Objective Mesurement of the Defocus Sensitivity: The Difference in the Transient P–VEP Between Emmetropes and Myopes

Abstract

Abstract: : Purpose:Previous studies examing the defocus thresholds of individuals with different refractive errors were mainly based on subjective measurement methods. The purpose of this study is to use the transient visually pattern evoked potentials (P–VEP) to objectively determine the defocus sensitivity and evaluate the difference in the sensitivity between individuals with emmetropia and myopia. Methods:Twenty nine optometry students (12 emmetropes and 17 myopes) participated in this study. Their ages were from 19–26 years. The subjects' transient VEPs were measured to contrast–reversing checkerboard patterns (3 cycle/degree, 1 Hz) using the Roland system. Testing was conducted monocularly (OD only) at a viewing distance of 1 meter. The subject wore his/her best correction. P–VEPs were recorded under 5 defocus conditions: 0, +0.25D, +0.50D, +1.00D, and +1.50D. Amplitude and latency of the P–VEP were measured for each viewing condition. A regression line was used to fit the amplitude data plotted vs. the defocus values for each subject and then the slope represented the sesitivity of the subject to image defocus on the retina. Results:The P–VEP latencies were constant across defocus conditions for each subject. The average slopes for emmetropes and myopes are –7.22 ± 2.41 (S.D.) (µv/D) and –5.21 ± 1.87 (µv/D), respectively. The difference in the slope between the emmetropes and myopes is significant (p = 0.017). Conclusions:This study suggests that the transient pattern VEP can be used to objectively estimate an individual’s sensitivity to defocus. Based on the data, individuals with myopia are less sensitive to defocus or blurred image than individuals with emmetropia. This results support the defocus theory for the development of myopia.

Keywords: myopia • electrophysiology: clinical • refractive error development