Fundaments of visual acuity applied to construction of concentric visual Stimuli. Guerrero´s Arorradial Stimulus (GAS)

Abstract

<em><strong>Objective:</strong></em> To compare the physiological and geometrical parameters of the Snellen E and Landolt C optotypes, through calculations and conceptual arguments, to formulate the Guerrero’s Arorradial Stimulus (GAS), based on the angular principle of the former and Landolt’s concentric arrangement. <em><strong>Methodology:</strong></em> It was based on a literature review of visual acuity, physiological basis, geometric-trigonometric calculation and anatomical principles applicable to the design of stimuli. The analysis revealed common elements and differences of the printed area and the perimeter area of the compared stimuli and made it possible to formulate a EAG matrix with homogeneous patterns of foveolar stimulation. <em><strong>Results:</strong></em> Common elements of angular configuration of the stimuli and differences in their morphological perimeter and geometric parameters were found. This suggests the reliability of the concentric matrix, which stimulates in a manner consistent with the anatomical distribution of photoreceptors and presumably provides greater reliability of visual responses. Conclusions: Perimeter and geometric differences found between alphanumeric stimuli of the same trial suggest that directional concentric tests gather the conditions of stimulation of homogeneous retinal areas to promote answers based on optical discrimination rather than readability. The GAS could become a modified alternative of the Landolt stimuli, which, by including radial elements that confer directionality to the stimulus and distractors, make it possible to conduct a qualitative study of refractive astigmatism and simulation.