earlier animal studies and with neurosurgical stimulations and recordings on convexial cortex in man. Representations of the five fingers were limited to a cortical strip of approximately 2 cm in length. Responses from SII peaked 100-140 ms after contra- and ipsilateral stimuli and varied considerably from one subject to another. Signs of somatotopical organization were seen also in SII. Responses of SII were not fully recovered at interstimulus intervals of 8 s.

Seeing faces activates three separate areas outside the occipital visual cortex in man.

We have examined magnetic cortical responses of 15 healthy humans to 46 different pictures of faces. At least three areas outside the occipital visual cortex appeared to be involved in processing this input, 105-560 ms after the stimulus onset. The first active area was near the occipitotemporal junction, the second in the inferior parietal lobe, and the third in the middle temporal lobe. The source in the inferior parietal lobe was also activated by other simple and complex visual stimuli.

Development of multichannel neuromagnetic instrumentation in Finland.

In this paper we describe the instrumentation for biomagnetic measurements available in our laboratory. The focus is on our 24-channel planar gradiometer system. In addition, a 122-channel system under construction will be discussed.

Neuromagnetic responses of human auditory cortex to interruptions in a steady rhythm.

We have recorded, with a 7-channel SQUID gradiometer, evoked magnetic responses of 6 healthy humans to interruptions of a steady rhythm of 50 ms 'standard' tone bursts repeated once every 610 ms. Ten percent of the tones occurred 'too early', 410 ms after the preceding stimulus. The response to standards peaked, on average, at 90 ms and that to the early tones at 148 ms. Field patterns were dipolar during both responses and the equivalent sources agreed with activation of the supratemporal auditory cortex, at slightly different locations. The dipole moments were more than twice as strong for the early tones as for the standards. The results emphasize the importance of temporal stimulation patterns in activating the human auditory cortex.

Cerebral magnetic responses to noise bursts and pauses of different durations.

We compared magnetic-evoked responses of human auditory cortex to short (5, 10, 20, 40, 80 and 160 ms) noise bursts and to pauses of identical durations in continuous noise. Onsets of both stimuli evoked responses with the most prominent deflection (N100m) peaking at about 100 ms. Both field maps could be explained by current dipoles, which agree with activity at the supratemporal cortex at slightly different locations. At the shortest 5-ms duration the noise bursts evoked a clear N100m whereas pauses elicited very low-amplitude responses or no response at all. For both stimuli, N100m increased in amplitude when the stimulus duration was increased from 5 up to 20-40 ms. The latencies were 10-20 ms longer for pauses than noise bursts with the longest latencies at the shortest stimulus durations. The differences in amplitudes and latencies as a function of stimulus duration and the slightly different source areas indicate that the generators of the on- and off-responses are not identical.