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A Gevins

Publications and source records attributed to A Gevins.

18 recordsLinked to original sources
PubMed1996

High resolution evoked potentials of cognition.

The precision of four methods of quantifying neuroelectric signals has been improved by increasing EEG spatial sampling, using up to 124 electrodes, and by accurate anatomical registration of the EEG with Magnetic Resonance Images (MRIs). One such method, equivalent dipole modeling, is a well-known form of source localization which is useful when the generator of the scalp recorded signal approximates a simple dipolar source, as is usually the case with early and mid-latency Evoked Potentials (EPs). Two methods of enhancing spatial detail which benefit from increased spatial sampling include the Laplacian Derivation and the Finite Element Deblurring method. The latter is a new technique which estimates the EP distribution at the superficial cortical surface. The fourth method, Evoked Potential Covariance, characterizes the spatiotemporal relationships among EP segments at different recording sites. This is useful when studying "functional neural networks" underlying higher cognitive functions. These methods are reviewed and examples of results of their application in recent experiments are presented.

Brain↗
PubMed1996

High resolution evoked potential imaging of the cortical dynamics of human working memory.

High resolution evoked potentials (EPs), sampled from 115 channels and spatially sharpened with the finite element deblurring method, were recorded from 8 subjects during working memory (WM) and control tasks. The tasks required matching each stimulus with a preceding stimulus on either verbal or spatial attributes. All stimuli elicited a central P200 potential that was larger in the spatial tasks than in the verbal tasks, and larger in the WM tasks than in the control tasks. Frequent, non-matching stimuli elicited a frontal, positive peak at 305 msec that was larger in the spatial WM task relative to the other tasks. Irrespective of whether subjects attended to verbal or spatial stimulus attributes, non-matching stimuli in the WM tasks also elicited an enhanced P450 potential over the left frontal cortex, followed by a sustained potential over the superior parietal cortex. A posterior P390 potential elicited by infrequent, matching stimuli was smaller in amplitude for both spatial and verbal WM tasks compared to control tasks, as was a central prestimulus CNV. These results indicate that WM is a function of a distributed system with both task-specific and task-independent components. Lesion studies and course temporal resolution functional imaging methods, such as PET and fMRI, tend to paint a fairly static picture of the cortical regions which participate in the performance of WM tasks. In contrast, the fine-grain time resolution provided by imaging brain function with EP methods provides a dynamic picture of subsecond changes in the spatial distribution of WM effects over the course of individual trials, as well as evidence for differences in the activity elicited by matching and non-matching stimuli within sequences of trials. This information about the temporal dynamics of WM provides a critical complement to the fine-grain spatial resolution provided by other imaging modalities.

Acoustic Stimulation↗
PubMed1995

Regional modulation of high resolution evoked potentials during verbal and non-verbal matching tasks.

Nine subjects performed a cued S1-S2 matching task in which two sequentially presented visual stimuli (either letter strings or non-verbal graphical patterns) were compared according to verbal (phonemic, semantic, syntactic) or non-verbal (graphic identity) criteria. The Laplacian derivation was used to spatially enhance the topography of averaged evoked potentials (EPs) recorded from 59 scalp electrodes. Several effects distinguished the non-verbal from the verbal conditions. For example, following S1 a P250 EP that reached maximum amplitude over the occipital area was larger for the non-verbal patterns, whereas word and word-like letter strings (but not unfamiliar characters) elicited an N470 in the left temporal region. In anticipation of S2, a CNV-like slow potential was enhanced over posterior regions for the non-verbal stimuli. During the matching interval following S2, a P475 peak was observed to be larger for non-verbal patterns than for letter strings over right frontal and temporal regions. Other effects distinguished the verbal conditions from one another. In particular, following S1 a left frontal P445 potential was enhanced to closed class versus open class words, and following S2 a P620 potential in the left temporal region was enhanced for phonological matching relative to semantic matching. These results suggest that processing of verbal and non-verbal stimuli depends on a network of subprocessors that are regionalized to functionally specialized cortical areas and that operate both sequentially and in parallel in order to extract and synthesize multiple forms of attribute-specific information. In contrast to neuropsychological approaches to the study of pattern recognition and reading, the fine-grain temporal resolution of EP measurements, in combination with the improved spatial resolution obtained through computation of Laplacian derivation wave forms from a large number of electrodes, permits characterization of both the regionalization of subprocesses and the subsecond dynamics of their engagement.

Adult↗
PubMed1995

Mapping cognitive brain function with modern high-resolution electroencephalography.

High temporal resolution is necessary to resolve the rapidly changing patterns of brain activity that underlie mental function. While electroencephalography (EEG) provides temporal resolution in the millisecond range, which would seem to make it an ideal complement to other imaging modalities, traditional EEG technology and practice provides insufficient spatial detail to identify relationships between brain electrical events and structures and functions that are visualized by magnetic resonance imaging (MRI) or positron emission tomography (PET). Recent advances overcome this problem by recording EEGs from more electrodes, by registering EEG data with anatomical information from each subject's MRI, and by correcting the distortion that is caused by volume conduction of EEG signals through the skull and scalp. Along with its ability to record how brains think when performing everyday activities in the real world, these advances make modern EEG an invaluable complement to other functional neuroimaging modalities.

Brain↗
PubMed1995

Towards measurement of brain function in operational environments.

In operational environments that demand sustained vigilance or that involve multiple tasks competing for limited attentional resources, continuous monitoring of the mental state of the operator could decrease the potential for serious errors and provide valuable information concerning the ergonomics of the tasks being performed. There is widespread discussion and appreciation of the basic feasibility of utilizing neurophysiological measurements to derive accurate, reliable, rapid and unobtrusive assessments of mental state. However, progress in transitioning this idea into practical applications has been impeded by the fact that at present no convenient, inexpensive and effective means exists to derive a meaningful index of brain activity outside of laboratory settings. In this paper, we review some recent advances in recording technology and signal processing methods that will help overcome this limitation. For example, rapid progress is being made in the engineering of recording systems that are small, rugged, portable and easy-to-use, and thus suitable for deployment in operational environments. Progress is also being made in the development of signal processing algorithms for detecting and correcting recording artifacts and for increasing the amount of useful information that can be derived from brain signals. Finally, results from basic research studies suggest that accurate and reliable inferences about the mental load and alertness of an individual can be derived from neurophysiological measures in a practical fashion. These research and engineering successes suggest that it is reasonable to expect that in the near term a basic enabling technology will be deployed that will permit routine measurement of brain function in operational environments.

Arousal↗
PubMed1994

High resolution EEG: 124-channel recording, spatial deblurring and MRI integration methods.

This paper describes a method for increasing the spatial detail of the EEG and for integrating physiological data with anatomical models based on magnetic resonance images (MRIs). This method includes techniques to efficiently record EEG data from up to 124 channels, to measure 3-D electrode positions for alignment with MRI-derived head models, and to estimate potentials near the outer convexity of the cortex using a spatial deblurring technique which uses a realistic model of the structure of the head and which makes no assumptions about the number or type of generator sources. The validity of this approach has been initially tested by comparing estimated cortical potentials with those measured with subdural grid recordings from two neurosurgical patients. The method is illustrated with somatosensory steady-state evoked potential data recorded from 5 healthy subjects. Results suggest that deblurred 124-channel topographic maps, registered with a subject's MRI and rendered in 3 dimensions, provide better spatial detail than has heretofore been obtained with scalp EEG recordings. The results also suggest that the potential for EEG as a functional neuroimaging modality has yet to be fully realized.

Adult↗
PubMed1994

Local estimate of surface Laplacian derivation on a realistically shaped scalp surface and its performance on noisy data.

A new implementation of the surface Laplacian derivation (SLD) method is described which reconstructs a realistically shaped, local scalp surface geometry using measured electrode positions, generates a local spectral-interpolated potential distribution function, and estimates the surface Laplacian values through a local planar parametric space using a stable numerical method combining Taylor expansions with the least-squares technique. The implementation is modified for efficient repeated SLD operations on a time series. Examples are shown of applications to evoked potential data. The resolving power of the SLD is examined as a function of the spatial signal-to-noise (SNR) ratio. The analysis suggests that the Laplacian is effective when the spatial SNR is greater than 3. It is shown that spatial low-pass filtering with a Gaussian filter can be used to reduce the effect of noise and recover useful signal if the noise is spatially incoherent.

Electricity↗
PubMed1994

Subdural grid recordings of distributed neocortical networks involved with somatosensory discrimination.

Previous studies suggest that evidence for the sub-second activation of distributed neural networks can be obtained by computing the covariance between segments of the scalp-recorded evoked potential. However, the cortical representation of such potentials is not known. Here we report a case study where the evoked potential covariance (EPC) measure was applied to data recorded from a 58-channel subdural grid implanted in an epilepsy patient. Recordings were made while the patient performed a task that required judging the somatosensory intensities of electrical stimuli and executing precise finger flexion responses in response to a subset of those stimuli. Post-stimulus EPC patterns involved covariances between somatosensory, motor, and temporal regions. Pre-stimulus EPC patterns involved these same regions, but only when it could be anticipated that the upcoming stimulus would likely require a response. The majority of the observed EPCs occurred with non-zero time-lags, and these EPCs often involved non-adjacent electrode pairs. Thus, the observed EPCs were unlikely to arise solely from volume conduction. Rather, they appeared to reflect the transient integration of activity across distinct cortical processing nodes.

Adult↗
PubMed1993

Spatiotemporal dynamics of component processes in human working memory.

Working memory (WM), the ability to momentarily maintain information in an active state, is central to higher cognitive functions. The processes involved in WM operate on a sub-second timescale, and thus evoked potential measures have an appropriate temporal resolution for studying them. In the experiment reported here, evoked potential covariances (EPC) between scalp recording sites were computed for a task requiring maintenance of numeric information in WM; these EPCs were compared to those observed in a control task which had the same stimuli and responses but less of a WM requirement. EPC patterns differed between conditions prior to the stimulus, and in an interval spanning the P300 peak in the match detection trials which required response inhibition. The pattern of prestimulus EPCs was more complex and left-sided in the WM task, when memory codes were being maintained and responses contingent on those codes were being prepared. P300 peak latency was 140 msec shorter in the WM task, and the P300 EPC pattern was more anterior and left-sided. In contrast, EPC patterns did not differ during early stages of stimulus processing or during response execution. These results suggest that distinct EPC patterns associated with WM only occur during intervals in which the information in an active state is being utilized for task performance.

Brain Mapping↗
PubMed1993

Method to reduce blur distortion from EEG's using a realistic head model.

A mathematical procedure, which we call "Deblurring," was developed to reduce spatial blur distortion of scalp-recorded brain potentials due to transmission through the skull and other tissues. Deblurring estimates potentials at the superficial cerebral cortical surface from EEG's recorded at the scalp using a finite element model of each subject's scalp, skull and cortical surface constructed from their magnetic resonance images (MRI's). Simulations indicate that Deblurring is numerically stable, while a comparison of deblurred data with a direct cortical recording from a neurosurgery patient suggests that the procedure is valid. Application of Deblurring to somatosensory evoked potential data recorded at 124 scalp sites suggests that the method produces a dramatic improvement in spatial detail, and merits further development.

Adult↗
PubMed1991

Seeing through the skull: advanced EEGs use MRIs to accurately measure cortical activity from the scalp.

There is a vast amount of untapped spatial information in scalp-recorded EEGs. Measuring this information requires use of many electrodes and application of spatial signal enhancing procedures to reduce blur distortion due to transmission through the skull and other tissues. Recordings with 124 electrodes are now routinely made, and spatial signal enhancing techniques have been developed. The most advanced of these techniques uses information from a subject's MRI to correct blur distortion, in effect providing a measure of the actual cortical potential distribution. Examples of these procedures are presented, including a validation from subdural recordings in an epileptic patient. Examples of equivalent dipole modeling of the somatosensory evoked potential are also presented in which two adjacent fingers are clearly separated. These results demonstrate that EEGs can provide images of superficial cortical electrical activity with spatial detail approaching that of O15 PET scans. Additionally, equivalent dipole modeling with EEGs appears to have the same degree of spatial resolution as that reported for MEGs. Considering that EEG technology costs ten to fifty times less than other brain imaging modalities, that it is completely harmless, and that recordings can be made in naturalistic settings for extended periods of time, a greater investment in advancing EEG technology seems very desirable.

Brain↗
PubMed1990

Beyond topographic mapping: towards functional-anatomical imaging with 124-channel EEGs and 3-D MRIs.

A functional-anatomical brain scanner that has a temporal resolution of less than a hundred milliseconds is needed to measure the neural substrate of higher cognitive functions in healthy people and neurological and psychiatric patients. Electrophysiological techniques have the requisite temporal resolution but their potential spatial resolution has been not realized. Here we briefly review progress in increasing the spatial detail of scalp-recorded EEGs and in registering this functional information with anatomical models of a person's brain. We describe methods and systems for 124-channel EEGs and magnetic resonance image (MRI) modeling, and present first results of the integration of equivalent-dipole EEG models of somatosensory stimulation with 3-D MRI brain models.

Brain↗
PubMed1981

Surface and deep EEG correlates of surgical outcome in temporal lobe epilepsy.

Interictal and ictal EEG characteristics derived from limited surface montages and medial temporal lobe sites were compared with long-term seizure relief following anterior temporal lobectomy in 52 epileptics. Patients were classified into one of four surgical outcome groups, ranging from seizure free to no clinical improvement. For each patient, interictal records were analyzed according to deep and surface spike characteristics and background activity. Ictal records were analyzed according to the proportion of episodes initiated in a unilateral or bilaterally synchronous fashion, the proportion of surface or deep onsets, the variability of onset location, and the morphology of seizures onsets. Interictal EEG variables that correlates with surgical outcome included: (a) various types of bilaterally synchronous surface/deep spikes; (b) diffuse background slowing; (c) sharp waves; and (d) the presence of multiple independent deep spike patterns in the lobe chosen for resection. Relevant ictal EEG variables included: (a) episodes initiated in a bilaterally synchronous fashion; (b) variability in seizure onset location; (c) the proportion of precisely focal onsets from deep sites; (d) the proportion of surface onsets; and (e) the proportion of onsets from the side chosen for resection. Multivariate analysis of these data with linear, stepwise, discriminate analysis and adaptive, nonlinear, distribution-free pattern recognition demonstrated that: (a) both interictal and ictal EEG characteristics can independently predict surgical outcome at levels significantly better than chance; (b) ictal and interictal EEG data contain nonredundant information for making such predictions; and (c) nonlinear pattern recognition techniques are capable of deriving the most accurate rules for predicting the effects of surgery.

Adolescent↗
PubMedDate not supplied

Dynamic functional topography of cognitive tasks.

Improved neuroelectric recording and analysis tools are yielding increasingly specific information about the spatial and temporal features of neurocognitive processes. Such tools include recordings with up to 125 channels, digital signal processing techniques, and correlation of neuroelectric measures with anatomical information from magnetic resonance images. These tools, and their application to the study of cognitive functions, are presented in this paper.

Adult↗