Faculty Bibliography

We assessed the clinical utility of objectively measured acuity using visual-evoked potentials. The technique was first standardized in normal emmetropic subjects and then applied to uncorrected myopic subjects. We found that visual-evoked potential acuity could accurately indicate Snellen acuity in emmetropia and corrected myopia; however, the two measures were highly correlated only in those uncorrected myopic subjects with visual acuities of 20/100 or better. In subjects with poorer than 20/200 uncorrected visual acuity caused by myopia, estimates of visual-evoked potential acuity could not be obtained. The correlation between these two measures of visual acuity was also lower in patients with decreased Snellen acuity attributable to retinal or ocular disease. We found that patients with unexplainable claims of decreased visual acuity could be diagnosed as having functional visual loss based on objective visual-evoked potential acuities

Sensitivity losses in patients with retinitis pigmentosa (RP) have been attributed to a decrease in photopigment density, to a reduction in the number of photoreceptors, and also to a change in temporal response properties of the receptors. The sensitivity losses in patients with macular degeneration have also been attributed to a loss of photoreceptors. To test these explanations for sensitivity loss we obtained electrophysiological and psychophysical temporal modulation transfer functions (MTFs) on normal subjects in response to varying stimulus luminances and retinal loci. These stimulus manipulations did not duplicate the changes observed in the temporal MTFs of patients. The temporal response properties of the receptors were tested electrophysiologically by manipulating stimulus presentation interval. The results provided evidence for sensitivity losses in RP patients being due to alterations in the temporal response properties of the receptors

Flicker sensitivity increases in the peripheral retina when relatively large targets are used. This enhancement of cone system-mediated temporal sensitivity persists even when corrections are made for cortical magnification factors. It has been suggested that the differences in temporal frequency response characteristics across the retina are based on differences in receptor morphology between the peripheral and central cones. We have examined a possible retinal origin of this phenomenon by obtaining psychophysical and electroretinographic data at a variety of locations on the temporal retina. Psychophysical results show an increased sensitivity for high temporal frequency stimuli (above 30 Hz) with retinal eccentricity whether or not the stimulus size was scaled. Focal electroretinograms recorded with a constant size stimulus did not show an increase in amplitude with eccentricity. However, when an equal number of receptors were stimulated by scaling the target size, focal amplitudes were larger in the periphery. The electrophysiological findings are consistent with a possible retinal origin for this flicker enhancement phenomenon

Conflicting results have been obtained concerning the parametric properties of the pattern electroretinogram. These discrepancies may be due to the large amount of variability inherent in recording amplitudes. We have found the variability within a single stimulus condition to be so large (ranging from 30% to 67% of the mean value) that it could mask any underlying spatial frequency tuning. Changing the stimulus conditions failed to significantly reduce the observed variability, although changing recording conditions produced some reduction. The use of a narrower rejection band, a greater number of sweeps, and placement of the reference electrode on the ipsilateral ear (as opposed to the ipsilateral temple) combined to decrease variability of the pattern electroretinogram within a single recording session; however, intersession variability remained high. Therefore one must be careful in evaluating data from this technique, and caution is advised in its clinical use

Visual evoked potential contrast adaptation was measured in normal subjects using a real-time-retrieval swept contrast analysis. Results demonstrated orientation specific adaptation suggesting a cortical locus. Conditions which disturb cortical inhibition (i.e., epilepsy and dopaminergic agents) altered this adaptation effect.

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

A patient with congenital stationary night blindness (CSNB) (Schubert-Bornschein type) transmitted as an autosomal recessive trait was studied with several tests of electrical function as well as a variety of psychophysical procedures. Comparison of the patient's present findings with those obtained 23 years earlier showed that while rod thresholds have remained the same, cone sensitivity has decreased. Subjective flicker thresholds obtained following a bleach were unchanged during the course of dark adaptation. The absence of rod-cone interaction, together with an absent scotopic b-wave, implies that the defect is in the mid-retinal layers. Further, the absence of oscillatory potentials in the photopic electroretinogram (ERG) suggests that the interplexiform cell may be implicated in some manner. The focal ERG of the CSNB patient showed normal amplitude and normal phase delays, supporting the idea that the focal ERG samples primarily cone photoreceptor activity