Self-reported ophthalmologically healthy subjects at least 20 years of age participated in the study. Ocular examination at the first visit comprised autorefractor keratometry without cycloplegic agents, best-corrected visual acuity measurements, axial length measurement using the IOLMaster (Carl Zeiss Meditec, Dublin, California), slitlamp examination, intraocular pressure measurement using a Goldmann applanation tonometer, dilated funduscopy, and visual field testing using the Humphrey 24-2 Swedish interactive testing algorithm standard strategy (Humphrey Field Analyzer; Carl Zeiss Meditec). Exclusion criteria included contraindication to dilation; intraocular pressure of 22 mm Hg or higher; unreliable Humphrey Field Analyzer results (fixation loss or false-positive or false-negative results >33%); abnormal findings in Humphrey Field Analyzer results suggesting glaucoma according to the Anderson and Patella criteria47; history of intraocular surgery; best-corrected visual acuity worse than 20/25; evidence of vitreoretinal diseases; and optic nerve or RNFL abnormality. Eyes with apparent tilted discs that were thought to be congenital deformation48 were cautiously excluded, while eyes with slightly tilted discs commonly found in low to moderate myopia were included. After eligibility for study entry was confirmed, all subjects underwent imaging with the 3-dimensional OCT-1000 version 2.13 (Topcon Inc, Tokyo, Japan) after mydriasis. This system uses a superluminescent diode with a center wavelength of 840 nm and a bandwidth of 50 nm as the light source and acquires 27 000 axial scans per second. The OCT data sets were obtained using the concentric scan protocol in which 7 concentric circles with diameters of 2.2, 2.5, 2.8, 3.1, 3.4, 3.7, and 4.0 mm are centered on the gravity center of the optic disc. Using the concentric scan protocol, the image acquisition time was approximately 0.26 second. Each measurement circle included 1024 axial scans. The scan points are closer to each other in the inner circles than in the outer circles, resulting in a higher spatial resolution in the inner circles. For analysis, mean 360° RNFLT was defined as the average of all 1024 points, and the mean quadrant RNFLT was defined as the average of the 256 points of the quadrant. To obtain more accurate circle sizes, the magnification effect in each eye was corrected according to the formula provided by the manufacturer (modified Littman method), based on the refractive error, corneal radius, and axial length, which is basically the same as the original Littman method,49 except for small differences in the coefficients of each parameter. To determine the optic disc edge (the inner edge of the scleral rim), 7 points were manually plotted on the optic disc edge of a color fundus photograph obtained by the fundus camera function of the OCT equipment by experienced examiners in each examination site. All of the contour lines of the optic disc were further confirmed by an investigator (A.T.). The center of the optic disc was determined as the barycenter of the closed spline curve fitted to the 7 points, and disc area was determined as the area of the closed curve. During measurements using the concentric scan protocol, the fundus image was continuously monitored to automatically place the concentric circles centered on the thus-determined center of the optic nerve. When eye movement during the measurement was found, the scan was excluded and the measurement was repeated. Criteria for acceptable SD-OCT fundus images were defined as no apparent eye movements during the measurements with a quality factor of more than 60%. The segmentation of the RNFL was performed automatically and an experienced investigator (A.T.) confirmed the segmentations in all scans.