Hal Weinberg

Magnetic fields of the human brain accompanying voluntary movement: Bereitschaftsmagnetfeld

SummaryA slow magnetic field shift has been detected in the human brain occurring in the foreperiod of a voluntary finger movement. This magnetic field accompanies a slow negative electrical cerebral potential which occurs in the same foreperiod, the Bereitschaftspotential (BP) of Kornhuber and Deecke. The present report is the first of a magnetic field associated with the BP, and has been named the Bereitschaftsmagnetfeld (BM) or readiness magnetic field. The BM is oriented with the field lines directed out of the head in the pre-rolandic region and with the field lines directed into the head in post-rolandic areas, suggesting a source in the sensorimotor area for the contralateral hand. Distribution of the magnetic fields has so far not revealed a source in the fronto-central midline where the BP is recorded maximally. The time course and morphology of the BP and BM are similar, but they have different topography over the skull.

Electrophysiological indices of persistent post-concussion symptoms

Mild head injuries (MHI) including concussion were once considered transient alterations of function that resulted in no long-term structural or functional effects. This opinion has changed somewhat in recent years, based on the scientific evidence and popular cases in the media that suggest MHI can result in damage that can, in some cases, lead to long-term cognitive sequelae. Purpose and methods: An EP/ERP assessment battery is presented as a method for detecting changes in brain function that form the organic basis of persistent post-concussion symptoms (PCS). The primary focus of the paper was directed towards using this battery to determine whether or not brain function in individuals with persistent PCS was different than responses of individuals that comprised a normative database. Results: Visual and auditory ERPs and visual EPs were beyond a 2.5 standard deviation normal limit in young and older groups of individuals with persistent post-concussion symptoms. Conclusions: Evidence of changes in brain function in individuals with persistent post-concussion symptoms is consistent with the position that the post-concussion syndrome has a substantial biological, as opposed to a psychological, basis.

Motor cortex activity and predicting side of movement: neural network and dipole analysis of pre-movement magnetic fields

Neuromagnetic fields were recorded from human subjects during the performance of left and right voluntary finger movements. Modeling of current dipole sources indicated symmetric activation of both motor cortices beginning 600 ms prior to movement onset. This activity became lateralized to the contralateral hemisphere 200-300 ms prior to movement onset, the period during which an artificial neural network showed increased ability to predict side of movement within single trials. The results describe the mechanism of lateralization of cortical brain activity preceding voluntary movement and provide further evidence of the involvement of ipsilateral motor cortex in unilateral movements.

Magnetic fields of the human brain (Bereitschaftsmagnetfeld) preceding voluntary foot and toe movements

SummaryVoluntary movements are preceded by a slow electrical potential of the brain (Bereitschaftspotential, BP) or readiness potential. The BP is accompanied by a magnetic field shift of similar time characteristics (Bereitschaftsmagnetfeld, BM). The BM preceding volitional right foot or toe movements was recorded from anterior, posterior, and lateral positions of the scalp using a SQUID (Superconducting Quantum Interference Device) third-order gradiometer. Controls were implemented to reduce head movements, which were simultaneously recorded with a mechanograph. The results showed that movements of the lower extremities are also preceded by a BM. However, contrary to finger movements, BMs with field lines directed into the head were found predominantly for foot movements and exclusively for toe movements. The BM preceding foot movements was maximum over a position 2 cm left of the vertex, i.e., contralateral to the movement. Two centimeters right of the vertex it was smaller, thus exhibiting a normal contralateral preponderance and not sharing the paradoxical side preponderance of the electrical BP preceding foot or toe movements. The BM preceding toe movements was only apparent at the vertex and was smaller than the one preceding foot movement. This may suggest a source that is located still deeper in the brain than with foot movements.

Magnetoencephalography Reveals Slowing of Resting Peak Oscillatory Frequency in Children Born Very Preterm

Resting cortical activity is characterized by a distinct spectral peak in the alpha frequency range. Slowing of this oscillatory peak toward the upper theta-band has been associated with a variety of neurological and neuropsychiatric conditions and has been attributed to altered thalamocortical dynamics. Children born very preterm exhibit altered development of thalamocortical systems. To test the hypothesis that peak oscillatory frequency is slowed in children born very preterm, we recorded resting magnetoencephalography (MEG) from school age children born very preterm (≤32 wk gestation) without major intellectual or neurological impairment and age-matched full-term controls. Very preterm children exhibit a slowing of peak frequency toward the theta-band over bilateral frontal cortex, together with reduced alpha-band power over bilateral frontal and temporal cortex, suggesting that mildly dysrhythmic thalamocortical interactions may contribute to altered spontaneous cortical activity in children born very preterm.

Neural network classifications and correlation analysis of EEG and MEG activity accompanying spontaneous reversals of the Necker cube

Abstract It has recently been suggested that reentrant connections are essential in systems that process complex information [A. Damasio, H. Damasio, Cortical systems for the retrieval of concrete knowledge: the convergence zone framework, in: C. Koch, J.L. Davis (Eds.), Large Scale Neuronal Theories of the Brain, The MIT Press, Cambridge, 1995, pp. 61–74; G. Edelman, The Remembered Present, Basic Books, New York, 1989; M.I. Posner, M. Rothbart, Constructing neuronal theories of mind, in: C. Koch, J.L. Davis (Eds.), Large Scale Neuronal Theories of the Brain, The MIT Press, Cambridge, 1995, pp. 183–199; C. von der Malsburg, W. Schneider, A neuronal cocktail party processor, Biol. Cybem., 54 (1986) 29–40]. Reentry is not feedback, but parallel signalling in the time domain between spatially distributed maps, similar to a process of correlation between distributed systems. Accordingly, it was expected that during spontaneous reversals of the Necker cube, complex patterns of correlations between distributed systems would be present in the cortex. The present study included EEG (n=4) and MEG recordings (n=5). Two experimental questions were posed: (1) Can distributed cortical patterns present during perceptual reversals be classified differently using a generalised regression neural network (GRNN) compared to processing of a two-dimensional figure? (2) Does correlated cortical activity increase significantly during perception of a Necker cube reversal? One-second duration single trials of EEG and MEG data were analysed using the GRNN. Electrode/sensor pairings based on cortico–cortical connections were selected to assess correlated activity in each condition. The GRNN significantly classified single trials recorded during Necker cube reversals as different from single trials recorded during perception of a two-dimensional figure for both EEG and MEG. In addition, correlated cortical activity increased significantly in the Necker cube reversal condition for EEG and MEG compared to the perception of a non-reversing stimulus. Coherent MEG activity observed over occipital, parietal and temporal regions is believed to represent neural systems related to the perception of Necker cube reversals.

Slow magnetic fields of the brain preceding movement and speech

SummaryMagnetic fields of the brain were recorded preceding selfinitiated finger, foot, toe movements, stimulus-triggered finger and toe movements, and speech. Slow magnetic-field shifts were found with morphology similar to coincident electrical potentials, but with very different distributions. Inferences from the data about localization of cerebral generators were discussed.RiassuntoSono stati registrati i campi magnetici del cervello che precedono i movimenti autonomi delle dita della mano, del piede e della dita del piede stesso, i movimenti delle dita della mano e del piede causati da uno stimolo e la parola. Sono stati trovati lenti spostamenti dei campi magnetici con morfologia simile ai coincidenti potenziali elettrici, ma con distribuzioni molto diverse. Si discutono le deduzioni ottenute dai dati sulla localizzazione dei generatori cerebrali.

Cerebral Potentials Preceding Visually Triggered Saccades

Saccades are ballistic movements of the eyes that direct the fovea to a selected part of the visual environment for maximum acuity. As with other ballistic movements, cerebral potentials have been reported to precede saccades that are made either in a self-paced, voluntary manner (Becker et al., 1972) or in response to a target light (Kurtzberg and Vaughan, 1979). One of the problems for investigation of the potentials preceding stimulus-elicited as compared to voluntary saccades is the contribution of the evoked potential to the visual stimulus. In the present experiment a target light appears a t the left or right and the subject is required simply to saccade to the target, or else move the eyes the same distance in the opposite direction. This paradigm has been used by Hallet ( 1 978) who referred to the two required responses as “normal” saccades and “anti-” saccades, respectively. It is thought that the normal task should activate whatever mechanism usually functions to direct the eyes to visual stimuli, and that the antitask requires more voluntary initiative to move the eyes. Even though for both of these conditions the visual stimulus is exactly the same prior to the start of the saccade, there might be some effect over blocks of trials of the difference in stimulation following the saccade (foveal versus peripheral). I t might be the case that a foveal stimulus in some sense “reinforces” the previous normal saccade. To control for this possibility the following experiment included stimulus conditions in which the target light changed positions at the onset of the saccade so that, for example, an antisaccade would result in a target appearing at the fovea.

The use of a squid third order spatial gradiometer to measure magnetic fields of the brain.

It appears to be clear from the results that the third order gradiometer is able to detect small biomagnetic signals from the brain which are related to evoked potentials and spontaneous electrical activity. The instrument operates reasonably well within a noisy environment, however further development is necessary to balance the first gradient. We intend to pursue this direction with software systems. Some of the data presented suggest that components of MEG evoked activity may change independently of EEG. One interpretation which may derive from this is that the same current dipoles are probably not responsible for the entire configuration of evoked fields. This interpretation is consistent with EEG evidence which indicates that analogous components in the evoked potential may vary independently as a function of stimulus parameters and information processing. Perhaps a model of magnetic dipoles due to small current loops would be more compatible with the electrophysiological data.

Altered Long-Range Phase Synchronization and Cortical Activation in Children Born Very Preterm

Children born very preterm, even with broadly normal IQ, commonly show selective difficulties in visuospatial processing and executive functioning. Very little, however, is known what alterations in cortical processing underlie these deficits. We recorded MEG while eight children born very preterm (≤32 weeks gestational age) and eight full-term controls performed a visual short-term memory task at mean age 7.5 years (range 6.4 - 8.4). Previously, we demonstrated increased long-range alpha and beta band phase synchronization between MEG sensors during STM retention in a group of 17 full-term children age 6-10 years. Here we present preliminary evidence that long-range phase synchronization in very preterm children, relative to controls, is reduced in the alpha-band but increased in the theta-band. In addition, we investigated cortical activation during STM retention employing synthetic aperture magnetometry (SAM) beamformer to localize changes in gamma-band power. Preliminary results indicate sequential activation of occipital, parietal and frontal cortex in control children, as well as reduced activation in very preterm children relative to controls. These preliminary results suggest that children born very preterm exhibit altered inter-regional functional connectivity and cortical activation during cognitive processing.