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Biomedical subjects

R Srebro

Publications and source records attributed to R Srebro.

At least 37 records · Page 2Linked to original sources
PubMed1991

Image segmentation and VEP topography.

VEP scalp fields were measured for four pattern onset-offset stimuli identical except for the single contrast attribute that allowed a square-wave grating to be seen. The contrast attribute used for each stimulus was either texture, binocular disparity, luminance, or color. All stimuli were presented as dynamic random dot displays. Scalp potentials were sampled at 1.5 cm intervals by 66 electrodes over the posterior scalp. Customized rigid helmets were constructed for each subject to ensure reproducible electrode placement. The scalp fields due to the four stimuli were significantly different in all cross-comparisons. This implies that in the human visual cortex there exists regional selectivity for the processing of the four different contrast attributes.

Brain Mapping↗
PubMed1990

Localization of visually evoked cortical activity using magnetic resonance imaging and computerized tomography.

Evoked scalp potentials, computerized tomography, and magnetic resonance imaging were used together to localize cortical activity evoked by visual stimuli in humans. The temporal resolution of evoked potential measurements is sufficient to track the flow of cortical activity which evolves in epochs of a few tens of msec. Spatial localization was enhanced by deconvolving scalp potential fields with a Laplacian operator. Markers glue to the scalp served to unify the three geometric reference frames into a single computer graphics database.

Brain Mapping↗
PubMed1990

Realistic modeling of VEP topography.

A finite element model of the human head was constructed from digitized contours of a cadaver head cut at 1 cm intervals parallel to the cantho-meatal plane. The boundary element method was used to calculate the potential fields produced by dipoles in the cortex at various locations and orientations. Field profiles and their Fourier transforms established the Nyquist Sampling interval necessary to faithfully reconstruct scalp fields from discrete measurements. The interval varies from approx. 1 cm for sources near the occipital pole to 2 cm for sources near the Rolandic fissure. The resolution of the VEP was also estimated. For signal-to-noise ratios ranging from 4 to 10, source displacements ranging from approx. 1.4 to 0.4 cm in the cortex can be resolved.

Brain↗
PubMed1987

The topography of scalp potentials evoked by pattern pulse stimuli.

Pattern pulse stimuli evoke activity in two distinct regions of the human visual cortex in temporal sequence. The earlier responding region (implicit time 93 msec) is localized to a relatively small region near Oz. Its topography is very sensitive to the position of the evoking stimulus in the visual field; it is dominated by a representation of the visual field near the vertical meridian. The later responding cortical region (implicit time 131 msec) is larger than the earlier one. Its medial border touches the lateral border of the earlier responding cortical region and it extends several cm laterally. Its topography is much less sensitive to the position of the evoking stimulus in the visual field. Stimuli presented to the ipsilateral field evoke either weak or no measurable activity in the earlier responding cortical region but often evoke measurable activity near the lateral boundary of the later responding cortical region. These results suggest that the earlier responding cortical region is striate cortex and that the later responding cortical region is composed of visual areas V2, V3, V3A, and V4.

Brain Mapping↗
PubMed1986

The relationship between log-complex transforms of stimuli and the cortical responses they evoke.

Two stimuli whose log-complex transform images in striate cortex are orthogonal square-wave gratings cause responses of equal magnitude in striate cortex but of unequal magnitude in extra-striate cortex. However, two other stimuli, orthogonal square-wave gratings in visual space, whose log transform images are identical except for phase shift, cause responses of equal magnitude in both striate and extra-striate cortex. A phase shift in the log-complex transform image represents a change either in the size or orientation of a form in visual space, parameters that do not affect its intrinsic shape. A change in the orientation of a contour line in the log-complex image represents either a change in the intrinsic shape of a form or its translation in visual space. Thus the results suggest that the extrastriate response may be related to the shape of an object in visual space.

Cerebral Cortex↗
PubMed1985

Localization of visually evoked cortical activity in humans.

The locations of cortical activity evoked by visual stimuli presented at different positions in the visual field are deduced from the scalp topography of visually evoked potentials in humans. To accomplish this, the Laplacian evoked potential is measured using a multi-electrode array. It is shown that the Laplacian response has the following useful attributes for this purpose. It is reference-free. Its spatial resolution is approximately 2 cm referred to the surface of the cortex. Its spatial sensitivity characteristic is that of a spatial band-pass filter. It is relatively insensitive to source--sink configurations that are oriented tangentially to the surface of the scalp. Only modest assumptions about the source--sink configuration are required to obtain a unique inversion of the scalp topography. Stimuli consisting of checkerboard-filled octant or annular octant segments are presented as appearance-disappearance pulses at sixteen different positions in the visual field in randomized order. The locations of evoked cortical activity in the occipital, parietal and temporal lobes are represented on a Mercator projection map for each octant or octant segment stimulated. Lower hemifield stimuli activate cortex which lies mainly on the convexity of the occipital lobe contralateral to the side of stimulus presentation in the visual field. The more peripheral the stimulus is in the visual field, the more rostral is the location of the active cortex. The rostral-to-caudal location of the evoked activity varies from subject to subject by as much as 3 cm on the surface of the occipital cortex. Furthermore, in any single subject there is a substantial amount of hemispheric asymmetry. Upper hemifield stimuli activate cortex that lies on the extreme caudal pole of the occipital lobe. This activity is relatively weak, and in some subjects it is almost unmeasurable. It is suggested that the representation of the upper hemifield in the cortex lies mostly on the inferior and mesial walls of the occipital lobe and possibly within the calcarine fissures. Those locations are inaccessible to the Laplacian analysis because the current generators therein may be oriented tangentially to the surface of the overlying scalp. Posterior parietal lobe activity and/or inferior temporal lobe activity is frequently evoked. Different subjects have different patterns of evoked activity. Unilateral or bilateral posterior parietal lobe activity is the most common pattern. Unilateral inferior temporal lobe activity is a less common pattern.(ABSTRACT TRUNCATED AT 400 WORDS)

Brain Mapping↗
PubMed1985

Localization of cortical activity associated with visual recognition in humans.

The Laplacian analysis described previously is used to localize cortical activity subserving visual object recognition in humans. In the first of two experiments, subjects are shown pictures of a human face corrupted by varying amounts of noise. After each picture has been presented for 34 ms against a large uniformly illuminated background, the subject is required to report whether or not he saw the face, by pressing a button. The Laplacian response associated with the report that the face is seen differs from that associated with the report that the face is not seen. The difference between these two Laplacian responses has a simple wave form with peak activity at approximately 206 ms after stimulus onset and approximately 196 ms before median reaction time for the button-press report. Its amplitude and polarity, which vary with centre-electrode location over the posterior scalp, are used to construct a map showing the location of cortical activity subserving recognition of the face. This cortical activity localizes to both temporal lobes with some degree of right hemispheric lateralization in right-handed subjects. In the second of the two experiments, subjects are shown the silhouette of a simple shape, such as a triangle, embedded in a large random dot field. Each silhouette is presented for 17 ms. The visibility of the shape is made to vary from trial to trial and the subject is required to report for each trial whether or not the shape is seen. The Laplacian response associated with the report that no shape is seen is very much smaller than that correctly identifying the shape. The difference between these two Laplacian responses has a simple wave form with peak activity at approximately 207 ms after stimulus onset. Its wave form is essentially the same as that associated with face recognition. Its amplitude and polarity, which vary with centre-electrode location over the posterior scalp, are used to construct a map showing the location of cortical activity subserving recognition of the simple shape. This cortical activity localizes to both temporal lobes with strong right hemispheric lateralization in right-handed subjects. The general topography of this activity is similar to that subserving face recognition. Although generally similar, there are measurable differences between the topographies of right temporal lobe activity associated with face recognition and that associated with simple shape recognition.(ABSTRACT TRUNCATED AT 400 WORDS)

Brain Mapping↗
PubMed1984

Effect of viewing high luminance gratings on the amblyopic visual system.

Five amblyopes viewed high contrast square wave gratings of 8 different spatial frequencies all presented at very high luminance. They were required to indicate the orientation of the grating, horizontal or vertical, and received immediate auditory feedback if incorrect. Each amblyope viewed 480 grating presentations daily for 15 days over 3 weeks. After this 'treatment' improvements in visual acuity and/or visually evoked potentials were observed and persisted after treatment.

Adult↗
PubMed1984

Visually evoked potentials in eccentrically and centrally fixing amblyopes.

Visually evoked potentials to checkerboard pattern reversal were found to be nearly five times larger in eccentrically fixing amblyopic eyes than in centrally fixing amblyopic eyes when compared with the fellow non-amblyopic eye. The two groups of amblyopes had comparably poor visual acuity and differed in no other way save in their fixation behaviour. This suggests that at least two neurodevelopmental mechanisms subserve human amblyopia and that only one of these resembles the animal model of visual deprivation.

Adult↗
PubMed1983

Fixation of normal and amblyopic eyes.

During attempts to maintain steady fixation on a small target for 6 s, amblyopic eyes drifted more than did normal eyes. Neither the magnitude of this long-term drift (LTD) nor the SE of eye position correlated with the visual acuity of the amblyopic eye. However, when the SE of eye position was corrected for the LTD, the corrected SE tended to be larger in amblyopic eyes than in normal eyes and correlated with the visual acuity of the amblyopic eye.

Adult↗
PubMed1983

Measurements of eccentricity of fixation in normals and in amblyopes by evoked potentials.

A method to estimate the eccentricity of fixation, i.e. the position of the center of the fovea relative to the point of fixation, based on visually evoked potentials is described and applied to 14 normal and 17 amblyopic subjects. Eye position was simultaneously recorded. In normal subjects, the estimates of fixational eccentricity distributed unimodally with mean 12.1' and range 1-36'. The estimates from the nonamblyopic eyes of amblyopic subjects distributed bimodally with peaks near 5 and 55' and those amblyopic subjects with larger estimates were anisometropic. The results suggest that the anisometropic amblyopes have an asymmetry of retinocortical projections. When corrected for the fixational eccentricity of the nonamblyopic eye. 5 of 17 amblyopic eyes had fixational eccentricity greater than 40'. Since only 1 of these amblyopic eyes was found to fixate eccentrically by conventional clinical testing, it is suggested that eccentric fixation may be more common in amblyopia than has heretofore been appreciated.

Amblyopia↗
PubMed1982

Estimation of the position of the center of the fovea relative to fixation in normal subjects and in patients with amblyopia. Use of the visually evoked potential.

A new method to measure eccentric fixation based on differences in the wave-forms of visually evoked potentials to stimuli presented to different parts of the visual field was studied in ten normal and 13 amblyopic subjects. In all of the 20 normal eyes, fixation corresponded to the horizontal position of the center of the fovea within the resolution of the method, approximately 30 minutes of arc. In seven of the 13 amblyopic eyes, fixation did not correspond to the horizontal position of the center of the fovea. Only one of these amblyopic eyes was found to have eccentric fixation by visuscopy. The vertical position of fixation relative to the center of the fovea was not well defined by the method in either the normal or the amblyopic eyes.

Amblyopia↗
PubMed1982

Reduction of EEG activity caused by a light flash.

Intertrial EP variance measured in response to a brief binocularly presented flash of light decreases with a time-course that overlaps the average EP. This decrease in variance is not exclusive to the alpha frequency band (8-12 c/sec) but it is selective for it. Subjects with large initial variance tend to show a large decrease in variance. There is no relationship between the magnitude of the decrease in variance and the consistency of the EP. The phenomenon reflected by the decrease in variance is probably identical with 'event-related desynchronization.' The results support the hypothesis that an EP may consist of two processes that overlap in time: a reduction in background EEG activity and an almost deterministic transient potential change.

Brain↗
PubMed1981

The power spectra of visually evoked potentials to pseudorandom contrast reversals of gratings.

Visually evoked potentials (VEPs) were recorded in response to the temporal pseudorandom contrast reversal of spatially sinusoidal gratings with different spatial frequencies. The stimulus mimicked bandwidth limited white noise in the temporal domain except for an inherent periodicity which permitted signal averaging. The power spectra of the resulting VEPs were calculated. For high spatial frequency gratings (7.5-15 c/degree) VEP power was localized to the temporal frequency range from 3 to 7 Hz. For medium spatial frequency gratings (0.75-3 c/degree) VEP power was distributed in a roughly bimodal way with peaks near 6 and 18 Hz. The VEP power spectra due to high frequency gratings were similar in 2 subjects studied in detail, but the power spectra to medium frequency gratings differed substantially.

Brain↗