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W R Webber

Publications and source records attributed to W R Webber.

14 recordsLinked to original sources
PubMed2005

Voyager 1 explores the termination shock region and the heliosheath beyond.

Voyager 1 crossed the termination shock of the supersonic flow of the solar wind on 16 December 2004 at a distance of 94.01 astronomical units from the Sun, becoming the first spacecraft to begin exploring the heliosheath, the outermost layer of the heliosphere. The shock is a steady source of low-energy protons with an energy spectrum approximately E(-1.41 +/- 0.15) from 0.5 to approximately 3.5 megaelectron volts, consistent with a weak termination shock having a solar wind velocity jump ratio r=2.6(-0.2)(+0.4). However, in contradiction to many predictions, the intensity of anomalous cosmic ray (ACR) helium did not peak at the shock, indicating that the ACR source is not in the shock region local to Voyager 1. The intensities of approximately 10-megaelectron volt electrons, ACRs, and galactic cosmic rays have steadily increased since late 2004 as the effects of solar modulation have decreased.

Journal Article↗
PubMed1999

Brief bursts of pulse stimulation terminate afterdischarges caused by cortical stimulation.

OBJECTIVE: To determine whether cortical electrical stimulation can terminate bursts of epileptiform activity in humans, we used afterdischarges (ADs) as a model of epileptiform activity. METHODS: Cortical stimulation was performed for clinical localization purposes using subdural electrodes implanted in patients undergoing preresection evaluations for treatment of medically intractable seizures. We used 0.3-millisecond pulses of alternating polarity, repeated at 50 pulses/second. When stimulation produced AD, we often applied short additional brief bursts of pulse stimulation (BPS). We examined the effectiveness of BPS in aborting ADs in 17 patients using survival analysis. RESULTS: With BPS, ADs stopped within 2 seconds in 115 cases, 2 to 5 seconds in 22 cases, and in more than 5 seconds in 89 cases. Without BPS, ADs stopped within 2 seconds in 21 cases, 2 to 5 seconds in 114 cases, and in more than 5 seconds in 340 cases. BPS was an effective method to abort ADs (Cox proportional hazards model: p<0.0001). At any time during the course of ADs, the instantaneous rate of stopping ADs within 2 seconds after BPS was applied was 4.6 times greater than when BPS was not applied (95% CI = 3.7, 5.7). In eight cases, ADs progressed to the occurrence of clinical seizures, always when BPS was not applied. CONCLUSIONS: Afterdischarges significantly decreased in duration after we applied brief bursts of pulse stimulation. Although afterdischarges are not identical to spontaneous epileptiform activity, these results support the idea that electrical stimulation, applied in an appropriate manner at seizure onset, could abort seizures in humans.

Adolescent↗
PubMed1998

Enhancing the detection of seizures with a clustering algorithm.

Automated detection algorithms of EEG seizures or similar clinical events typically analyze a finite epoch a given channel at a time, producing a probability or a weight estimating how likely it is for the event to resemble a clinical pattern. Epochs are normally shorter than the duration of a seizure, which may spread to more than one electrode. This may result in a weak correspondence between the seizure pattern in the record and its calculated detector event counterpart. As a result, such algorithms suffer from a high rate of false detections. We show that the weights/probabilities of a generic detector can be described as a weight function embedded in a directed graph (digraph). Extended objects such as seizures therefore correspond to the connected components of the digraph. We introduce a clustering algorithm that accounts for the shortcomings of a generic detector of the type described above. By correlating detector results with respect to both time and channel, we effectively extend the detection to an unlimited number of electrodes over an indefinite time. The algorithm is fast (linear - O(m)) and may be implemented in real time. We argue that the algorithm enhances the detection of seizure onset and lowers the rate of false detections. Preliminary results demonstrate a strong correlation between the seizure and the cluster's boundaries and over 50% reduction of false detection rate.

Algorithms↗
PubMed1997

Neuronal generators of visual evoked potentials in humans: visual processing in the human cortex.

PURPOSE: We wished to define the localization of cortical generators of visual (pattern) evoked potentials (VEP) and the temporal sequence of activation in the occipital region. METHODS: In 4 candidates for epilepsy surgery, a large array of subdural electrodes was placed over occipital areas. Checkerboard pattern reversal stimuli were generated and the epileptogenic focus was localized and functionally mapped. Magnetic resonance imaging did not show any occipital lesions in any of the 4 patients. RESULTS: The area first activated was the lingual gyrus in the mesial occipital lobe (negative potential peaks at approximately 70 ms), followed by an area superior to the calcarine fissure (negative peaks at approximately 80 ms). Later (starting at approximately 90 ms), there were positive potentials over the occipital pole and lingual gyrus, followed by potentials at the lateral occipital lobe. CONCLUSIONS: These data support the idea that VEP are generated in the mesial and lateral occipital cortex by different circumscribed neuronal generators with different latencies of activation. The scalp-recorded N1 and P1 potential peaks most likely derive from the progressive activation of neuronal masses in different regions of the occipital lobe.

Brain Mapping↗
PubMed1996

An approach to seizure detection using an artificial neural network (ANN).

We have developed an EEG seizure detector based on an artificial neural network. The input layer of the ANN has 31 nodes quantifying the amplitude, slope, curvature, rhythmicity, and frequency components of EEG in a 2 sec epoch. The hidden layer has 30 nodes and the output layer has 8 nodes representing various patterns of EEG activity (e.g. seizure, muscle, noise, normal). The value of the output node representing seizure activity is averaged over 3 consecutive epochs and a seizure is declared when that average exceeds 0.65. Among 78 randomly selected files from 50 patients not in the original training set, the detector declared at least one seizure in 76% of 34 files containing seizures. It declared no seizures in 93% of 44 files not containing seizures. Four false detections during 4.1 h of recording yielded a false detection rate of 1.0/h. The detector can continuously process 40 channels of EEG with a 33 MHz 486 CPU. Although this method is still in its early stages of development, our results represent proof of the principle that ANN could be utilized to provide a practical approach for automatic, on-line, seizure detection.

Electroencephalography↗
PubMed1994

Practical detection of epileptiform discharges (EDs) in the EEG using an artificial neural network: a comparison of raw and parameterized EEG data.

We have developed and tested "off-line" an artificial neural network (ANN) that successfully detects epileptiform discharges (EDs) when trained on EEG records marked by an electroencephalographer (EEGer). The system was trained on both parameterized and raw EEG data and can process 49 channels of EEG data in real time on an 80486/33 MHz personal computer, making it capable of processing EEG on-line in long-term monitoring units. Our detector consists of 2 stages: (1) a threshold detector identifies candidate EDs in 4-channel bipolar chains within the recording montage, parameterizes them and then passes these data to the second stage; (2) a 3-layer feed-forward ANN decides if a candidate wave form is an ED. The intersection of detector sensitivity and selectivity curves, or crossover threshold, for 10 patients from our Epilepsy Monitoring Unit occurred at 73% for parameterized EEG data and at 46% for "raw" EEG data. The ANN could be adapted to different EEGers' styles by changing the ANN output threshold for accepting candidate wave forms as EDs. In this "proof of principle" study the detector was trained on EEGs from 10 Johns Hopkins Hospital Epilepsy Monitoring Unit (JHH EMU) patients. We used different EEGs from the same patients for testing. Current testing should demonstrate that the ANN detector can generalize to previously "unseen" patients. This study shows that ANNs offer a practical solution for automated, real time ED detection that uses, standard, inexpensive computers, is easily adjustable to individual EEGer style and can produce sensitivities and selectivities similar to those of EEGers.

Brain↗
PubMed1994

A program to measure new energetic particle nuclear interaction cross sections.

The Transport Collaboration, consisting of researchers from institutions in France, Germany, Italy and the USA, has established a program to make new measurements of nuclear interaction cross sections for heavy projectiles (Z > or = 2) in targets of liquid H2, He and heavier materials. Such cross sections directly affect calculations of galactic and solar cosmic ray transport through matter and are needed for accurate radiation hazard assessment. To date, the collaboration has obtained data using the LBL Bevalac HISS facility with 20 projectiles from 4He to 58Ni in the energy range 393-910 MeV/nucleon. Preliminary results from the analysis of these data are presented here and compared to other measurements and to cross section prediction formulae.

Cosmic Radiation↗
PubMed1993

Automatic EEG spike detection: what should the computer imitate?

We conducted a study to explore how electroencephalographers (EEGers) read EEGs and reach clinical impressions based upon them. Eight EEGers and a rule-based computerized "spike" detector marked epileptiform discharges ("EDs") in 12 test records. Of all marked events, 18% were marked by all readers and 38% were marked by only one reader. Readers agreed on basic clinical features of the records, such as whether a record demonstrated EDs, the rank order of ED sources by location, and the ranking of test records in order of the number of EDs detected. Readers marked records in a consistent pattern that was independent of an objective measure of expertise and experience. Our computerized ED detector had lower sensitivity and selectivity than human readers, but either parameter could be adjusted to be comparable to human EEGers at the expense of the other. We propose that EEGers employ reproducible, quantitatively different styles of reading EEG tracings to reach qualitatively similar clinical impressions. In practice, EDs are not absolutely defined, but appear to represent a continuum of activity which lends itself better to description and rank ordering than to absolute quantitation. More than just counting EDs, a successful computerized ED detector should be adaptable to the style of individual readers in order to help them efficiently formulate their clinical impressions.

Brain↗
PubMed1992

High-frequency EEG activity at the start of seizures.

Frequencies above 35-40 Hz are poorly visualized on conventional EEG scalp recordings. We investigated frequency components up to 150 Hz in digitally recorded EEGs of seizures in five patients with implanted subdural grids, as part of their evaluation for epilepsy surgery. Amplifier bandpass was set from 0.1 to 300 Hz, and EEG was digitized at 2,000 samples per second. Seizures with electrodecremental patterns at the start showed a significant increase in spectral power above 35 Hz, with a twofold increase in the 40-50-Hz range, and up to a fivefold increase in the 80-120-Hz portion of the spectrum. Activity above 40 Hz could represent summed action potentials, harmonics of synaptic potentials or transient sharp components of synaptic potentials. High-frequency increases were largely localized to the region of the seizure focus. Grid sites remote from the focus did not show significant energy in the EEG band above 40 Hz at baseline, nor at time of seizure onset. Our findings suggest that high-frequency recordings may be of use in localizing seizure foci.

Adolescent↗
PubMed1987

Fluorophotometric measurements of tear turnover rate in normal healthy persons: evidence for a circadian rhythm.

The determination of tear turnover rate in a total of 16 normal healthy volunteers is described. A continuous reading fluorophotometer is used to measure the fluorescence resulting from the instillation of 1 microliter of 2 per cent fluorescein into the eye. A mean (+/- s.d.) turnover rate of 14.9 +/- 5.6 per cent per minute is found, in agreement with previously reported values. Morning and afternoon measurements on the same subject show that morning tear turnover rates are significantly higher than those in the afternoon (p less than 0.01), suggesting a circadian rhythm in tear flow. It is also concluded that use of a 10 per cent fluorescein solution can lead to a significant underestimation of tear turnover rates and that any resulting errors are likely to be larger for light coloured irises.

Adult↗
PubMed1982

Ophthalmic fluorophotometry: a new solid state fluorophotometer.

A new low-voltage ophthalmic fluorophotometer is described. A solid state photodetector has been combined with a high-gain, low-noise amplification and synchronous detection system to develop a stable, relatively inexpensive instrument. Reliable measurements of sodium fluorescein concentration in the eye can be made down to 100 ng/ml (0.26 mumol/l) which allows the estimation of corneal permeability and tear flow rates. Preliminary measurements of tear turnover rate using the fluorophotometer are presented.

Cornea↗