Visual fatigue effects on vergence dynamics in asymptomatic individuals

Functional Neuroanatomy of the Human Accommodation Response to an "E" Target Varying from -3 to -6 Diopters

Background: We aimed to identify the functional brain networks involved in the regulation of visual accommodation by contrasting the cortical functional areas evoked by foveal fixation to an "E" target, which were subservient to the accommodation responses to a -3/-6 diopter stimulus. Methods: Neural activity was assessed in healthy volunteers by changes in blood oxygen level-dependent (BOLD) signals measured with functional magnetic resonance imaging (fMRI). Twenty-five right-handed subjects viewed the "E" target presented in a hierarchical block design. They participated in two monocular tasks: (i) sustained foveal fixation upon an "E" target on a white background at 33 cm (-3.03D accommodative demand); and (ii) sustained fixation through an attached -3D concave lens (-6D accommodative demand) in front of the fixated eye; each condition cycled through a standard alternating 30-s eye open/30-s eye closed design to provide the BOLD contrast. The total sustained period was 480 s. Results: The contrast between the -3D and the rest condition revealed activation in the occipital lobe (Lingual gyrus, Cuneus, Calcarine_L, and Calcarine_R); cerebellum (Cerebellum_Crus1_L and Cerebellum_6_L); precentral lobe (Precentral_R); frontal lobe (Frontal_Inf_Oper_R and Frontal_Mid_R); and cingulate cortex (Cingulum_Ant_L). With the -3D concave lenses (-6D accommodative demand) in front of the fixated eye, the voxel size and peak intensity of activation in the occipital lobe and cerebellum were greater than with the -3D accommodative demand; emergent activated brain areas included the parietal lobe (bilateral precuneus gyrus and right supramarginal gyrus); the precentral lobe and cingulate cortex failed to reach the threshold in the -6D vs. rest contrast. In the -3D and -6D contrast comparison, the frontal lobe (Frontal_Sup_Medial_L) and parietal lobe (Precuneus_L and Precuneus_R) passed the significance threshold of cluster-level family-wise error (FWE) correction. The mean activation in the -3D and -6D contrast revealed an incremental summation of the activations than that found in the previous -3D vs. rest and -6D vs. rest comparisons. Conclusions: Neural circuits were selectively activated during the -3D/-6D accommodative response to blur cues. Cognitive-perceptual processing is involved in signal regulation of ocular accommodative functions.

Binocular vision and abnormal head posture in children when watching television

AIM

To determine the association between the binocular vision and an abnormal head posture (AHP) when watching television (TV) in children 7-14y of age.

METHODS

Fifty normal children in the normal group and 52 children with an AHP when watching TV in the AHP group were tested for spherical equivalents, far and near fusional convergence (FC) and fusional divergence (FD) amplitudes, near point of convergence, far and near heterophoria, accommodative convergence/ accommodation ratio and stereoacuity. The values of these tests were compared between the two groups. The independent t test was applied at a confidence level of 95%.

RESULTS

The far and near FC amplitudes and far FD amplitudes were lower in the AHP group (the far FC amplitudes: break point 13.6±5.4(Δ), recovery point 8.7±5.4(Δ). The near FC amplitudes: break point 14.5±7.3(Δ), recovery point 10.3±5.1(Δ). The far FD amplitudes: break point 3.9±2.7(Δ), recovery point 2.6±2.3(Δ)) compared with those in the normal group (the far FC amplitudes: break point 19.1±6.2(Δ), recovery point 12.4±4.5(Δ). The near FC amplitudes: break point 22.3±8.0(Δ), recovery point 16.1±5.7(Δ). The far FD amplitudes: break point 7.0±2.1(Δ), recovery point 4.6±1.9(Δ)). Other tests presented no statistically significant differences.

CONCLUSION

An association between the reduced FC and FD amplitudes and the AHP in children when watching TV is proposed in the study. This kind of AHP is considered to be an anomalous manifestation which appears in a part of puerile patients of fusional vergence dysfunction.

Asthenopia in schoolchildren

Objective

To assess asthenopia prevalence and associated factors in schoolchildren aged 6–16.

Methods

This was a cross-sectional study of all children attending the first to eighth grades at two public schools in the urban region of a medium-sized town in Southern Brazil between April and December 2012. A questionnaire on socioeconomic and cultural matters was answered by parents, while the children answered a questionnaire on asthenopia-related symptoms. The children underwent a complete visual function examination, including measurement of visual acuity, refraction test, cover test, stereopsis, heterophoria assessment, near point of convergence, and accommodative convergence/accommodation ratio.

Results

Asthenopia prevalence was 24.7% in a total sample of 964 children. Visual acuity of 20/25 or better in both eyes was found in 92.8% of the children. The stereopsis test was normal in 99.4% of them, and some kind of strabismus was found in 3.5%. About 37.8% had astigmatism, 71.6% had mild hyperopia, 13.6% had moderate hyperopia, and 6.1% were myopic. Near point of convergence was abnormal in 14.0% of the children, and the accommodative convergence/accommodation ratio was found to be altered in 17.1% of them.

Conclusion

Children and adolescents have expressive prevalence of asthenopia. The prevalence of visual function alterations does not differ from the general population, and, therefore, they are not prerequisites. It is very important that its mechanisms and risk factors be better defined. Health professionals need to be on the lookout for complaints of visual fatigue because of its potential to influence learning and school performance.