Electrophysiological and Psychophysical Studies of Meridional Anisotropies in Children With and Without Astigmatism

Abstract

PURPOSE. We investigated the pattern of meridional anisotropies, if any, for pattern onsetoffset visual evoked potential (POVEPs) responses and psychophysical grating acuity (GA) in children with normal letter visual acuity (20/20 or better). METHODS.A total of 29 children (aged 3-9 years), nine of whom were astigmatic (AS), were recruited. Orientation-specific monocular POVEPs were recorded in response to sinewave grating stimuli oriented along the subjects' principal AS meridians. Horizontal and vertical gratings were designated Meridians 1 and 2, respectively, for nonastigmatic patients (Non-AS). Binocular POVEPs in response to the same stimuli, but oriented at 458, 908, 1358, and 1808, were recorded. Psychophysical GAs were assessed monocularly and binocularly along the same meridians using the same stimuli by a 2-alternative-forced-choice staircase technique. The C3 amplitudes and peak latencies of the POVEP and GAs were compared across meridians using linear mixed models (monocular) and ANOVA (binocular).RESULTS. There were significant meridional anisotropies in monocular C3 amplitudes regardless of astigmatism status (P ¼ 0.001): Meridian 2 (mean 6 SE Non-AS, 30.13 6 2.07 lV; AS, 26.53 6 2.98 lV) was significantly higher than Meridian 1 (Non-AS, 26.14 6 1.87 lV; AS, 21.68 6 2.73 lV; P ¼ 0.019), but no meridional anisotropies were found for GA or C3 latency. Binocular C3 amplitude in response to horizontally oriented stimuli (1808, 29.71 6 3.06 lV) was significantly lower than the oblique (458, 36.62 6 3 .05 lV; P ¼ 0.03 and 1358, 35.95 6 2.92 lV; P ¼ 0.04) and vertical (908, 37.82 6 3.65 lV; P ¼ 0.02) meridians, and binocular C3 latency was significantly shorter in response to vertical than oblique gratings (P 0.001).CONCLUSIONS. Meridional anisotropy was observed in children with normal vision. The findings suggest that horizontal gratings result in a small, but significantly lower POVEP amplitude than for vertical and oblique gratings.

Do reduced visual acuity and refractive error affect classroom performance?

“…Up until approximately school age, childhood astigmatism can potentially affect normal visual development and is associated with amblyopia, abnormal binocular vision and myopia development . Some authors recommend that astigmatism as low as 0.50 D should be corrected, particularly if associated with oblique or against‐the‐rule (ATR) axes, or if asthenopic symptoms are present aligning with findings from a recent study that reported a trend of better grating acuity (and visually evoked potential amplitude) in children without astigmatism, compared to those with astigmatism as low as 0.50 DC (who were corrected) . Other authors suggest that astigmatism of 0.75 D or more should always be corrected in school children, irrespective of symptoms .…”

Effect of Stimulus Orientation on Visual Function in Children with Refractive Amblyopia

“…43,44 The oblique effect can be defined as more sensitive cardinal meridians compared with the oblique meridians, which is normally observed in adults; whereas the horizontal effect is defined by less sensitive horizontal meridians compared with the vertical and oblique meridians. Our previous study in children with normal vision found a horizontal effect, 41 where electrophysiological signals in response to binocular stimulation using horizontal gratings of 4 cpd were significantly poorer than the vertical and oblique gratings. This type of meridional anisotropy has been postulated to be an adaptative strategy for the visual system to optimize the perception of orientations that are less naturally encountered.…”

“…Their visual processing in response to orientation-specific grating stimuli was evaluated using electrophysiological and psychophysical techniques, as published previously for non-amblyopic children. 41 The research study adhered to the tenets of Helsinki and ethical approval was obtained from the Centralized Institutional Review Board (CIRB) (Registration number: R1083/98/2013) at SingHealth and ratified by the human research ethics committees at the University of New South Wales, Sydney, New South Wales, Australia (Approval number: 09364). Parents and guardians gave their informed consent and children six years of age and above provided assent.…”

“…Normally developing non-amblyopic children aged 3 to 9 years with normal 20/20 VA have been found to produce a horizontal effect under electrophysiological testing, regardless of their astigmatism status. 41 In newborn infants, poorer sensitivity to horizontal than vertical square-wave gratings was observed psychophysically at very low spatial frequencies (0.06 to 0.10 cycles per degree [cpd]). 42 This type of meridional anisotropy differs from the oblique effect, which is a physiologically normal phenomenon in adults.…”

“…This type of meridional anisotropy has been postulated to be an adaptative strategy for the visual system to optimize the perception of orientations that are less naturally encountered. 41 This is thought to provide more efficient neural coding 45,46 and was found psychophysically in adults viewing natural scenes [47][48][49][50] containing both broad spatial frequencies and orientation content.…”