We propose a novel l1l2-norm inverse solver for estimating the sources of EEG/MEG signals. Based on the standard l1-norm inverse solver, the proposed sparse distributed inverse solver integrates the l1-norm spatial model with a temporal model of the source signals in order to avoid unstable activation patterns and "spiky" reconstructed signals often produced by the original solvers.
Rapid progress in effective methods to image brain functions has revolutionized neuroscience. It is now possible to study noninvasively in humans neural processes that were previously only accessible in experimental animals and in brain-injured patients. In this endeavor, positron emission tomography has been the leader, but the superconducting quantum interference device-based magnetoencephalography (MEG) is gaining a firm role, too.
Whole-head magnetoencephalographic recordings revealed two parietal epileptic foci in homotopic areas of the hemispheres. The discharges occurred 17-20 ms later on the left than on the right hemisphere, implying the existence of a left-sided mirror focus. The foci were about 1 cm posterior to the hand primary somatosensory area, identified by evoked response measurements, and thus suggested epileptic activity at the parietal association cortex, in agreement with the observed callosal conduction time.
We propose a novel l(1)l(2)-norm inverse solver for estimating the sources of EEG/MEG signals. Based on the standard l(1)-norm inverse solvers, this sparse distributed inverse solver integrates the l(1)-norm spatial model with a temporal model of the source signals in order to avoid unstable activation patterns and "spiky" reconstructed signals often produced by the currently used sparse solvers.
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.
Magnetic responses to frequent and infrequent auditory stimuli, all presented in the same stimulus block in randomized order, were recorded. The standard stimuli, comprising 90% of all the stimuli, were 100-ms, 1000 Hz, 90dB sinusoidal tone bursts. There were three deviant tones, each presented at a probability of 3.3%, which differed from the standard tone on one dimension only: frequency deviant (1500 Hz), intensity deviant (67dB SPL), or duration deviant (50 ms).
Neuromagnetic responses were recorded over the left hemisphere to find out in which cortical area the heard and seen speech are integrated. Auditory stimuli were Finnish/pa/syllables presented together with a videotaped face articulating either the concordant syllable/pa/(84% of stimuli, V = A) or the discordant syllable/ka/(16%, V not equal to A). In some subjects the probabilities were reversed. The subjects heard V not equal to A stimuli as/ta/ or ka.
A persistent problem in developing plausible neurophysiological models of perception, cognition, and action is the difficulty of characterizing the interactions between different neural systems. Previous studies have approached this problem by estimating causal influences across brain areas activated during cognitive processing using structural equation modeling (SEM) and, more recently, with Granger-Geweke causality.
The effect of selective attention on activity of the right human auditory cortex was studied with a 24-channel planar SQUID-gradiometer. Two conditions were used, favoring either a late attention effect following N100m, or an early effect, overlapping with N100m. In experiment 1 (15 subjects), a randomized tone sequence of 1 and 3 kHz tones was delivered to the left ear with a constant interstimulus interval (ISI) of 405 msec. The subjects' task was to count infrequent longer tones of one of these pitches among shorter standards.
Neuromagnetic responses to 600-ms binaural click trains, presented once every 1.1 s, were recorded with a 24-channel gradiometer from 6 healthy humans. During the first 300 ms, the left-ear stimulus led the right by 0.7 ms and the sound was lateralized to the left ear. At 300 ms, the interaural time difference (ITD) changed and the lateralization moved to one of 5 different locations between the ears. An N100m response peaked about 110 ms after the sound onset and an N130mc response (c to stress a response to the change) about 135 ms after the ITD change.
