Fabian A. Abraham

Single P100 visual evoked potential analyses in man.

Standards evoked potential averaging leads to a loss of information inherent in the moment to moment variability of the amplitude and latency of single evoked potentials. In order to extract this information, attempts were made to develop procedures which would allow recording of single visual evoked potentials in at least a selected group of subjects. In 11 out of 25 non-selected subjects, 43% to 86% of all stimuli (reversal checkerboard pattern elicited single visual evoked potentials (VEP) which were recognized as such by a group of independent observers. The mean amplitude and latency obtained by directly measuring the peak to peak amplitudes and peak latencies of these single VEPs (arithmetic averaging) were compared to those obtained following conventional time-locked averaging of the same data. During long-term continuous stimulation, the arithmetic averaged VEPs increased in amplitude to a steady state while the time-locked averages of the same sets of responses decreased in amplitude. This reduction was found to be closely related to the latency jitter. It may provide a better understanding of the phenomenon of habituation. This finding was confirmed in model single VEPs obtained by summing on-going pre-stimulus EEG activity with a time-locked average VEP stationary wave form. The variability of the true single VEPs was found to be less than the variability of the model single VEPs. The latency and amplitude parameters of the true single VEPs were strongly correlated with each other while those of the model single VEPs were not. These findings show that single VEPs have an inherent variability which may reflect brain processing.

Visual Thresholds following Posterior Chamber Lens Implants

The dark-adapted threshold for a 2 degree white test imaged 10 degrees parafoveal and the foveal threshold luminance for Landolt ring gap discrimination were measured on 23 pseudophakic eyes with 6/9 or better visual acuity, 6 months to 3 years following posterior chamber lens implantation, and on 15 phakic eyes from an appropriately matched control group. Both scotopic and photopic thresholds were higher for the pseudophakic than for the control group by 0.3 and 0.6 log, respectively. These parafoveal and foveal thresholds, when measured under the influence of glare, were likewise significantly higher for the pseudophakic group.

Retinal Function in Mucolipidosis IV

In an 18-month-old girl affected by mucolipidosis IV (ML IV) with mild corneal clouding, normal retinal activity was documented by electroretinogram (ERG) and slightly delayed optic pathway conduction was revealed by visual evoked potential (VEP). Re-examination 9 years later disclosed severe retinal alterations resulting in atrophy with reduced photopic and missing scotopic ERG components and flat VEP. These fundoscopic and electrophysiologic deteriorations in ML IV suggest progressive rod-cone impairment similar to tapetoretinal dystrophy.

Optic Nerve Trauma with Prolonged Blindness Followed by Visual-Evoked Potential

An accidental deep intraorbital penetration of a radio antenna tip damaged the optic nerve and caused immediate amaurosis in a 19-year-old female with normal funduscopic, electroretinographic and computerized tomographic orbital findings. A small visual-evoked potential (VEP) recorded within 2 h of injury and in the following days revealed persistent electrical nerve conduction despite loss of vision and afferent pupillary reaction. The VEP constituted the only anticipating sign for the visual recovery which occurred after 3 days. Further increase of the VEP amplitudes preceded the visual improvement, resulting, finally, in 6/24 visual acuity with only lower nasal quadrant scotoma and slight optic disk pallor.

A Device for Easy Slitlamp Funduscopy with a +90-Diopter Lens

We developed a device which places the +90-dptr double aspheric lens in the slitlamp common axis channel at 11 mm further from the microscope focal plane, enabling easy visualization of the human fundus.

Visual thresholds in aphakia and pseudophakia

ABSTRACT Following extracapsular senile cataract extraction, visual thresholds were measured in 15 aphakic eyes corrected with glasses and compared with an appropriately matched group of 23 eyes with poly (methyl methacrylate) posterior chamber lenses. All subjects were six months to three years postoperative and had 20/30 or better visual acuity. The paramacular dark adapted threshold, the foveal luminance threshold, and the glare effect on both thresholds were not significantly different in the two groups despite the optical differences between aphakic and pseudophakic correction.

Visual Performance After Senile Cataract Surgery

Visual thresholds were measured following extracapsular senile cataract extraction in a group of15 aphakic eyes corrected with glasses, and compared with an appropriately matched group of 23 eyes with polymethylmethacrylate posterior chamber implants. All patients were seen 6 months to 3 years after surgery and had 6/9 or better visual acuity. The dark-adapted threshold at 10° above the fovea, the foveal luminance threshold for Landolt ring discrimination and the glare effect onto both thresholds were similar, revealing similar visual performance in the two groups, despite fundamentally different post-surgery optical correction.

Scotopic threshold of the human retina during dynamic physical effort

The scotopic threshold of 7 subjects was measured with a 1 degree circular white light located 15 degrees below fixation during 45 min at rest followed by 10 min of running on an exercise treadmill. The slope of the treadmill was adjusted individually so that the effort extended by each subject required 60-75% of his maximum oxygen uptake. Thresholds showed a sudden elevation of about 0.55 nLa (logarithmic scale) at the onset of the exercise period, and remained at this level throughout the following 10 min. It was concluded that the head movements and consequent eye instability imposed by the run were the main cause for the increase in scotopic threshold.

The Scotopic Threshold of the Human Retina during Static Physical Effort

The scotopic threshold of 15 subjects was measured with a 1 degree circular white stimulus light placed at 15 degrees below the fixation level during 45 min at rest followed by 20 min constant pedalling on an ergometric bicycle with the head stationary. Loads were adjusted individually so that the effort extended by each subject required 60-75% of his maximum oxygen uptake. Similar results were obtained under both resting and effort conditions, indicating that the dark adaptation threshold remained unaffected by vigorous static effort.

Meridional Variations of the Scotopic Sensitivity in the Human Retina

The absolute threshold of the dark-adapted retina for a 1 degree circular white light stimulus located at 15 degrees eccentric to the fovea was measured on four meridians. In each of the 11 tested subjects the sensitivity was found to be higher on the 90 degrees suprafoveal vertical meridian, on average by 0.3 log nLa compared to the subfoveal vertical meridian. On the two oblique 45 degrees and 135 degrees meridians of the upper retina the sensitivity was only very slightly lower compared to the 90 degrees meridian.