Localization of brain activity during auditory verbal short-term memory derived from magnetic recordings

Beamformer source analysis and connectivity on concurrent EEG and MEG data during voluntary movements

Electroencephalography (EEG) and magnetoencephalography (MEG) are the two modalities for measuring neuronal dynamics at a millisecond temporal resolution. Different source analysis methods, to locate the dipoles in the brain from which these dynamics originate, have been readily applied to both modalities alone. However, direct comparisons and possible advantages of combining both modalities have rarely been assessed during voluntary movements using coherent source analysis. In the present study, the cortical and sub-cortical network of coherent sources at the finger tapping task frequency (2–4 Hz) and the modes of interaction within this network were analysed in 15 healthy subjects using a beamformer approach called the dynamic imaging of coherent sources (DICS) with subsequent source signal reconstruction and renormalized partial directed coherence analysis (RPDC). MEG and EEG data were recorded simultaneously allowing the comparison of each of the modalities separately to that of the combined approach. We found the identified network of coherent sources for the finger tapping task as described in earlier studies when using only the MEG or combined MEG+EEG whereas the EEG data alone failed to detect single sub-cortical sources. The signal-to-noise ratio (SNR) level of the coherent rhythmic activity at the tapping frequency in MEG and combined MEG+EEG data was significantly higher than EEG alone. The functional connectivity analysis revealed that the combined approach had more active connections compared to either of the modalities during the finger tapping (FT) task. These results indicate that MEG is superior in the detection of deep coherent sources and that the SNR seems to be more vital than the sensitivity to theoretical dipole orientation and the volume conduction effect in the case of EEG.

Event-related potential correlates of the word length effect in working memory

It has been proposed that a frontally distributed ERP negativity reflects rehearsal within the phonological loop component of working memory. This study investigated the relationship between phonological rehearsal and frontal negativity, by examining the effects of word length and articulatory suppression (continuously uttering an irrelevant word) on memory for auditorily presented words while ERPs were recorded. P2 amplitude, thought to reflect word identification, was increased for long compared to short words. However, this difference did not remain under conditions of suppression. A centrally maximum early negativity was larger in the short than long word silent conditions and this word length effect was reduced under suppression. The early negativity was interpreted as reflecting the transfer of the information from input to output buffers in the silent conditions that was prevented by the suppression. There was only a word length effect for the late frontally distributed negativity in the suppression conditions, suggesting that this component was not associated with phonological loop rehearsal but rather other working memory processes that operate under high load conditions.

Magnetoencephalography in Epilepsy

Magnetoencephalography (MEG)-also known as magnetic source imaging when combined with magnetic resonance imaging-has developed to the point that it has now entered routine clinical application. Epilepsy MEG studies show that it can accurately localize spike sources--both ictal and interictal--as compared to both direct (intracranial EEG) and indirect (imaging abnormalities) measures. Challenges remain with difficulties in detecting complex or deep sources when recording spontaneous cerebral activity. Magnetoencephalography not only provides a novel tool to localize and characterize epileptiform disturbances, it also has an important role in determining the significance of abnormalities seen on both structural and functional imaging. Combined with mapping of normal or eloquent brain function, MEG should ultimately play a major role in the totally noninvasive epilepsy surgery evaluation.

Early occipito-parietal activity in a word recognition task: an EEG and MEG study

OBJECTIVE

We investigated early brain activity (under 200 ms after the stimulus onset) related to the encoding and the retrieval of verbal information.

METHODS

First, we compared ERPs produced by words which were encoded to ERPs produced by words from following test phases (correctly identified repetitions and correctly classified new words) in two different experiments. Experiment 1 consisted of an intentional learning paradigm and experiment 2 consisted of an incidental learning paradigm. In addition, we conducted a control experiment (experiment 3), which was a continuous recognition task with two different repetition intervals. Secondly, we conducted a magnetoencephalographic (MEG) study to further investigate early brain activity (experiment 4). The same intentional learning paradigm as in experiment 1 was used.

RESULTS

We found that ERPs elicited by correctly classified test words (repeated words and new words) of both experiment 1 and experiment 2 were significantly more negative going than the ERPs elicited by the study words. This effect was apparent between 100 ms and 200 ms after the stimulus onset and was distributed over occipital and parietal scalp locations. In the control task (experiment 3), these early potential differences were missing (for both repetition intervals). Early event-related fields (ERFs) were also found to depend on the situation of the study phase and the test phase. This activity difference peaked at 120 ms after the stimulus onset. The distributions of the difference magnetic fields were occipito-parietal and thus consistent with the findings of experiment 1 (EEG-experiment).

CONCLUSION

Whether the effect we defined in the present study is due to an increase of activity during the test situation or due to a decrease of activity during the study situation remains unclear. However, it might reflect attentional processes within a word recognition task depending on whether a word is encoded or an effort of word retrieval has to be made.

Different temporal stability and partial independence of EEG asymmetries from different locations: implications for laterality research

In order to investigate a possible regional specificity of activation asymmetries of different parts of the cortex and the stability of these asymmetries across time and conditions, EEG was recorded during rest and stimulation conditions in a large sample of right-handed university students (n = 60). Recordings were made in two sessions (interval between sessions 2-4 weeks). In the first session two rest periods were conducted (interval 30 min) and one under stimulation. In the second session EEG was again recorded at rest. Reliability analyses show that the long-term stability of orbitofrontal asymmetries is markedly lower than that of dorsolateral, temporal, and parietal scores. Intercorrelations of EEG asymmetries at different electrode positions suggest that anterior and posterior EEG asymmetries are largely independent measures of cortical laterality. Both this partial independence of activation asymmetries and the differences in temporal stability underscore the significance of the anterior-posterior dimension in laterality research and may be one reason for several contradictory observations in studies on brain laterality.

Consciousness and cognition may be mediated by multiple independent coherent ensembles

Short-term or working memory (WM) provides temporary storage of information in the brain after an experience and is associated with conscious awareness. Neurons sensitive to the multiple stimulus attributes comprising an experience are distributed within many brain regions. Such distributed cell assemblies, activated by an event, are the most plausible system to represent the WM of that event. Studies with a variety of imaging technologies have implicated widespread brain regions in the mediation of WM for different categories of information. Each kind of WM may thus be expected to involve many brain regions rather than a local, uniquely dedicated set of cells. Neurons in a distributed "cell assembly" may be self-selected by their temporally coherent activations. The process by which this fragmented representation of the recent past is reassembled to accomplish essentially automatic and reliable recognition of a recurrent event constitutes an important problem. One plausible mechanism to achieve the identification of past with previous events would require that the representational system mediating WM must coexist in spatial extent and somehow overlap in temporal activation with cell ensembles registering input from subsequent events. The detection of such a postulated mechanism required an experimental approach which would focus upon spatial patterns of coherent activation while information about different events was stored in WM and retrieved, rather than focusing upon the temporal sequences of activation in localized regions of interest. For this purpose, the familiar delayed matching from sample (DMS) task was modified. A series of information-free flashes, or "noncontingent probes," was presented before an initial series of visual information items, the Priming Sample, which were to be held in WM during a Delay Period. A second series of visual information items were then presented, the Matching Sample. The task required detection of any item in the second series which had been absent from the initial series. Thirty such trials with a particular category of visual information constituted a single task. Several DMS tasks with this standardized design, but with different categories of visual information, were presented within each test session. The information categories included letters of the alphabet, single digit numbers, or faces from a school yearbook. Event-related potentials (ERPs), were computed from 21 standardized electrode placements, separately for information-free probes and for information items in each interval of the trials within a task. Because each electrode is particularly sensitive to coherent activation of neurons in the immediately underlying brain regions, topographic maps were constructed and interpolated across the surface of the scalp. The momentary fluctuations of the resulting voltage "landscapes" throughout the task were then subjected to quantitative analysis. Distinctive landscapes sometimes persisted for prolonged periods, implying sustained engagement of very large populations of neurons. "Difference landscapes" were constructed by subtraction of topographic maps evoked by noncontingent probes during the Delay Period from maps of probe ERPs before the presentation of the initial information in the Priming Sample. Such probe difference landscapes displayed recurrent high similarity to momentary landscapes elicited during subsequent presentation of the information items in the Matching Sample. It seemed as if the distributed cell assembly continuously engaged by mediation of WM of the diverse attributes of the initial stimuli was being dynamically compared to the ensembles engaged by registration of the subsequent stimuli. Spatial Principal Component Analysis was applied to the sequences of momentary voltage landscapes observed throughout trials of each task. This method sought a small number of spatial patterns with which these large sets of inhomogeneous spatial distributions of voltage co

Magnetoencephalographic study on the cerebral neural activities related to the processing of visually presented characters

Neuromagnetic fields were recorded from normal subjects to study the time course of cerebral neural activation while they performed a matching task of visual stimuli in which sequentially presented Japanese characters or unreadable pseudo-characters were compared according to phonological (reading of the characters) or graphical (geometry of the pseudo-characters) identity. In response to the single real-character or pseudo-character which was presented the latest distinct magnetic field components were observed, from which current dipole sources of the fields were localized in the individual magnetic resonance images of the brain. In the phonological identification, the sources were found in the parieto-occipital extrastriate cortex at 155-210 ms following the character presentation, and in the posterior temporal region (part of the Wernicke's area) and the posterior superior temporal region of the visual/auditory association cortex at 210-410 ms. The activity in these temporal regions was left hemisphere dominant, and may be the neural basis of phonological processing of the visual characters. In the graphical identification, sources occurring at 125-250 ms were noted in the inferior temporo-occipital region, and those at 180-460 ms in the posterior temporal and posterior superior temporal regions of the right hemisphere. These results indicate that the activities in the temporal area are lateralized to the left for the phonological processing and to the right for the graphical processing.

Working memory in temporal lobe epilepsy: an event-related potential study

Event-related potentials (ERPs) were recorded to a digit-probe identification and matching task (modified 'Sternberg paradigm') in 29 patients with temporal lobe epilepsy (TLE) and 26 healthy subjects. Our main aim was to identify the neurophysiological correlates of abnormal short term memory function in patients with TLE. Neuropsychological tests allowed the definition and comparison of two patient groups according to the presence or absence of memory dysfunction. These two groups did not differ significantly in mean age, education years, IQ, seizure duration, seizure frequency, anti-epileptic drug (AED) regimes, or on findings on neuroimaging. ERPs recorded under different levels of memory load were analysed both by conventional component identification and by an objective computer method of determining mean amplitudes of multiple 50 ms epochs (MMA analysis). We found that some significant abnormalities were common to both groups of patients; these included slow reaction times, a reduced amplitude of the N170 wave (and the corresponding 157-210 ms epoch in the MMA analysis) and a broad late negative shift between 577 and 735 ms. Other findings, including a significantly reduced performance accuracy as the level of memory load increased, were restricted to patients with abnormal memory function. The ERP changes that were specific to these patients occurred within a latency band of 200-420 ms and included a relatively preserved, but delayed P250 component and a delayed and attenuated N290 wave. When compared with either healthy subjects or with patients with normal memory, the responses in patients with abnormal memory showed an abnormal 'positive shift' between 262 and 315 ms after probe presentation and a further positive shift between 315 and 420 ms as memory load increased. These abnormalities of 'memory scanning' ERPs in patients with TLE which paralleled neuropsychological and behavioural evidence of memory dysfunction, and which occurred in the section of the response that is sensitive to memory loading in healthy subjects, provide further objective evidence that abnormalities of short term memory processes contribute to the memory deficits of TLE.

Human brain potentials of spatial location encoding into memory

Event-related potentials (ERP) were elicited by little vertical bars located randomly in 1 of 4 positions (top, bottom, left or right) relative to a central fixation point. There were 4 experimental conditions requiring the subjects either to press a button if the stimulus was in the target location, count the number of stimuli appearing in the target location, look at the stimuli passively or memorize the location of the stimulus. The interstimulus interval was 2 sec for all tasks. In the memory condition subjects had to consider stimuli as pairs, memorize the location of the first stimulus of every pair and press a button if the second stimulus was in the same location. Our results indicate that ERPs corresponding to the memorization and retention of spatial location are different from those of the other 3 tasks in the presence of a long duration negativity mainly distributed bilaterally over O1, O2, T5 and T6 electrodes. This negativity seems to develop gradually several milliseconds before stimulus onset, reaches its highest value when the spatial location is assumed to be analyzed, and continues with uniform scalp distribution until the end of the recording (822 msec after stimulus onset).

Spatio-temporal stages in face and word processing. I. Depth-recorded potentials in the human occipital, temporal and parietal lobes [corrected]

Evoked potentials (EPs) were used to help identify the timing, location, and intensity of the information-processing stages applied to faces and words in humans. EP generators were localized using intracranial recordings in 33 patients with depth electrodes implanted in order to direct surgical treatment of drug-resistant epilepsy. While awaiting spontaneous seizure onset, the patients gave their fully informed consent to perform cognitive tasks. Depth recordings were obtained from 1198 sites in the occipital, temporal and parietal cortices, and in the limbic system (amygdala, hippocampal formation and posterior cingulate gyrus). Twenty-three patients received a declarative memory recognition task in which faces of previously unfamiliar young adults without verbalizable distinguishing features were exposed for 300 ms every 3 s; 25 patients received an analogous task using words. For component identification, some patients also received simple auditory (21 patients) or visual (12 patients) discrimination tasks. Eight successive EP stages preceding the behavioral response (at about 600 ms) could be distinguished by latency, and each of 14 anatomical structures was found to participate in 2-8 of these stages. The earliest response, an N75-P105, focal in the most medial and posterior of the leads implanted in the occipital lobe (lingual g), was probably generated in visual cortical areas 17 and 18. These components were not visible in response to words, presumably because words were presented foveally. A focal evoked alpha rhythm to both words and faces was also noted in the lingual g. This was followed by an N130-P180-N240 focal and polarity-inverting in the basal occipitotemporal cortex (fusiform g, probably areas 19 and 37). In most cases, the P180 was evoked only by faces, and not by words, letters or symbols. Although largest in the fusiform g this sequence of potentials (especially the N240) was also observed in the supramarginal g, posterior superior and middle temporal g, posterior cingulate g, and posterior hippocampal formation. The N130, but not later components of this complex, was observed in the anterior hippocampus and amygdala. Faces only also evoked longer-latency potentials up to 600 ms in the right fusiform g. Words only evoked a series of potentials beginning at 190 ms and extending to 600 ms in the fusiform g and near the angular g (especially left). Both words and faces evoked a N150-P200-PN260 in the lingual g, and posterior inferior and middle temporal g.(ABSTRACT TRUNCATED AT 400 WORDS)