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R Srebro

Publications and source records attributed to R Srebro.

At least 19 recordsLinked to original sources
PubMed2000

Subspace averaging of steady-state visual evoked potentials.

A new algorithm for doing signal averaging of steady-state visual evoked potentials (VEP's) is described. The subspace average is obtained by finding the orthogonal projection of the VEP measurement vector onto the signal subspace, which is based on a sinusoidal VEP signal model. The subspace average is seen to out-perform the conventional average using a new signal-to-noise-ratio-based performance measure on simulated and actual VEP data.

Algorithms↗
PubMed1998

Optimal detection of visual evoked potentials.

We consider the problem of detecting visual evoked potentials (VEP's). A matched subspace filter is applied to the detection of the VEP and is demonstrated to perform better than a number of other evoked potential detectors. Unlike single-harmonic detectors, the matched subspace filter (MSF) detector is suitable for detecting multiharmonic VEP's. Moreover, the MSF is optimal in the uniformly most powerful sense for multiharmonic signals with unknown noise variance.

Algorithms↗
PubMed1997

Estimating cortical activity from VEPS with the shrinking ellipsoid inverse.

An iterative inverse method using Tikhonov regularization (the shrinking ellipsoid method) previously tested in a model system is used to invert the sequence of bioelectric scalp fields evoked by the onset of a checkerboard pattern in either the right or left lower hemifield. The shrinking ellipsoid method is modified from its original description to accommodate simultaneously inverting a sequence of thirteen VEP scalp fields measured from 65 to 125 ms after stimulus onset. This allows the evoked cortical activity to be tracked in 5-ms intervals without distortion due to occasional VEP scalp fields in the sequence that have too low a signal-to-noise ratio to be reliably inverted in isolation. A new method is described to identify the surface of the cortex from MRI data. This is required to implement the shrinking ellipsoid inverse. Results from two subjects studied in detail are presented. The earliest cortical activity occurs either in area MT (the middle temporal area) or simultaneously in MT and striate cortex (V1). However when it does occur in both areas, the activity in V1 is relatively weak and quickly subsides. Seventy-five ms after stimulus onset activity is seen mainly near MT corresponding to a region identified from PET studies as one that subserves motion processing. Activity moves to V1 by 90-100 ms after stimulus onset. Near 120 ms after stimulus onset, cortical activity returns to the region near MT. Virtually all activity identified in this time epoch occurs in the cortical hemisphere contralateral to the location of the stimulus in the visual field.

Brain↗
PubMed1996

Apparent motion confounds early vernier visual evoked potentials.

Human scalp potentials evoked by vernier stimuli have been recorded for offsets less than the diameter of a foveal cone, but always for abruptly moving stimuli. Those evoked potentials were related to the magnitude of vernier offset. Here we report results for stimuli containing no apparent motion confound, using multichannel recordings and multivariate analysis methods which stress the concept of a sampling of the scalp field. We found evidence of cortical activity dependent on the direction of vernier offset, at much shorter latencies (ca. 75 ms) than previously reported, but no evidence of early cortical activity related to the magnitude of offset. Repeating the experiments and analysis using a stimulus containing apparent motion, we found evidence of cortical activity dependent on the magnitude of offset at both 75 and 200 ms, but none related to direction of offset. These findings suggest that previous studies which contained the apparent motion confound might not have obtained visual evoked potentials entirely due to vernier offset.

Electroencephalography↗
PubMed1996

An iterative approach to the solution of the inverse problem.

The bioelectric inverse problem is framed as a search through a feasible set of solutions for one that is physiologically plausible. The definition of the feasible set of solutions takes into account the important effects of measurement noise. This leads to an iterative approach. At each step, a regularized inverse is used to limit the amount of brain that need be searched in the next step. The approach is tested in a model system that incorporates a digitized cadaver cortex into a 3-shell spherical replica of the head.

Artifacts↗
PubMed1996

A bootstrap method to compare the shapes of two scalp fields.

A method is described to compare two evoked potential scalp fields in order to decide if the two fields are the same or different. The method uses Efron's bootstrap technique which avoids potential errors due to assumptions about the underlying stochastic process. It is configured to focus only on the shape of the evoked potential scalp field. The method is applied to a simple visually evoked potential paradigm and results are compared to the chi-square test using data from 7 normal subjects.

Evoked Potentials, Visual↗
PubMed1996

Event-related potential scalp fields during parallel and serial visual searches.

Event-related potential (ERP) scalp fields generated during parallel and serial searches were compared using a bootstrap resampling technique. Two different parallel search tasks required the detection of a single feature either color or orientation. A serial search task required the detection of the feature conjunction target: color and orientation. Identical stimuli were used for both parallel searches and a similar stimulus for the serial search. No evidence for scalp field differences earlier than 150 ms were discovered, suggesting that "low-level' visual processing is the same in both types of searches. ERP scalp fields that distinguished parallel from serial searches were identified between 150 and 250 ms. It is proposed that these different scalp fields represent timing and/or magnitude differences in the regions of cortex activated during parallel and serial searches.

Adult↗
PubMed1996

Iterative refinement of the minimum norm solution of the bioelectric inverse problem.

Functional brain imaging based on the bioelectric scalp field depends on the solution of the inverse problem. The object is to estimate the current distribution in the cortex from a measured noisy scalp potential field. A minimum norm least squares solution is often used for this purpose. Although this approach leads to a unique solution, it represents only one of a set of feasible solutions. In particular, it leads to a solution in which current is found to be generated widely in the cortex. There are other feasible minimum norm solutions for which cortical current is generated only in a restricted region, and it is likely that in many cases, these solutions are of physiological importance. This study presents a method to uncover these solutions. It is based on the idea that a minimum norm solution can be used to define a region of interest, an ellipsoid, within which it is worthwhile to search for another feasible solution. The minimum norm approach is used iteratively and the ellipsoid shrinks. As it shrinks it provides evidence pointing to the location of the active cortical region. The method explicitly recognizes the role of measurement noise in defining feasible solutions. It is tested in a model of the human head that incorporates a realistic cortex in a three-shell sphere. The method improves the localization of cortical activity mimicking physiological processing.

Brain Mapping↗
PubMed1996

A modified boundary element method for the estimation of potential fields on the scalp.

A modified boundary element method (BEM) for the calculation of potential fields on the scalp is proposed and tested in a three-shell model. The method greatly reduces the computational burden with only a small cost in accuracy. The resistivity of the skull is taken as infinite and the potential field on the inner skull surface is calculated using the standard BEM method for a one-surface system. Then the potential at any single selected point on the scalp is calculated directly from Green's theorem. The method does not give an accurate estimate of the magnitude of the potential on the scalp, but it does preserve correct relative magnitudes over all source locations in the head.

Electroencephalography↗
PubMed1995

The Duffing oscillator: a model for the dynamics of the neuronal groups comprising the transient evoked potential.

Thirty years ago Zeeman conjectured that the dynamics of the EEG might be modeled by the "equation of motion" of a Duffing oscillator, a pendulum with a nonlinear, cubic, restoring force. In this study the idea is extended to the evoked potential (EP). A transient sensory EP reflects activity from several neuronal groups, pools of neurons that fire in synchrony, with voltage-time curves that overlap appreciably. When the dynamics of each neuronal group is modeled by a Duffing oscillator, multi-electrode transient VEPs are well predicted. Predictions based on Duffing oscillator dynamics are substantially better than those based on the assumption that each neuronal group follows a simpler exponentially damped sinusoid or a function that simulates a post-synaptic potential. The component voltage-time curves are reasonably consistent over 7 subjects, suggesting sequential activation of neuronal groups with delays of several tens of milliseconds between them. The scalp topographies of the components suggest their origins in the occipital cortex.

Electroencephalography↗
PubMed1995

Perfluorinated organic liquid as an intraocular oxygen reservoir for the ischemic retina.

PURPOSE: Liquid perfluorocarbons are used as temporary vitreous substitutes in the surgical management of complicated retinal detachment. The purpose of this study was to determine if physiologic benefits could also be derived from the high oxygen solubility of perfluorochemicals relative to vitreous, especially during retinal ischemia. METHODS: The normal vitreous humor of the rabbit eye was replaced with either perfluorotributylamine (FTBA) or balanced salt solution (BSS). Retinal ischemia was then induced by increasing the intraocular pressure above the peak systolic blood pressure for intervals of 10, 30, or 90 minutes. RESULTS: Over a 10- or 30-minute period of ischemia, during which electroretinographic (ERG) responses were recorded, FTBA-filled eyes and BSS-filled eyes showed decreases in the a- and b-wave amplitudes. However, wave amplitudes were significantly greater in FTBA-filled eyes at most times examined (P < .05). ERG responses were maintained throughout a 30-minute ischemic interval in oxygenated FTBA-filled eyes, but not in oxygenated BSS- or deoxygenated FTBA-filled eyes. When examined 1 day after a 90-minute interval of ischemia, oxygenated FTBA-filled eyes maintained 45% and 57% of the preischemic ERG a- and b-wave amplitudes, respectively, compared to a 5% and 3% retention of wave amplitudes in oxygenated BSS-filled eyes. On light microscopic examination of these eyes, FTBA-exposed retinas showed less ischemic damage than BSS-exposed retinas. CONCLUSIONS: When used as a vitreous substitute, FTBA exerts a neuroprotective effect on the ischemic retina that appears to relate to an increased retinal oxygen supply compared to BSS.

Animals↗
PubMed1994

Human brain responses to different image contrasts.

Human brain activity was evoked by a dynamic random-dot display in which a square-wave grating appeared and disappeared at regular intervals. Grating visibility was determined by one of four different contrasts: texture, stereo disparity, luminance, or color. Scalp fields measured with 31 electrodes were used to estimate epicortical potential fields. The estimation procedure required detailed anatomical data for each subject. These were obtained from magnetic resonance images. A three-dimensional digitizer and a stereotactic headgear were used to accurately merge the frame of reference of the magnetic resonance image with that of the evoked potential. Epicortical potential fields provided a better indicator of where brain activity is evoked than did scalp fields. These procedures also corrected for anatomical variations between scalp and brain from subject to subject. In two right-handed female subjects, evoked activity was observed in the left posterior parietal and the right occipital, parieto-occipital and posterior temporal cortices. Evoked activity was observed in the left parietal cortex for luminance processing, in the right parietal cortex for texture processing and in the right temporal cortex for color processing, which was selective for the particular contrast.

Brain↗
PubMed1994

Continuous current source inversion of evoked potential fields in a spherical model head.

This study explores the efficacy of a physiological constraint on cortical current generators in promoting a robust solution for the inverse problem in evoked potentials. It is proposed that the current sources responsible for the evoked potential be modeled as a set of dipoles oriented orthogonal to the surface of the cortex. Rather than using a minimum norm approach, the solution space is searched for a vector that minimizes the error of the predicted evoked potential scalp field.

Animals↗
PubMed1993

Functional brain imaging: dipole localization and Laplacian methods.

The performance of two methods, used to localize brain activity from evoked potential fields measured on the scalp, was assessed in a tank model of the human head. This physical model contained a human skull encased in a polymer simulating the resistivity and geometry of brain and scalp. The dipole localization method mislocalized the positions of known dipole sources by several centimeters. The mislocalization was systematic. The dipoles were localized too deeply in the head. The Laplacian method yielded a field resembling the brain surface field (epicortical potential field) provided that the iso-potential contours of the scalp field closed within the measurement range. Clipping resulted in a serious mislocalization of the position of the peak of the epicortical potential field.

Brain Mapping↗
PubMed1993

Inferring regional brain activity from evoked potential fields on the scalp.

A new method is described to calculate epicortical potential fields from scalp fields based on linear algebra. It requires detailed anatomical information, for each subject, obtained from MR images. The calculation is validated in a physical model of the human head and applied to human subjects. The results suggest that the method yields reliable epicortical fields that help to localize evoked cortical activity in humans.

Brain↗
PubMed1993

Estimating regional brain activity from evoked potential fields on the scalp.

Potential fields on the surface of the brain were estimated from discretely sampled scalp fields in human subjects. Relatively simple methods of linear algebra were combined with detailed anatomical information from magnetic resonance imaging. The method was verified using a tank model of the human head that encased a fully hydrated human skull in a polymer matrix of controlled resistivity matching that of human brain and scalp. Brain surface fields evoked by checkerboard contrast reversal, spread less than their scalp field counterparts, and provided information helpful in localizing brain activity.

Brain↗
PubMed1992

An analysis of the VEP to luminance modulation and of its nonlinearity.

Identification of the VEP luminance modulation system was carried out using a relatively long pseudorandom binary m-sequence stimulus. The complete first order cross-correlation function provides a "fingerprint" of the nonlinearity in the time domain. Volterra kernel slices up to third order are recognized in the complete first order cross-correlation function. They all lie close to the major diagonal. Higher order kernel slices are not measurable. A cascade block model (sandwich model) analysis shows the nonlinearity to be an asymmetric rectification. Luminance increase causes a stronger response than does luminance decrease. The pre-filter gain function is low-pass. The post-filter gain function is band-pass tuned near 10 Hz. Coherency spectra are used to examine the channel structure of the system. Two channels operating near 8 and 15 Hz are easily identified. Another broad channel (or possibly multiple channels) is present above 30 Hz. In many subjects, the pseudorandom stimuli allow a complete system identification to be carried out in less than 82 sec.

Evoked Potentials, Visual↗