Jukka Knuutila

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.

122-channel squid instrument for investigating the magnetic signals from the human brain

A 122-channel d.c. SQUID magnetometer with a helmet-shaped detector array covering the subjects head has been operational in the Low Temperature Laboratory of the Helsinki University of Technology since June 1992. The new system allows simultaneous recording of magnetic activity all over the head. The probe employs 122 planar first-order thin-film gradiometers in dual units with two exactly orthogonal channels at 61 measurement sites. The performance of the device is analyzed and compared with more conventional axial gradiometer arrays by considering signal-to-noise ratios, spatial sampling theory, confidence intervals for the estimated equivalent current dipole positions, and information-theoretical channel capacity. The signal-to-noise ratio and the resolution of the planar and axial arrays with the same number of channels are found practically equal. The number of channels and their spacing in our new Neuromag-122 system are found fully adequate for neuromagnetic measurements. An example of whole cortex recordings of auditory evoked brain activity is presented and analyzed.

Activation of the human posterior parietal cortex by median nerve stimulation

We recorded somatosensory evoked magnetic fields from ten healthy, right-handed subjects with a 122-channel whole-scalp SQUID magnetometer. The stimuli, exceeding the motor threshold, were delivered alternately to the left and right median nerves at the wrists, with interstimulus intervals of 1, 3, and 5 s. The first responses, peaking around 20 and 35 ms, were explained by activation of the contralateral primary somatosensory cortex (SI) hand area. All subjects showed additional deflections which peaked after 85 ms; the source locations agreed with the sites of the secondary somatosensory cortices (SII) in both hemispheres. The SII responses were typically stronger in the left than the right hemisphere. All subjects had an additional source, not previously reported in human evoked response data, in the contralateral parietal cortex. This source was posterior and medial to the SI hand area, and evidently in the wall of the postcentral sulcus. It was most active at 70–110 ms.

Visual cortex activation in blind humans during sound discrimination

We used a whole-scalp magnetometer with 122 planar gradiometers to study the activity of the visual cortex of five blind humans deprived of visual input since early infancy. Magnetic responses were recorded to pitch changes in a sound sequence when the subjects were either counting these changes or ignoring the stimuli. In two of the blind subjects, magnetic resonance images were also obtained, showing normal visual cortex macroanatomy. In these subjects, the magnetic responses to counted pitch changes were located at visual and temporal cortices whereas ignored pitch changes activated the temporal cortices almost exclusively. Also in two of the other three blind, the visual-cortex activation was detectable in the auditory counting task. Our results suggest that the visual cortex of blind humans can participate in auditory discrimination.

Bilateral activation of the human somatomotor cortex by distal hand movements

We recorded cortical magnetic signals, simultaneously over the whole scalp, from 6 healthy subjects during 3 motor tasks to track the varying proportion of contra- vs. ipsilateral activation. The subjects performed self-paced index finger flexions, simultaneous flexion of 4 fingers, and a sequence of rapid digit movements in different sessions. Index finger and 4-finger movements were associated with phasic bilateral dampening of spontaneous 10 and 20 Hz rhythms along the central sulcus, starting approximately 1 sec before the movement in the contralateral hemisphere. A rebound occurred within 1 sec after the index finger and 4-finger flexions; the rapid finger movements resulted in a persistent blocking of the rhythms. Averaging with respect to movement onset showed a slow bilateral frontal readiness field starting about 0.5 sec prior to motion onset. It was followed, within 200 msec after movement onset, by phasic movement-evoked fields (MEFs) which were bilateral during the tasks involving several fingers. The contra- vs. ipsilateral MEF amplitude ratio C/I decreased from 4.0 during index finger movements to 0.6 during rapid finger flexions, reflecting the enhanced activation of the ipsilateral primary somatomotor cortex with increasing complexity of movement.

SQUID magnetometers for low-frequency applications

We present a novel formulation for SQUID operation, which enables us to evaluate and compare the sensitivity and applicability of different devices. SQUID magnetometers for low-frequency applications are analyzed, taking into account the coupling circuits and electronics. We discuss nonhysteretic and hysteretic single-junction rf SQUIDs, but the main emphasis is on the dynamics, sensitivity, and coupling considerations of dc-SQUID magnetometers. A short review of current ideas on thin-film, dc-SQUID design presents the problems in coupling and the basic limits of sensitivity. The fabrication technology of tunnel-junction devices is discussed with emphasis on how it limits critical current densities, specific capacitances of junctions, minimum linewidths, conductor separations, etc. Properties of high-temperature superconductors are evaluated on the basis of recently published results on increased flux creep, low density of current carriers, and problems in fabricating reliable junctions. The optimization of electronics for different types of SQUIDs is presented. Finally, the most important low-frequency applications of SQUIDs in biomagnetism, metrology, geomagnetism, and some physics experiments demonstrate the various possibilities that state-of-the-art SQUIDs can provide.

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 122-channel whole-cortex SQUID system for measuring the brain's magnetic fields

A 122-channel neuromagnetometer with a helmet-shaped detector array covering the entire head allows simultaneous recording of magnetic fields over the whole cortex. The instrument has 122 planar first-order gradiometers in dual units at 61 measurement sites. The SQUIDs are directly coupled to the read-out electronics, with amplifier noise cancellation to eliminate the need for separate preamplifiers inside the magnetically shielded room. The authors analyze the performance of the device and compare it with traditional axial gradiometer arrays by considering signal-to-noise ratios, spatial sampling theory, confidence intervals for equivalent current dipole fits, and information-theoretical channel capacity. The analysis includes the fact that instrument noise is smaller than the background activity of the brain; the signal-to-noise ratio and the resolution of the planar array are in that case equal to or better than that of an axial array. The number of channels and their spacing are very suitable for neuromagnetic measurements. >

Interaction between representations of different features of auditory sensory memory.

The neurophysiological basis of sensory memory was studied by measuring the magnetic counterpart (MMNm) of the mismatch negativity (MMN) with a whole-head 122-channel magnetometer. The MMNm is a response to a difference in the presented stimulus and a neuronal memory trace formed by repeated standard stimuli. This trace must contain information about the feature differing in the deviant. Keeping one feature (frequency) constant, we studied how other stimulus features affect the strength of the MMNm. The MMNm to a frequency change was weaker when the other features varied than when they were constant. This suggests that the MMNm to a frequency change is not independent of other stimulus features.