Visa Antero Vilkman

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 ∼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.

Large-area low-noise seven-channel dc SQUID magnetometer for brain research

The design, construction, and performance of a new high‐sensitivity dc SQUID magnetometer, covering a circular area of 93‐mm diameter, is described. The device, now used routinely in our brain research, comprises seven asymmetric first‐order gradiometers, located on a spherical surface of 125‐mm radius and with the symmetry axis tilted 30° with respect to the vertical. The pickup coil diameter is 20 mm, and the channels are separated by 36.5 mm from each other in a hexagonal array. The overall field sensitivity of the system, measured inside our magnetically shielded room, is 5 fT/(Hz)1/2, mainly limited by the thermal noise in the radiation shields of the Dewar. The optimization of the coil configuration and the measurement system is discussed in detail, and a system to determine automatically the position and orientation of the Dewar with respect to certain fixed points on the subject’s head is described. Finally, some examples of measurements carried out with the new device are given.

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 B/sub z/ perpendicular to the planar grid of measurement points, or the two components partial B/sub z//partial x and partial B/sub z//partial 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 B/sub z/ 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.<<ETX>>

A 24-Channel Magnetometer for Brain Research

This paper describes the hardware of the 24-channel SQUID magnetometer being completed in the Low Temperature Laboratory. The overall system, including computer hardware and software, is discussed elsewhere (Hamalainen 1989). The instrument will be used in a magnetically shielded room for brain research. We hope that this apparatus will enable us to locate current sources underlying evoked magnetic fields without moving the dewar.

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.

A 24-SQUID gradiometer for magnetoencephalography

Abstract In this contribution we briefly describe our newest 24-channel neurogradiometer, employing dc SQUDs as magnetic flux sensors, and present an example of its performance. The instrument is used in a magnetically shielded room. This state-of-the-art apparatus is able to locate the stimulus-activated area in the brain by just one measurement, without moving the dewar.

A Low-Noise Seven-Channel DC SQUID Magnetometer for Brain Research

The design, construction, and performance of a high-sensitivity DC SQUID magnetometer, covering a circular area of 93 mm diameter, is described. The device, now used routinely in our brain research, comprises seven asymmetric first-order gradiometers, located on a spherical surface of 125 mm radius and with the symmetry axis tilted 30° in respect to the vertical. The overall field sensitivity of the system, measured inside our magnetically shielded room, is 5 – 6 fT/√Hz ,mainly caused by thermal noise in the radiation shields of the dewar.