Non-astigmatic children's contrast sensitivities differ from anisotropic patterns of adults

Contrast detection and orientation discrimination thresholds associated with meridional amblyopia

Humans who have astigmatism resulting in meridional amblyopia exhibit deficits in performing visual tasks at or near detection thresholds. However, there is mounting evidence supporting the idea that performance at threshold may not reliably predict visual capabilities at supra-threshold levels of stimulation. In this study the threshold and supra-threshold performance of six meridional amblyopes were compared. A difference in the pattern of oblique effects was observed between contrast detection thresholds and supra-threshold orientation discriminations. This suggests there exists an independence between populations of neurons subserving contrast detection and the discrimination of visual stimuli. Meridional amblyopia may primarily involve a degradation in those mechanisms subserving visual contrast detection. Populations of cells subserving supra-threshold abilities such as orientation discrimination remain relatively unaffected in humans exhibiting meridional amblyopia.

Visual evoked potentials in three-dimensional color space: correlates of spatio-chromatic processing

Visual evoked potentials (VEPs) were measured for sinusoidal gratings with spatio-chromatic modulation defined in a three-dimensional color space. The spatio-chromatic modulation of the gratings can be decomposed into contributions from an achromatic luminance varying component, an isoluminant component which modulates only the activities of L cones and M cones, and an isoluminant component corresponding to modulation of only S-cone activity. The emphasis of this report is the nature of VEPs resulting from isoluminant spatio-chromatic modulation. The VEP response was characterized along a number of spatial, temporal, and chromatic stimulus dimensions: contrast, spatial frequency, chromaticity in the isoluminant plane, chrominance/luminance ratio, orientation, and temporal frequency. Isoluminant VEPs resulting from stimuli modulating L and M cones are compared with those from S-cone modulation. When appropriate spatiotemporal conditions are employed, both types produce robust VEPs; however, the S-pathway VEPs show considerably longer latencies than do those from LM-pathway activation. The VEP results are compared to psychophysical and single unit electrophysiological observations. VEP latencies exhibit the lowpass character of psychophysical chromatic contrast sensitivity functions but VEP amplitudes show bandpass tuning along both the S and LM axes. An oblique effect, i.e. shorter latencies for horizontal and vertical gratings than for diagonal, is observed in the isoluminant VEP. S-pathway VEPs are used to demonstrate an electophysiological correlate of transient tritanopia. Normative amplitude and latency data for VEPs from selectivity stimulated chromatic mechanisms provide a baseline for clinical electrodiagnostic applications.

Astigmatism, accommodation, the oblique effect and meridional amblyopia

The variation with accommodation in the dimensions of the geometrical blur ellipse that is formed on the retina of the astigmatic eye is explored in relation to orientation dependence of the corresponding line-spread function (LSF) and modulation transfer function (MTF). It is shown that if theta is the orientation of the line or grating with respect to the more powerful meridian of an eye having C dioptres of astigmatism, the optimal accommodation response varies as 0.5 C cos2 theta. The analysis is extended to include the effects of diffraction. Even at optimal focus, the diffraction-limited MTF at 45 degrees to the principal meridians of an eye with small amounts of astigmatism is substantially degraded. For example, with a 3 mm pupil and only 0.50 DC of astigmatism, the first zero in monochromatic modulation transfer at 555 nm occurs at approximately 32 c/deg in the oblique meridian as compared with 94 c/deg in the principal meridians. The results are used as the basis of a discussion of the possible role of infant and juvenile astigmatism in the development of the oblique effect and of meridional amblyopia.

Orientation anisotropy: some caveats in interpreting group differences and developmental changes

We tested 220 children aged 7-12 yr and 20 university students for visual contrast sensitivity at spatial frequencies of 1.2-21.9 c/deg for vertical, horizontal and oblique sinusoidal gratings. Males had higher mean sensitivity then females and adults had higher mean sensitivity than children. Sensitivity was greater for cardinal than oblique orientations, and this oblique effect increased with spatial frequency and mean sensitivity, and with age in children. Developmental growth of oblique anisotropy can be explained by growth in contrast sensitivity. Different sizes of oblique effect were found between groups living in different locations, but variations were mainly linked to differences in age and contrast sensitivity. Genetic or environmental interpretations of group differences should be treated with caution. Other anisotropies occurred (horizontal was better than vertical and right than left obliques), but these differences were small and irregular and may represent response biases rather than neurological effects.

Evoked potential and preferential looking correlates of the oblique effect in 3-month-old infants

Sensitivity for vertical and horizontal gratings is higher than for obliquely oriented gratings, a phenomenon known as the oblique effect. We compared visual evoken potential and preferential looking estimates of the oblique effect in nonastigmatic 3-month-old infants and found that orientation effects occurred more frequently by visual evoked potential measure than by preferential looking.

Visual fields of young children

A light-emitting diode (LED) perimeter and forced-choice procedures are described that enable quantitative assessment of peripheral visual fields of young children. The visual fields of normal children, aged 2-5 years, and adults are compared. There are no significant differences in the extent of the LED visual field of the children and the adults. For selected neuro-ophthalmology patients, results of LED and kinetic Goldmann perimetry have been compared. The patients' LED and Goldmann fields are similar in overall extent. In all cases, field defects demonstrated by Goldmann perimetry are detected by LED perimetry. However, discrepancies in the severity of quandrantanopic defects of a patient have been discovered that may be stimulus-dependent.

The effect of practice on the oblique effect in line orientation judgments

Line orientation discrimination improves with selective practice for oblique orientations and not for principal orientations. This training effect was observed with an identification task as well as with two alternative forced choice tasks. Despite the improvement for oblique orientations, just noticeable differences in orientation are still larger for the practised oblique orientation than for the principal orientations after 5000 practice trials. These findings suggest that the oblique effect in line orientation has at least two sensorial components, one of which is attributed to the meridional variations in the preferred orientation of area 17 S-cells.

Anisotropic resolution in children's vision

Grating acuity was measured for 4 orientations in 111 children aged 3-8 yr. Results showed that 73% of the children had better acuity for horizontal and vertical compared to oblique gratings, 14% showed no anisotropy, and 13% had better acuity for oblique gratings. The magnitude of the oblique effect was 0.2 octaves, similar to that found in both younger children and adults.

Practice improves adults' sensitivity to diagonals

Adults who were initially less sensitive to a diagonal, 10 c/deg sinusoidal grating, practiced detecting it for 3000 yes-no signal detection trials. Following practice all observers had improved their relative sensitivity to the diagonal and most were as sensitive to the diagonal as to cardinal (horizontal or vertical) orientations. Practicing a cardinal axis, on the other hand, caused no improvement in sensitivity unless the pre-practice threshold for that orientation was elevated with respect to other orientations. Three hypotheses are proposed to account for the improved sensitivity. The results are also related to the typical pattern of adult anisotropic contrast sensitivity which favors the cardinal orientations.