Claudia D. Tesche

Frontal theta activity in humans increases with memory load in a working memory task

Recent theoretical work has suggested that brain oscillations in the theta band are involved in active maintenance and recall of working memory representations. To test this theoretical framework we recorded neuromagnetic responses from 10 subjects performing the Sternberg task. Subjects were required to retain a list of 1, 3, 5 or 7 visually presented digits during a 3‐s retention period. During the retention period we observed ongoing frontal theta activity in the 7–8.5‐Hz band recorded by sensors over frontal brain areas. The activity in the theta band increased parametrically with the number of items retained in working memory. A time–frequency analysis revealed that the task‐dependent theta was present during the retention period and during memory scanning. Following the memory task the theta activity was reduced. These results suggest that theta oscillations generated in frontal brain regions play an active role in memory maintenance.

dc SQUID: Noise and optimization

A computer model is described for the dc SQUID in which the two Josephson junctions are nonhysteretic, resistively shunted tunnel junctions. In the absence of noise, current-voltage(I–V) characteristics are obtained as functions of the applied flux, Φa, SQUID inductanceL, junction critical currentI0, and shunt resistanceR. The effects of asymmetry inL, I0, andR are discussed.I–V characteristics, flux-voltage transfer functions, and low-frequency spectral densities of the voltage noise are obtained at experimentally interesting values of the parameters in the presence of Johnson noise in the resistive shunts. The transfer functions and voltage spectral densities are used to calculate the flux and energy resolution of the SQUID operated as an open-loop, small-signal amplifier. The resolution of the SQUID with ac flux modulation is discussed. The flux resolution calculated for the SQUID of Clarke, Goubau, and Ketchen is1.6 × 10−5Φ0 Hz−1/2, approximately one-half the experimental value. Optimization of the SQUID resolution is discussed: It is shown that the optimum operating condition is β=2LI0/Φ0≈1. Finally, some speculations are made on the ultimate performance of the tunnel junction dc SQUID. When the dominant noise source is Johnson noise in the resistive shunts, the energy resolution per Hz is4kBT(πLC)1/2, whereC is the junction capacitance, and the constraintR=(Φ0/2πCI0)1/2 has been imposed. This result implies that the energy resolution is proportional to (junction area)1/2. In the limiteI0R ≫kBT, the dominant noise source is shot noise in the junctions; for β=1, the energy resolution per Hz is then approximatelyh/2.

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.

Signal-space projections of MEG data characterize both distributed and well-localized neuronal sources

We describe the use of signal-space projection (SSP) for the detection and characterization of simultaneous and/or sequential activation of neuronal source distributions. In this analysis, a common signal space is used to represent both the signals measured by an array of detectors and the underlying brain sources. This presents distinct advantages for the analysis of EEG and MEG data. Both highly localized and distributed sources are characterized by the components of the field patterns which are measured by the detectors. As a result, a unified description of arbitrary source configurations is obtained which permits the consistent implementation of a variety of analysis techniques. The method is illustrated by the application of SSP to auditory, visual and somatosensory evoked-response MEG data. Single-trace evoked responses obtained by SSP of spontaneous activity demonstrate that a considerable discrimination against both system noise and uncorrelated brain activity may be achieved. Application of signal-space projections determined in the frequency domain to spontaneous activity illustrates the possibility of including temporal relationships into the analysis. Finally, we demonstrate that SSP is particularly useful for the description of multiple sources of distributed activity and for the comparison of the strengths of specific neuronal sources under a variety of different paradigms or subject conditions.

Consensus paper: roles of the cerebellum in motor control--the diversity of ideas on cerebellar involvement in movement.

Considerable progress has been made in developing models of cerebellar function in sensorimotor control, as well as in identifying key problems that are the focus of current investigation. In this consensus paper, we discuss the literature on the role of the cerebellar circuitry in motor control, bringing together a range of different viewpoints. The following topics are covered: oculomotor control, classical conditioning (evidence in animals and in humans), cerebellar control of motor speech, control of grip forces, control of voluntary limb movements, timing, sensorimotor synchronization, control of corticomotor excitability, control of movement-related sensory data acquisition, cerebro-cerebellar interaction in visuokinesthetic perception of hand movement, functional neuroimaging studies, and magnetoencephalographic mapping of cortico-cerebellar dynamics. While the field has yet to reach a consensus on the precise role played by the cerebellum in movement control, the literature has witnessed the emergence of broad proposals that address cerebellar function at multiple levels of analysis. This paper highlights the diversity of current opinion, providing a framework for debate and discussion on the role of this quintessential vertebrate structure.

Anticipatory cerebellar responses during somatosensory omission in man.

The traditional view of cerebellum is a structure that modifies and synchronizes elements of motor performance. Recent evidence indicates that human cerebellum is involved in a wide range of nonmotor sensory and cognitive functions. A common feature in these diverse motor and nonmotor tasks may be the capacity of cerebellar neuronal circuits to process and anticipate sensory input with high temporal acuity. We present evidence supporting this hypothesis from measurements of the magnetic field at the scalp evoked by neuronal population activity in human cerebellum. Intermittent electrical stimulation of the finger and the median nerve elicited stimulus‐locked cerebellar responses with oscillatory components at 6–12 Hz and 25–35 Hz. Sustained oscillatory activity followed random stimulus omissions, with initiation of cerebellar responses prior to the next overt stimulus. These responses indexed processing of temporal features of somatosensory input independent of motor performance or response. The refractory behavior of the responses suggested that a neuronal trace of the temporal pattern of somatosensory stimulation remained in cerebellar circuits for 2–4 s. The cerebellar activity elicited by violation of an established temporal pattern was enhanced when attention was directed to somatosensory stimuli, in concordance with recent imaging studies suggesting participation of cerebellum in attentional networks. The attentional enhancement of the cerebellar responses supports the salience of cerebellar activity in the processing of purely somatosensory input. The short‐term maintenance of cerebellar templates for predictable sensory input may reflect a physiological substrate for fine‐grained temporal tuning and optimization of performance in large‐scale sensory and integrative systems. Hum. Brain Mapping 3:119–142, 2000.

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.

OPTIMIZATION OF DC SQUID VOLTMETER AND MAGNETOMETER CIRCUITS

We calculate the signal-to-noise ratio in a dc SQUID system as a function of source impedance, taking into account the effects of current and voltage noise sources in the SQUID. The optimization of both tuned and untuned voltmeters and magnetometers is discussed and typical sensitivities are predicted using calculated noise spectra. The calculations are based on an ideal symmetric dc SQUID with % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacqaHYoGycqGH9a% qpcaaIYaacbaGaa8htaiaa-LeadaWgaaWcbaacbiGaa4hmaaqabaGc% caGGVaGaeuOPdy0aaSbaaSqaaiaa+bdaaeqaaOGaeyypa0JaaGymaa% aa!3D23!\[\beta = 2LI_0 /\Phi _0 = 1\] and moderate noise rounding % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaaiiGacqWFOaakcq% WFtoWrcqWF9aqpcqWFYaGmcqaHapaCcaWGRbWaaSbaaSqaaiaadkea% aeqaaGqaaOGaa4hvaiaac+cacaGFjbWaaSbaaSqaaerbbjxAHXgaiu% GacaqFWaaabeaakiab-z6agnaaBaaaleaacaqGGaacbiGaaWhmaaqa% baGccqGH9aqpcaaIWaGaaiOlaiaaicdacaaI1aGaaiykaaaa!471A!\[(\Gamma = 2\pi k_B T/I_0 \Phi _{{\rm{ }}0} = 0.05)\], where Φ0 is the flux quantum, T is the temperature, L is the SQUID inductance, and I0 is the critical current of each junction. The optimum noise temperatures of tuned and untuned voltmeters are found to be 2.8(ΩL/R)T and 8(ΩL/R)T (1 + 1.5α2 + 0.7α4)1/2/α2 respectively, where Ω/2π is the signal frequency, assumed to be much less than the Josephson frequency, and α is the coupling coefficient between the SQUID and its input coil. It is found that tuned and untuned magnetometers can be characterized by optimum effective signal energies given by (16kBTLE/α2R)[1 + (1 + 1.5α2 + 0.7α2)1/2 + 0.75α2] and 2kBTiRiB/Ω2Lp respectively, where B is the bandwidth, Ri is the resistance representing the losses in the tuned circuit at temperature Ti and Lp is the inductance of the pickup coil.

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. >

Neural processing of human faces: a magnetoencephalographic study

Abstract This is a whole head magnetoencephalographic (MEG) study of the neural processing of briefly presented images of human faces in 14 normal subjects. The experiments involved three tasks of increasing complexity, involving image categorisation, image comparison and the identification of emotion. The analyses were based on average responses to repeated stimuli in the different image categories. These averages were processed to give numerical measures of the power within defined regions and latency spans. The only statistically significant difference in these data between the response to faces and other images is in the right occipito-temporal channels at a latency of 140 ms. The face-specific response is largely independent of the task. Source modelling suggests an extended source in the ventral occipito-temporal region. The analysis supports the notions of both face-specificity and right hemisphere dominance for all image types at early latencies.