Wolfgang Wesemann

Measurement of spatial contrast sensitivity with the swept contrast VEP

Contrast response functions (CRFs) for the VEP were obtained with a Discrete Fourier Transform (DFT) technique employing swept contrast gratings. VEP CRFs in infants were found to have a form similar to those observed in adults, being linear functions of log contrast over a range of near-threshold contrasts. CRFs with low and high contrast lobes were present in infants, as they are in adults. Contrast thresholds were estimated by extrapolation of the CRF to zero microvolts. The effects of additive EEG noise and of the DFT data window on the shape of the measured CRF are considered. For large signals, the measured CRF is nearly independent of the additive noise, but at small signal values additive noise introduces a small bias towards larger amplitudes. The VEP signal-plus-noise distribution was modeled as a family of Rice distributions in order to evaluate the effects of bias on the estimates of threshold. The amount of bias depends inversely upon the slope of the CRF. The amount of bias introduced by a smoothing window also depends upon slope of the CRF as well as the sweep rate. The combined effects of additive noise and window bias were such that the total bias was nearly independent of CRF slope. Sweep VEP contrast thresholds were shown empirically to be unaffected by changes in the range of contrast swept.

Accuracy and accommodation capability of a handheld autorefractor

PURPOSE To determine the accuracy of measurement by the Nikon Retinomax handheld autorefractor and its ability to relax accommodation. SETTING Pediatric Section, Department of Ophthalmology, University of Mainz, Germany. METHODS To perform a series of comparative measurements, autorefractor readings were obtained on healthy young adults (students) and on children aged 2 to 12 years. The autorefractor readings were compared with subjective refractions of the young adults and with cycloplegic retinoscopy of the children. RESULTS In adults, the accuracy of the handheld autorefractor measurements was comparable to that of conventional tabletop autorefractors. In children, the autorefractor measurements performed under cycloplegia were reliable; when cycloplegic agents were not administered, 24% were overcorrected by more than -2.0 diopters. CONCLUSION Cycloplegia is often necessary to obtain accurate autorefractor results.

Electrophysiological correlates of vernier and relative motion mechanisms in human visual cortex

Vernier onset/offset thresholds were measured both psychophysically and with the steady-state VEP by introducing a series of horizontal breaks in a vertical square-wave luminance grating. Several diagnostic tests indicated that the first harmonic component of the evoked response generated by periodic modulation of offset gratings taps mechanisms that encode the relative position of spatial features. In the first test, a first harmonic component was only found with targets that contained transitions between collinear and noncollinear states. VEP vernier onset/offset thresholds obtained with foveal viewing were in the range of 15-22 arc sec. Control experiments with transitions between symmetrical, noncollinear patterns (relative motion) did not produce first harmonic components, nor did full-field motion of a collinear grating. A second series of experiments showed that VEP thresholds based on the first harmonic component of the vernier onset/offset response had an eccentricity dependence that was very similar to that found in a psychophysical discrimination task that required a left/right position judgment (vernier acuity). Other recordings showed that the first harmonic of the vernier onset/offset VEP was degraded by the introduction of a gap between stimulus elements, as is the displacement threshold. The vernier onset/offset target also produced a second harmonic component that was virtually identical to the one produced by a relative motion stimulus. Displacement thresholds based on these second harmonic components showed a more gradual decline with retinal eccentricity than did the first harmonic component elicited by vernier offsets. The second harmonic of the vernier onset/offset VEP was relatively unaffected by the introduction of gaps between the stimulus elements. The first and second harmonic components of the vernier onset/offset VEP thus tap different mechanisms, both of which support displacement thresholds that are finer than the resolution limits set by the spacing of the photoreceptors (hyperacuity).

Clinical evaluation of refraction using a handheld wavefront autorefractor in young and adult patients

Purpose: To determine the accuracy of measurement by the SureSight autorefractor (software version 2.0) and the influence of accommodation. Setting: Pediatric Section, Department of Ophthalmology, University of Hamburg, Hamburg, Germany. Methods: In a series of comparative measurements, autorefractor readings were compared with cycloplegic retinoscopy in 195 eyes of 108 patients (1 to 81 years) measured under cycloplegia. Ninety‐six eyes were also measured without cycloplegia. Results: The wavefront autorefractor was able to refract human eyes from a distance of 0.35 m. The accuracy was lower than that with conventional tabletop autorefractors. A difference of less than 0.51 diopter (D) was found in 68% of the spherical equivalents under cycloplegia. Many emmetropic and hyperopic children accommodated during the noncycloplegic measurements and were minus‐overcorrected up to −6.13 D. In our group of young patients (2 to 17 years), 47% were minus‐overcorrected by more than −2.00 D. Conclusions: The wavefront autorefractor uses a new method to determine the refractive state of the eye from a distance. It was less accurate than other conventional autorefractors. A benefit is its application in infants and disabled and uncooperative subjects. Cycloplegia is necessary in young hyperopic patients.

Theory of eccentric photorefraction (photoretinoscopy): astigmatic eyes

An optical analysis of eccentric photorefraction (photoretinoscopy) of astigmatic eyes is presented. The size and the angular tilt of the dark crescent appearing in the subjects pupil are derived as a function of five variables: the ametropia of the eye (Dsph, Dcyl, axis), the eccentricity of the flash, e, and the distance of the camera from the subjects eye, dc. A simplified solution and a solution of the inverse problem, which enable one to calculate the degree of ametropia from the size and the tilt of the crescent, are also presented. If the crescent is smaller than the pupil, both the size and the tilt of the dark crescent are independent of the pupil size. The angular tilt of the crescent is also independent of the eccentricity. Characteristic changes of the crescent as a function of the cylinder axis are illustrated for compound and mixed astigmatisms. The validity of the theoretical predictions was experimentally verified on a model eye.

Contrast dependence of the oscillatory motion threshold across the visual field

Observer sensitivity to oscillatory step displacements of sine-wave gratings was investigated at various loci in the visual field (0-30 degrees) as a function of contrast. Detection thresholds at 10 Hz and high grating contrasts were approximately 11-15 arcsec in the fovea and 37-47 arcsec at 30 degrees eccentricity. At any given contrast, threshold displacement increases linearly with eccentricity. The data provide evidence against an interpretation based on cortical magnification, because the slope and the scale-free x intercept of the eccentricity function vary strongly with contrast. While foveal thresholds for high-contrast gratings are in the range of the hyperacuities, the oscillatory motion threshold falls off an order of magnitude more slowly than the traditional hyperacuities. Rather than conceiving of the oscillatory motion threshold as a spatial acuity limited by cortical magnification, we suggest an alternative approach that is based on a form of contrast discrimination. Oscillatory motion can be decomposed into the sum of a modulating counterphase grating and a static masking grating, both of which are in spatial quadrature (i.e., 90 degrees out of phase). At low grating contrast, oscillatory motion can be detected when the counterphase component exceeds a constant contrast value. Above a critical contrast value of the static component Cscrit, threshold rises as a power function of contrast with a slope near 1.0. The critical contrast value Cscrit increases linearly with eccentricity, indicating that oscillating gratings observed with the peripheral visual field are less easily masked compared with foveally fixated gratings.

Incoherent image formation in the presence of scattering eye media

Incoherent image formation in human eyes that have scattering eye media is investigated as a function of the particle size and the optical density of the scattering medium and for test targets that differ in form and size. For single scattering by large particles (much greater than lambda), a point-spread function and the associated modulation-transfer function of the scattered light are derived from diffraction theory. It is shown that object structures with low spatial frequencies are also imaged by the scattered light. Following single scattering by small particles and/or multiple scattering, the scattered light forms an approximately uniform background. Consequently, the retinal contrast is reduced regardless of spatial frequency. The image quality is, contrary to what is found in normal image formation, extremely sensitive to the form and size of the test target. It is shown that the optimal readability of white-on-black letters is obtained at intermediate spatial frequencies. For an extended layer of arbitrary optical density and particle size, the influence of multiple scattering is approximated by using Hartels scattering theory. It is shown that wavelength has only a small influence on retinal contrast for scattering by particles greater than lambda.