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Biomedical subjects

M Brigell

Publications and source records attributed to M Brigell.

At least 19 recordsLinked to original sources
PubMed1995

Locating VEP equivalent dipoles in magnetic resonance images.

Pattern-reversal and diffuse flash visual evoked potentials (VEPs) were obtained from 4 normal adults. A spatiotemporal dipole model was used to determine the location of the hypothetical equivalent dipoles consistent with the scalp distribution of the VEPs. Equivalent dipoles representing ERG and VEP activity were placed within 3-D magnetic resonance images of the brain. Most of the localization error appeared to be due to inadequate sampling of the potential field in frontal and occipital areas by the 10-20 system of electrode placement. Locating electrophysiologic dipoles within magnetic resonance images of brain structure allows evaluation of dipole localization techniques.

Adult↗
PubMed1995

Supranuclear eye movement dysfunction in mitochondrial myopathy with tRNA(LEU) mutation.

A patient with multiple neurological deficits and biopsy-proven mitochondrial myopathy with mutation of tRNA(LEU) at nucleotide 3243 was referred for eye movement evaluation. He had restricted range of voluntary motions in all directions and full range of eye movements on passive rotation of head while fixating a visual target. Eye movement recordings revealed decreased horizontal and vertical saccadic velocities and markedly decreased smooth pursuit gain in both directions. The vestibulo-ocular reflex showed gain abnormalities with many saccadic intrusions on the smooth reflex response. His brother, with similar mutation, was clinically asymptomatic. However, his eye movement recordings revealed slow horizontal saccadic velocities leftward and normal saccadic velocities rightward in both eyes as well as in upward and downward direction. Smooth pursuit and vestibulo-ocular reflexes were within normal limits. Although eye movement abnormalities are seen commonly in mitochondrial myopathies, the exact mechanism is not known. Our cases suggest supranuclear dysfunction as one of the mechanisms for ophthalmoparesis.

Biopsy↗
PubMed1994

The pattern visual evoked potential. A multicenter study using standardized techniques.

The peak latency of the pattern-reversal visual evoked potential is a sensitive measure of conduction delay in the optic nerve caused by demyelination. Despite its clinical utility, the pattern-reversal visual evoked potential has not previously been used in multicenter clinical trials, presumably because of difficulty in standardizing conditions between centers. To establish whether the pattern-reversal visual evoked potential could be adequately standardized for use as a measure in multicenter therapeutic trials for optic neuropathy or multiple sclerosis, stimulus and recording variables were equated at four centers and pattern-reversal visual evoked potentials were recorded from 64 normal subjects and 15 patients with resolved optic neuritis. Results showed equivalent latency and amplitude data from all centers, suggesting that stimulus and recording variables can be satisfactorily standardized for multicenter clinical trials. N70 and P100 peak latencies and N70-P100 interocular amplitude difference were sensitive measures of resolved optic neuritis.

Adolescent↗
PubMed1994

Dipole source localization in a case of epilepsia partialis continua without premyoclonic EEG spikes.

A 72-year-old woman with epilepsia partialis continua (EPC) of the right foot is presented. Rhythmic myoclonic jerks were localized to the 1st and 2nd toes of the right foot and persisted for 72 h. EEG/video monitoring did not show any epileptiform transient in association with myoclonic jerks. MRI and MRA demonstrated an arterio-venous malformation involving the left fronto-parietal parasagittal area. Using the EMG signal from the myoclonic jerk we back-averaged the EEG 640 msec before and after the onset of the twitch. A negative-positive deflection was observed preceding the myoclonic jerks by 128-188 msec. Voltage topographic mapping showed a negative maximum in the left centro-parietal region. A multiple spatio-temporal dipole model was applied to the back-averaged deflection preceding the myoclonus. The patient's MRI was used to determine the center of the best fitting sphere, and the model was corrected accordingly. The best dipole solution consisted of 3 dipoles localized in the parasagittal frontal cortex, in the location of the motor representation for the foot. The utilization of a combined technique of back-averaging from the myoclonus and dipole source localization supported the epileptogenic etiology in this case.

Aged↗
PubMed1993

Pattern visual evoked potentials in cases of ambiguous acuity loss.

Thirty patients were referred to our visual evoked potential laboratory with complaints of profound acuity loss in one or both eyes. However, the objective ophthalmologic findings were normal, including pupillary reaction, and anterior segment and fundus examinations. Transient visual evoked potentials to a 2.3-c/deg sinusoidal grating pattern were found to be present in 26 of these 30 patients. Visual evoked potentials may be used as a measure of afferent pathway integrity.

Adolescent↗
PubMed1993

An electrophysiological study of visual processing in Alzheimer's disease.

Visual processing of sinusoidally modulated gratings was studied in a group of patients (n = 11) with Alzheimer's disease (AD) and an elderly normal control group (n = 9). Spatial square wave gratings (1.47 c/d) were reversed at a temporal frequency of 4 or 8 Hz. EEG recordings at rest and during visual stimulation were obtained from 20 channels using the 10/20 international system. The power spectrum of the 2nd and 4th harmonic of the stimulation frequency was calculated by Fast Fourier Transform (FFT) at a resolution of 0.25 Hz. Association of activity between occipital, temporal, parietal and central regions was measured by intra- and inter-hemispheric coherence and phase at harmonics of the stimulation frequency. A significant difference (P < 0.01) in evoked activity of the 4th harmonic at O1 and O2 was found between the two groups with less activity in the AD patients. In the AD group there was a significant correlation (P < 0.05) between evoked activity at the 2nd harmonic of the 8 Hz visual stimulation and Mini-Mental State (MMS) score. This correlation was independent of the age effect on MMS. Response phase between O1 and O2 for both 4 and 8 Hz stimuli was close to 0 degree C and coherence had similar values in both groups. Occipital and central regions showed a phase reversal for all harmonic responses to both visual stimuli. The AD patients showed statistically significant (P < 0.05) phase dispersion at O1-P3 and O2-P4 not seen in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1993

Identification of the hemisphere activated by hemifield visual stimulation using a single equivalent dipole model.

Topographic amplitude distribution of the hemifield pattern visual evoked potential (PVEP) shows substantial intersubject variability. Many subjects have larger P100 amplitudes paradoxically over the hemisphere ipsilateral to the stimulated field, whereas others show larger responses over the stimulated hemisphere. The present study was designed to determine whether a single equivalent dipole model could correctly identify the field of stimulation, and therefore the hemisphere activated, under conditions in which the surface distribution is variable. Under conditions used in the present study, visual examination of the surface amplitude distribution of the P100 peak could not be reliably used to identify the hemifield that was stimulated. Of 28 hemifield PVEPs, obtained from 14 normal subjects, only 13 showed higher amplitude ipsilateral to the field of stimulation. Thus, neither examination of EP wave forms nor topographic maps provided an accurate means for determination of which hemifield was stimulated or which hemisphere was activated. The single equivalent dipole model correctly identified the stimulated hemisphere for 25/28 hemifield PVEPs. Orientation of the equivalent dipole accounted for much of the variability in surface amplitude distribution, with tangential orientations obtained in subjects with ipsilaterally predominant P100 surface topography. Although the dipole model improved identification of the stimulated hemisphere, dipoles were located anterior or inferior to the occipital lobe in some subjects. Results suggest that dipole modeling can provide useful information regarding the source of surface recorded potentials.

Adult↗
PubMed1993

Effects of age and hemianopic visual field loss on driving.

PURPOSE: With the use of an interactive driving simulator, we examined the driving performance of older patients with either homonymous or quadrantic hemianopsia with primarily occipital lobe damage resulting from cerebrovascular accidents (CVA's). METHODS: We compared the performance of these patients with that of a normally sighted, age-similar control group and that of a normally sighted younger group. RESULTS: The driving performance of the patients was either worse than, or similar to, that of the older control group; all of the older individuals (both patients and normally sighted subjects) had worse performance than the younger group. CONCLUSIONS: Age-related effects combined with the effects of visual field losses in older patients with cerebrovascular accidents had a negative impact on driving skills.

Aged↗
PubMed1992

Spatial frequency evoked visuograms in multiple sclerosis.

We obtained steady-state visual evoked potentials (VEPs) to sinusoidal gratings alternating at 4 Hz with spatial frequencies varying from 0.5 to 8 cpd in 21 normal controls and 21 patients with multiple sclerosis (MS), and analyzed responses by fast Fourier transform. Amplitude- and phase-spatial frequency functions were obtained and referred to as amplitude and phase "visuograms." We observed two types of abnormalities in the phase visuograms of MS patients: (1) abnormal responses at all spatial frequencies tested (37%), and (2) abnormal responses only at selective spatial frequencies (52%). Some patients had phase lag limited to low, middle, or high spatial frequencies. Steady-state and transient VEPs to 2 and 4 cpd showed a similar percent of abnormalities. The use of more than one spatial frequency stimulus increased the diagnostic yield by 17%. Our data confirm that MS may selectively affect specific neuronal channels within the visual pathways.

Adolescent↗
PubMed1992

Event-related potentials.

Event-related potentials (ERPs) are scalp recorded electrophysiological responses that are related to an internal cognitive event. This review summarizes recent findings on the effects of neurologic disease and the origin of ERPs obtained in expectancy, attention, memory, and linguistic tasks. Cognitive ERPs allow the physiologic activity of the brain to be analyzed with exquisite temporal resolution. Our understanding of these potentials is incomplete at present. Advances in cognitive psychology and source localization of these surface potentials, however, may result in an increased understanding of both the organization of cognitive processing in the brain and cognitive deficits that result from neurologic disease.

Arousal↗
PubMed1990

Optic neuritis: a prospective study.

We studied 20 patients with acute optic neuritis prospectively for 12 months. Visual fields, color vision, and VEPs to 15' checks were initially abnormal in all patients. Visual acuity was abnormal in 90% and contrast sensitivity in 95% of patients. We devised a graded visual impairment scale (GVIS) to include all visual functions tested. Complete recovery of visual function occurred in 65% of cases. Recovery in the majority of patients was rapid and complete within the first 2 months. In some patients, improvement continued over 6 months. The initial classification on the GVIS was significantly correlated with the final outcome. Patients initially classified as having moderate visual impairment recovered completely or improved to near normal vision. Sixty percent of patients initially classified as total or severe blindness had permanent visual impairment. VEP latency remained prolonged in 19 patients, even when their vision had returned to normal, and is a reliable indicator of resolved optic neuritis.

Adult↗
PubMed1990

Olivopontocerebellar atrophy presenting with hemiparkinsonian ocular motor signs.

A patient with olivopontocerebellar atrophy presented with an asymmetric parkinsonian syndrome consisting of right limb rigidity, bradykinesia, masked facies, and a hypophonic, monotonous voice. Right limb clumsiness suggested corticospinal tract involvement. Gait was slightly wide based, but no other cerebellar or brainstem signs were present. Eye movements were characteristic of parkinsonism with low-gain pursuit and hypometric rightward saccades. Saccadic velocity and the vestibular ocular reflex were within normal limits. There was no pathologic nystagmus or saccadic dysmetria. Magnetic resonance imaging showed cerebellar and lower brainstem atrophy virtually diagnostic of olivopontocerebellar atrophy. Olivopontocerebellar atrophy may present uncommonly with asymmetric parkinsonian features, including parkinsonian eye movement abnormalities.

Humans↗
PubMed1989

Hemi-field pattern visual evoked potentials: a comparison of display and analysis techniques.

Three methods for analyzing the spatial organization of visual evoked potentials were compared. Pattern reversal visual evoked potentials were obtained from a single subject under three viewing conditions: stimulation of the left, right, and both visual fields. The scalp distribution of the VEP to 1 deg checks was displayed using three recording and analysis techniques: a conventional horizontal occipital array of electrodes, topographic mapping, and 3-dimensional evoked potentials. All three techniques revealed "paradoxical" lateralization of P100. The relative merits of each technique are discussed.

Brain Mapping↗
PubMed1988

Visual evoked potentials to multiple temporal frequencies. Use in the differential diagnosis of optic neuropathy.

The usefulness of the visual evoked potential (VEP) in differential diagnosis increases when stimulus parameters such as check size and grating orientation are varied. In this study we varied the stimulation frequency. Temporal frequency-specific abnormalities were compared in three patient categories, including retrobulbar optic neuritis (eight patients), pseudotumor cerebri (11 patients), and thyroid eye disease (seven patients). All patients had minimal clinical evidence of optic nerve damage when tested. A 2.3 cycle-per-degree sinusoidal grating of 55% contrast was phase reversed at either 1 or 4 Hz. The P1 latency of the 1-Hz data and the phase at 8 Hz, the second harmonic of the 4-Hz input frequency, were measured. In retrobulbar neuritis, latency (phase) was severely abnormal at both temporal frequencies. In thyroid eye disease, VEP phase was abnormal at 8 Hz while the P1 latency was normal at 1 Hz. The P1 latency and phase were normal in most cases of pseudotumor cerebri. The results suggest differing mechanisms for damage in compressive vs primary demyelinating neuropathies.

Adolescent↗
PubMed1984

Hypometric saccades and low-gain pursuit resulting from a thalamic hemorrhage.

Saccadic and pursuit eye movements were measured in a patient with a right thalamic hemorrhage using an infrared reflection technique. Saccades were hypometric when directed away from the side of the lesion, and pursuit was low in gain, with many interposed saccades, when directed toward the side of the lesion. This eye movement pattern is similar to that observed in hemidecorticate patients. We postulate that the ocular abnormalities observed in our patients were secondary to an interruption of the efferent pathways from the right cerebral areas mediating saccadic and smooth-pursuit eye movements.

Eye Movements↗
PubMed1981

Effects of stimulus location and pattern upon the visually evoked cortical potential and the electroretinogram.

The pattern properties of the visually evoked potential and the electroretinogram have been investigated for phase alternated patterns of checks presented to the near periphery of the retina. When the eye was light-adapted, coarse patterns became relatively more effective for eliciting the evoked potential as the stimulus was moved away from the fovea. Fine patterns were most effective in the center of the field. The electroretinogram responded best to coarse spatial frequencies at all retinal locations. At a lower level of light adaptation the sensitivity of the evoked potential shifted to coarse spatial frequencies in the center of the field, but did not change appreciably in the periphery. The results may reflect some of the dynamic properties of visual fields.

Adaptation, Physiological↗
PubMed1979

The relational determination of length illusions and length aftereffects.

When two lines were presented simultaneously, in an illusion paradigm, the judged length of one of the lines (the focal line) assimilated toward the length of the contextual line. When the lines were presented sequentially, in an aftereffect paradigm, the apparent length of the focal line was displaced away from the length of a previously presented contextual line (i.e. contrast). The largest effects obtained at inverse contextual-to-focal length ratios: 1.67:1 for contextual lines longer than the focal line and 0.60:1 for contextual lines shorter than the focal line. Overestimation of focal length was larger than underestimation for both illusions and aftereffects despite the fact that identical stimuli produced effects in opposite directions in the two paradigms. Results were discussed in terms of a neural length-coding model and were related to visual size constancy.

Figural Aftereffect↗