Sampling theory for neuromagnetic detector arrays

Ahonen, Antti; Hämäläinen, Matti S.; Ilmoniemi, Risto J.; Kajola, M.; Knuutila, Jukka; Simola, J.; Vilkman, V.

doi:10.1109/10.245606

Cited by 106 publications

(86 citation statements)

References 14 publications

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“…The planar field gradient approximates the signals measured by planar gradiometers. This is often advantageous in MEG signal processing, because the strongest field of the planar gradient usually is situated above the neural sources (43). The vertical and horizontal components were estimated using the fields of each sensor and its neighboring sensors.…”

Section: Methodsmentioning

confidence: 99%

Modulations in oscillatory activity with amplitude asymmetry can produce cognitively relevant event-related responses

Dijk

Werf

Mazaheri

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

143

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Event-related responses and oscillatory activity are typically regarded as manifestations of different neural processes. Recent work has nevertheless revealed a mechanism by which slow eventrelated responses are created as a direct consequence of modulations in brain oscillations with nonsinusoidal properties. It remains unknown if this mechanism applies to cognitively relevant eventrelated responses. Here, we investigated whether sustained event-related fields (ERFs) measured during working memory maintenance can be explained by modulations in oscillatory power. In particular, we focused on contralateral delayed activity (CDA) typically observed in working memory tasks in which hemifield specific attention is manipulated. Using magnetoencephalography, we observed sustained posterior ERFs following the presentation of the memory target. These ERFs were systematically lateralized with respect to the hemisphere in which the target was presented. A strikingly similar pattern emerged for modulations in alpha (9-13 Hz) power. The alpha power and ERF lateralization were strongly correlated over subjects. Based on a mechanistic argument pertaining to the nonsinusoidal properties of the alpha activity, we conclude that the ERFs modulated by working memory are likely to be directly produced by the modulations in oscillatory alpha activity. Given that posterior alpha activity typically reflects disengagement, we conclude that the CDA is not attributable to an additive process reflecting memory maintenance per se but, rather, is a consequence of how attentional resources are allocated.cognition | evoked-responses | electroencephalography | magnetoencephalography | oscillations N oninvasive electrophysiological recordings in the human brain are typically carried out by using either electroencephalography (EEG) or magnetoencephalography (MEG). In cognitive studies, data from these techniques are studied by means of event-related responses or by modulations in oscillatory power (1). Event-related responses are recorded by averaging 30 or more trials time-locked to a given stimulus, assuming that oscillatory brain activity that is not phase-locked to the stimulus is "averaged out." To determine task-related modulations in oscillatory activity, the power is calculated for each trial separately and then averaged. This allows for quantification of stimulus-induced changes in oscillatory activity. The relation between event-related responses and oscillatory activity remains a fundamental question in human electrophysiological research (1).Recently, it was reported that human oscillatory activity in the alpha band has "amplitude fluctuation asymmetry" (AFA) (2, 3). This refers to a nonsinusoidal property implying that amplitude changes are reflected stronger in the peaks than in the troughs of the ongoing oscillations (or vice versa). As a consequence, the signal distribution is skewed such that the mean covaries with magnitude. This finding challenges the dogma that oscillations are averaged out when event-related responses are ca...

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Section: Methodsmentioning

confidence: 99%

Modulations in oscillatory activity with amplitude asymmetry can produce cognitively relevant event-related responses

Dijk

Werf

Mazaheri

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

143

View full text Add to dashboard Cite

show abstract

“…Signal strength was visually controlled during the entire measurement. In 12 subjects, neuromagnetic activity was recorded noninvasively with a helmet-shaped 122-channel whole-head Neuromag TM system (Ahonen et al 1993). Due to a change of the MEG system (Elekta Oy, Helsinki, Finland), 15 further subjects were measured by means of a 306-channel system.…”

Section: Meg Recordingsmentioning

confidence: 99%

Changes of cortico-muscular coherence: an early marker of healthy aging?

Kamp

Krause

Butz

et al. 2011

AGE

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Cortico-muscular coherence (CMC) at beta frequency (13-30 Hz) occurs particularly during weak to moderate isometric contraction. It is a well-established measure of communication between the primary motor cortex (M1) and corresponding muscles revealing information about the integrity of the pyramidal system. Although the slowing of brain and muscle dynamics during healthy aging has been evidenced, functional communication as determined by CMC has not been investigated so far. Since decline of motor functions at higher age is likely to be associated with CMC changes, the present study aims at shedding light on the functionality of the motor system from a functional interaction perspective. To this end, CMC was investigated in 27 healthy subjects aging between 22 and 77 years during isometric contraction of their right forearm. Neuromagnetic activity was measured using whole-head magnetoencephalography (MEG). Muscle activity was measured by means of surface electromyography (EMG) of the right extensor digitorum communis (EDC) muscle. Additionally, MEG-EMG phase lags were calculated in order to estimate conducting time. The analysis revealed CMC and M1 power amplitudes to be increased with age accompanied by slowing of M1, EMG, and CMC. Frequency changes were particularly found in subjects aged above 40 years suggesting that at this middle age, neurophysiological changes occur, possibly reflecting an early neurophysiological marker of seniority. Since MEG-EMG phase lags did not vary with age, changes cannot be explained by alterations of nerve conduction. We argue that the M1 power amplitude increase and the shift towards lower frequencies might represent a neurophysiological marker of healthy aging which is possibly compensated by increased CMC amplitude.

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“…The calculated planar field gradient approximates the signals measured by physical planar gradiometers [e.g., as in Elekta (Stockholm, Sweden) Neuromag systems]. This is often advantageous when analyzing MEG signals, because the strongest field of the planar gradient signal usually is situated above the neural sources (Ahonen et al, 1993). The horizontal and vertical components of the planar gradients were estimated at each sensor location using the fields from the sensor and its neighboring sensors.…”

Section: Methodsmentioning

confidence: 99%

Prestimulus Oscillatory Activity in the Alpha Band Predicts Visual Discrimination Ability

Dijk¹,

Schoffelen²,

Oostenveld³

et al. 2008

J. Neurosci.

760

577

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Although the resting and baseline states of the human electroencephalogram and magnetoencephalogram (MEG) are dominated by oscillations in the alpha band (ϳ10 Hz), the functional role of these oscillations remains unclear. In this study we used MEG to investigate how spontaneous oscillations in humans presented before visual stimuli modulate visual perception. Subjects had to report if there was a subtle difference in gray levels between two superimposed presented discs. We then compared the prestimulus brain activity for correctly (hits) versus incorrectly (misses) identified stimuli. We found that visual discrimination ability decreased with an increase in prestimulus alpha power. Given that reaction times did not vary systematically with prestimulus alpha power changes in vigilance are not likely to explain the change in discrimination ability. Source reconstruction using spatial filters allowed us to identify the brain areas accounting for this effect. The dominant sources modulating visual perception were localized around the parieto-occipital sulcus. We suggest that the parieto-occipital alpha power reflects functional inhibition imposed by higher level areas, which serves to modulate the gain of the visual stream.

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Sampling theory for neuromagnetic detector arrays

Cited by 106 publications

References 14 publications

Modulations in oscillatory activity with amplitude asymmetry can produce cognitively relevant event-related responses

Modulations in oscillatory activity with amplitude asymmetry can produce cognitively relevant event-related responses

Changes of cortico-muscular coherence: an early marker of healthy aging?

Prestimulus Oscillatory Activity in the Alpha Band Predicts Visual Discrimination Ability

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