Neural binocular summation and the effect of defocus on the pattern electroretinogram and visual evoked potentials for different pupil sizes.

PURPOSE: To evaluate the influence of defocus and pupil size on subjective (visual acuity [VA]) and objective (electrophysiology) descriptors of human vision and their effect on binocular visual performance by means of neural binocular summation (BS). METHODS: Fifteen healthy young subjects were recruited in this crossover study. Pattern electroretinogram (PERG) and visual evoked potentials (VEP) were measured under two levels of positive (+1.5 and +3.0 D) spherical and astigmatic defocus (axis 90°). Pupil size was controlled to reduce the inter-individual variability factor. RESULTS: Low- and high-contrast VA showed poorer visual performance in the monocular versus the binocular condition. Positive BS (for VA) was higher with greater pupil size and higher levels of defocus. In the visual electrophysiology tests (i.e., VEP and PERG), peak time and amplitude were affected by pupil size and defocus. The increase in peak time was larger and the reduction in amplitude was more significant with greater levels of defocus and smaller pupil sizes. For the VEP, positive BS was found in all conditions, being stronger with larger amounts of defocus and pupil size (for the P100 amplitude). Significant negative correlations were observed between the P100 amplitude and VA BSs. CONCLUSION: Smaller pupil size and levels of defocus produced greater changes in cortical activity as evidenced by both the PERG and VEP. Considering these changes and the obtained positive BS, the mechanism could be initiated as early as the retinal processing stage, then being modulated and enhanced along the visual pathway and within the visual cortex.

Pupil size effect on binocular summation for visual acuity and light disturbance.

PURPOSE: Smaller pupil size under binocular conditions could justify partially the improvement of visual performance when compared to monocular conditions. The purpose of this study was to assess the binocular summation for increasing levels of spherical and cylindrical defocus when the pupil size factor is ruled out as a confounding factor. METHODS: Fifteen young subjects were recruited in this crossover study. Light disturbance index (LDI) was evaluated with the light disturbance analyzer and low (LCDVA) and high (HCDVA) contrast visual acuity with the ETDRS test. Two positive spherical and cylindrical defocus levels (+ 1.5 and + 3.0 D) were used to induce a controlled degradation of the retinal image for two pupil sizes (3 and 5 mm). RESULTS: Our results showed poorer visual performance in monocular than the binocular condition. An increasing deterioration was observed with the level of spherical defocus. Positive binocular summation for visual acuity was found in all the parameters studied and was stronger under larger pupil size and for higher levels of spherical defocus. It was observed a positive binocular summation for the LDI under all the conditions studied. Strong and significant correlations were found between LDI and LCDVA and between LDI and HCDVA for all the conditions. Higher correlations were obtained between disturbance index and visual acuity for spherical defocus compared to cylindrical. CONCLUSION: Binocular summation was observed under different conditions of spherical and cylindrical optical degradation of the image quality when the pupil size was fully controlled. This suggests that a neural factor is involved.

Assessing the influence of cannabis and alcohol use on different visual functions: A comparative study.

This study aimed to assess and compare the influence of alcohol intake and cannabis smoking on different visual functions. A total of 64 young and healthy volunteers took part in the study. All undertook several randomised experimental sessions in which different visual functions, namely distance stereopsis, retinal straylight, visual discrimination capacity, and contrast sensitivity, were tested. Cannabis smokers (N = 30) took a baseline session and a session after smoking a cannabis cigarette, whereas alcohol users (N = 34) underwent a baseline session, a session after a low alcohol intake (Alcohol 1), and a session after a moderate to high alcohol intake (Alcohol 2). All visual functions were impaired by cannabis and alcohol use, particularly for the Cannabis and Alcohol 2 groups. The deterioration of all visual variables was higher for the Alcohol 2 than for the Alcohol 1 and Cannabis groups, except for retinal straylight, the deterioration of which was equal for the Cannabis group, and distant stereopsis, which was more impaired for the Cannabis group. The Alcohol 2 group experienced the most impairing conditions, although very similar to the cannabis group, and that factors other than the experimental conditions, such as sex and age, also influenced these visual changes. Alcohol and cannabis use clearly impair vision. The deterioration caused by cannabis is similar to, but slightly lower than, that produced by a moderate to high alcohol intake, with the experimental conditions, sex and age all having an impact on the variability of visual deterioration.

Anterior Chamber and Retinal Morphological Changes During Accommodation in Different Age Ranges.

PURPOSE: Accommodation mainly affects the lens, a structure of the eyeball that degrades with age. The aim of this work was to study the morphological changes of different ocular structures during accommodation, both in the anterior pole and the posterior pole, which may also be involved in the accommodation process. METHODS: The study will be carried out by stimulating accommodation through lenses of -1.00, -3.00 and -5.00 D starting from the spherical equivalent (M) of each participant in different age groups, from 18 to 66 years. To obtain the M value, aberrometry was achieved, and retinal optical coherence tomography and anterior pole tomography were performed to evaluate the possible structural modifications (central and peripheral), while accommodation was progressively stimulated. RESULTS: It showed that as the accommodative demand increased, morphological changes were produced in retinal thickness, both in the central and peripheral retina, in all age groups. A thinning of the retina was observed in the central 3 mm, while significant progressive thickening was observed closer to the periphery (up to 6 mm from the fovea) as the required accommodative power increased. A decrease in the anterior chamber depth (ACD) and anterior chamber volume (ACV) was observed with increasing lens power. CONCLUSION: Structural changes were observed in the central and peripheral retina, as well as in the ACD and ACV, while progressively greater accommodation was stimulated, showing that these structures were modified in the accommodation process even in advanced presbyopes.

Alcohol effects on drivers' speed management: The influence of visual performance and road complexity.

OBJECTIVE: The aim of this work is to assess how drivers adapt their driving speed as a self-regulation mechanism when driving under the influence of alcohol (DUIA), and the influence of alcohol consumption, visual performance, road complexity, and personal traits. METHOD: Thirty one volunteers took part in the study. All of them underwent three experimental sessions: Baseline (no alcohol), Alcohol 1 (low-moderate dose), and Alcohol 2 (moderate-high dose). Vision was tested by means of contrast sensitivity and retinal straylight. Driving performance was assessed using a driving simulator. The difference between the driving speed and the speed limit was calculated in 10 road scenarios of different complexity. RESULTS: Drivers adapted their driving speed less (i.e., drove faster) in Alcohol 1 condition compared to Alcohol 2 (p = 0.007). This indicate that participants felt more confident under the influence of a low-moderate dose of alcohol. Participants with better contrast sensitivity drove faster (p = 0.021). The complexity of the road and other factors such as driving experience, gender, or DUIA frequency, also influenced speed choice. CONCLUSIONS: Drivers under the influence of a low-moderate dose of alcohol seem to be less aware of the risk. Contrast sensitivity is a good predictor of the speed choice when DUIA. A better understanding of drivers' behavior under the influence of substance use may be useful to adjust and improve the traffic laws and driving regulations.

Deterioration of binocular vision after alcohol intake influences driving performance.

In this study, we assessed the influence of moderate alcohol intake on binocular vision, vergence system and simulated driving performance by analyzing the interactions between visual deterioration and driving variables. Thirty young healthy subjects were recruited. For the analysis, we measured: visual function (visual acuity and stereoacuity), phorias and fusional reserves. Also, we checked Sheard's and Percival's criteria at near and far. The accommodative convergence/accommodation (AC/A) ratio was calculated and vergence facility was also obtained at near. A driving simulator was used to assess driving performance under natural conditions and after alcohol consumption with a breath alcohol content of 0.40 mg/l. Alcohol intake significantly reduced binocular visual performance and vergence function, except for vertical phorias, horizontal phoria at near and Sheard's and Percival's criteria at near. Driving performance parameters also presented a statistically significant deterioration after alcohol consumption. A statistically significant correlation was found between the deterioration in overall visual function and overall driving performance, highlighting the influence of the visual deterioration on the driving performance. Moderate alcohol consumption impairs binocular visual and simulated driving performances, implying a greater safety hazard. In addition, deteriorations in binocular visual function and vergence correlated with simulated driving impairment, which indicates that the deterioration of binocular vision due to alcohol consumption affects driving, thus reducing road safety.