Abstract
Abstract: :
Purpose: Previous studies of high-pass filtered optotypes reported in the literature were conducted under various luminance and contrast conditions and with different presentation methods, factors which are all known to affect subject performance. The purpose of this study was to evaluate peripheral thresholds for detection and resolution of three high-pass filtered optotypes under identical experimental conditions. Methods: We used three computer generated high-pass filtered optotypes - tumbling E, Landolt C and the circular Ring perimeter stimulus. All tests were performed at the same lighting (darkened room), luminance (average 20 cd/m2) and contrast levels (25%, 50%, and 90%). Measurements were taken at 10°, 20° and 30° in the temporal visual field of the right eye of two observers at a distance of 1.5 m. Resolution thresholds, expressed as minimum angle of resolution (MAR), were measured using a 3/1 four alternative forced choice (4AFC) algorithm and detection thresholds using a 3/1 two alternative forced choice (2AFC) algorithm. A stimulus step size of 1 dB was used. Trials were performed until seven reversals were obtained, and threshold was determined by the mean of the last six reversals. Results: Detection thresholds, similar for the three optotypes, were lower than resolution thresholds. Landolt C resolution thresholds were greater than tumbling E resolution thresholds at all three measured eccentricities except at 90% contrast, where thresholds overlapped. A linear proportionality between both detection and resolution thresholds was found for all optotypes at a contrast of 25% in both observers. Further measurements on one observer confirmed this relationship at 50% and 90% contrast. All measured thresholds also showed a linear proportionality versus ganglion cell separation. Conclusions: We find that thresholds for all stimuli, whether being used to measure detection or resolution, were proportional to ganglion cell separation and dependant on contrast. This indicates that classical sampling theory, which applies solely to invariant performance and stationary, perfectly regular sampling arrays, needs to be modified before it can be applied to high-pass filtered optotypes.
Keywords: visual acuity • spatial vision