Contrast sensitivity for flickering and static letters and visual acuity at isoluminance in glaucoma.

PURPOSE: Psychophysical testing was used to assess the effects of early glaucomatous damage on visual functions mediated by the magnocellular and parvocellular pathways. METHODS: We studied 57 eyes with early glaucoma, 92 eyes suspect for glaucoma, and 88 normal eyes. Tests were designed to target the functions of the magnocellular (M) or the parvocellular (P) pathways, and included measurements of contrast sensitivity for letter recognition, using both static presentation, and counterphase modulation at 25 Hz. We assumed that ability to perform the latter task would depend on the integrity of the M-cell pathway. To evaluate P-cell function we measured spatial acuity for a green letter presented against an isoluminant white background. Tests were carried out in the fovea, and at 3 degrees nasal, superior, temporal and inferior to the fovea. RESULTS: Glaucomatous eyes showed, on average, defects in almost all of the functions tested. Threshold elevations for static and isoluminant presentations were generally as large as those for flickering stimuli. The inferior paracentral part of the retina was generally more severely affected than the other locations tested. The defects observed in the suspect population were similar to, although smaller in magnitude than, those found in eyes with glaucoma. A discriminant analysis identified a subset of five tests, including P as well as M pathway tests, which was able to correctly classify 89% of normal and 79% of glaucomatous eyes. CONCLUSIONS: Putative M- and P-cell functions are both impaired at a relatively early stage of glaucoma. Deficits for both types of test are most severe in the inferior paracentral area of the retina (superior visual fields). Psychophysically based detection of the condition can benefit if results from different tests, done at different retinal locations, are combined.

Vernier acuity for edges defined by flicker.

We investigated the effects of contrast and temporal frequency on vernier edge alignment thresholds. Edges were defined by the presence of a 180 deg phase difference in the temporal modulation waveform of adjacent rectangles with spatially uniform luminances. Thresholds of 10 arc sec or less could be obtained at high contrasts, and flicker rates up to 8 Hz. Above this range, thresholds increased rapidly with decreasing contrast and increasing rates of flicker. Thresholds also increased with increasing temporal frequency over the range 0-20 Hz for contrasts normalized to thresholds for the detection of either flicker or the edge. Linear regression on log-log plots of threshold vs contrast at different temporal frequencies showed that the relationship between threshold and contrast was well described by a power law with an exponent of about -0.5 at temporal frequencies of 8 Hz or lower. About 8 Hz the slope more than doubled and thresholds increased. Thresholds also increased when the relative phase (i.e. the instantaneous sign of the contrast) of the upper and lower edges was reversed, and this effect was observed at all temporal frequencies. Measurements of threshold as a function of the size of a gap between the upper and lower edges suggested that the integration region was larger at 16 Hz or above than at 8 Hz. The results suggest that the channels which mediate vernier hyperacuity are phase sensitive and attenuate frequencies higher than 8 Hz.

Visual remapping in infantile nystagmus.

The possibility that patients with idiopathic infantile nystagmus achieve spatial constancy by visual remapping was investigated by comparing subjective localization of flashed test targets to their absolute position in space and to their absolute position on the retina. Nystagmats first viewed a screen-stationary reference target that was followed by a test flash. A computer used eye movement feedback to precisely control the test flash position on the retina. All six nystagmats detected test flashes throughout their nystagmus cycle. For three nystagmats test flashes (total N = 48) were delivered to the same retinal locus that were, at different times, to the right and left of the reference target. More than two-thirds of such crossover stimuli were correctly located in space: when only those stimuli at least 0.5 deg from the reference were considered, two of three subjects correctly located all stimuli. Taken together these results argue that our subjects could see throughout the nystagmus cycle and shifted their visual map in synchrony with their nystagmus as an explicit means of avoiding oscillopsia.

Visual-evoked cortical potentials in dissociated vertical deviation.

It is debatable whether chiasmatic misrouting of temporal optic-nerve fibers (similar to that found in ocular albinism) is also characteristic of dissociated vertical deviation. Pattern appearance, pattern reversal, and diffuse-flash, monocular full-field visual-evoked cortical potentials were recorded from albino and normal human subjects and subjects with dissociated vertical deviation. Pattern appearance was the most reliable stimulus for evaluating lateralization (albino-type misrouting) in adult albino patients, and diffuse-flash stimulation was almost as reliable in children. Pattern reversal was found to be an unreliable indicator. Lateralization was not evident among patients with dissociated vertical deviation, as determined by the three modes of stimulation. Our data supported earlier findings that pattern appearance is the most appropriate technique to detect lateralization. Our findings differed from those of previous reports in demonstrating that reliability of the lateralization phenomenon increases with age up to approximately 15 years. Pattern reversal stimulation was not reliable in patients with horizontal nystagmus.

Visual acuity measurements by swept spatial frequency visual-evoked-cortical potentials (VECPs): clinical application in children with various visual disorders.

Previous studies have indicated that visual acuities of normal infants can be estimated with good accuracy using swept spatial frequency visual-evoked-potentials (VECPs). In this report we describe acuity measurements obtained with this technique from 304 examinations performed on 135 children having various visual disorders. When possible, two or more different stimulation frequencies (8, 12, 15 or 24 contrast reversals/sec) were used in each patient, and three to eight sweep VECPs were obtained from each patient under each simulation and recording condition. High correlation coefficients (0.94 - 0.96) between the acuity estimated on each patient from either the single sweep giving the best visual acuity (BSS) or from vector averages (VeA) of the EEG data obtained from several sweeps confirmed previous findings in normal infants. We also found high correlation coefficients among BSS recorded at different temporal frequencies (0.79-0.97) and among comparisons of BSS or VeA acuity to optotype visual acuity (0.6-0.89). Children with clinically undetectable optokinetic responses showed lower visual acuity estimated by BSS than those who demonstrated optokinetic nystagmus. We conclude that the sweep VECP is a valid method, giving estimates of acuity which correlate well with optotype acuity and correspond well to other clinical findings, and that it can be useful in the clinical management of nonverbal patients.