Biomedical subjects

M G Brigell

Publications and source records attributed to M G Brigell.

6 recordsLinked to original sources

PubMed1991

The effects of age on steady-state pattern electroretinograms and visual evoked potentials.

Steady-state pattern-reversal electroretinograms and visual evoked potentials were simultaneously recorded in two groups of young and elderly normal volunteers. The young group consisted of 23 subjects (13 women and 10 men) aged 18 to 28 years, and the elderly group consisted of 24 subjects (11 women and 13 men) aged 58 to 77 years. Stimuli were square-wave gratings ranging in spatial frequency from 0.5 to 6 c/deg and phase reversed at a frequency of 4 Hz. Pattern-reversal electroretinograms and visual evoked potentials consisted of a prominent second and a smaller fourth harmonic response. Spatial frequency-amplitude functions of the pattern-reversal electroretinogram second and fourth harmonics were similar for the young and elderly groups. The mean fourth harmonic phase was significantly shifted in elderly subjects compared with young subjects for all spatial frequencies tested. Spatial frequency tuning was observed for amplitude and phase functions of the visual evoked potential second and fourth harmonic responses for both age groups. Age had a significant effect on phase for spatial frequencies above 1.5 c/deg. Amplitude of the fourth harmonic was significantly lower for the elderly group at 1.5-4 c/deg. Phase was significantly different between groups for spatial frequencies below 3 c/deg. Our results suggest that aging influences both retinal and central visual pathways. Aging differentially affected the visual evoked potential second and fourth harmonic responses, suggesting different neuronal origins for these components.

Adolescent↗

PubMed1991

Cortical blindness and residual vision: is the "second" visual system in humans capable of more than rudimentary visual perception?

We studied 12 patients with static cortical blindness to evaluate residual vision after destruction of area 17 and to assess the visual capacity of the subcortical "second" visual system in humans. In each case, the cause was bilateral infarction of the occipital lobes. Five patients had total blindness, and four had residual rudimentary vision (RRV), characterized by homonymous areas of light perception in the peripheral field and ability to detect moving objects. Only three patients had the ability to read; two of these had spared macular vision, and the other had spared left homonymous hemimaculae and spared temporal crescent. Neuroimaging and visual evoked potentials (VEPs) correlated with the extent of the visual dysfunction. Total destruction of area 17 bilaterally was associated with total permanent visual loss. The larger the amount of spared visual cortex, the better the vision. Positron emission tomography (PET) or single photon emission computed tomography (SPECT) demonstrated retained metabolic activity in islands of preserved area 17 in patients with some residual vision. VEPs were present in totally blind individuals. We conclude that, in humans, useful visual function is preserved only when a critical amount of area 17 is spared. The subcortical second system may participate in the generation of VEPs, but is incapable of conscious visual perception.

Aged↗

PubMed1990

Topographic mapping of electrophysiologic measures in patients with homonymous hemianopia.

We analyzed electroencephalographic (EEG) activity and spatial distribution of the pattern-reversal visual evoked potential (PVEP) in 20 patients with unilateral lesions in the retrochiasmal visual pathways. Focal abnormalities that were consistent with lesion location were present in the topographically analyzed EEG or VEP of 85% of the patients, compared with a 70% detection rate for conventional analysis techniques. Quantitative analysis of spectrally analyzed EEG revealed focal abnormality in 7 patients whose conventional EEG was interpreted as either normal or diffusely slow. However, focal paroxysmal spikes present in the EEG of 1 patient were missed by EEG mapping, and a false localization of quantitative EEG abnormality contralateral to the lesion occurred in 1 patient. Topographic analysis of the PVEP was no more sensitive to retrochiasmatic lesions than conventional analysis of 2 lateral occipital electrodes. We conclude that topographic mapping is a valid technique in the detection of localized cerebral lesions.

Adult↗

PubMed1988

Saccadic latency as a measure of afferent visual conduction.

Latency to initiate a saccadic eye movement to a visual target, and visual evoked potential, were measured in seven patients with resolved unilateral optic neuritis. Saccades were delayed when the target was presented to the clinically involved eye, but were normal when the contralateral eye was tested. With binocular target presentation, saccades were symmetric between eyes and normal in latency. In two patients with pituitary adenoma and low-grade bitemporal field defects, saccades were delayed when targets were presented in the temporal field, but were within normal limits when presented in the nasal field. These results cannot be attributed to lesions in the motor pathways. It is concluded that saccadic latency to visual targets is a valid measure of afferent conduction. If the robust delays found in this study prove to have test-retest reliability, saccadic latency may provide a measure of afferent function which is sensitive to the demyelination that preceeds neuronal degeneration and sensitivity loss in patients suspected of having optic neuropathy.

Adenoma↗

PubMed1987

Psychophysical and electroretinographic findings in X-linked juvenile retinoschisis.

We compared psychophysical and electroretinographic test results of four patients with X-linked juvenile retinoschisis who had clinically apparent lesions isolated to the foveal area. The b-wave of the flash electroretinogram was selectively reduced, while the a-wave was within the normal range. Oscillatory potentials generated by either the rod or cone systems were markedly reduced. Absolute thresholds outside the fovea were either normal or only moderately elevated, indicating that the sensory neural pathways were, by and large, operating with limited dysfunction. Therefore, the oscillatory potentials do not directly reflect, to any appreciable extent, the function of bipolar or ganglion cells. Finally, a sequence of pathologic events is proposed for X-linked juvenile retinoschisis that initiates with Müller cell dysfunction.

Adolescent↗

PubMed1987

Pattern electroretinogram threshold does not show contrast adaptation.

Pattern electroretinogram (PERG) thresholds were examined using a swept contrast stimulus method. Stimulus contrast was either continuously changed (swept) from high to low (descending sweep), or from low to high (ascending sweep). Visual evoked potential (VEP) contrast threshold was higher when measured using descending sweeps than when using ascending sweeps. This VEP threshold difference has been attributed to cortical adaptation. Although previous work has reported changes in the PERG amplitude as a function of pre-exposure, we have found no analogous effect on the PERG threshold.

Adaptation, Physiological↗