Abstract

Previous research has shown that vernier acuity improves slowly with stimulus contrast (approximately a square-root law). However, the localization accuracy of an ideal observer, limited only by a source of constant variance noise, increases linearly with contrast. The weaker contrast dependence of human vernier acuity means that people are less efficient in visual localization at high contrast than low contrast. What accounts for this decline in efficiency? We used a noise-masking method to partition sources of inefficiency in vernier localization into two generic parts: the existence of internal noise in the visual system, and suboptimal sampling by the observer. Vernier thresholds were measured for Gaussian-windowed sine wave gratings in 1-D static visual noise. Sine wave contrast ranged from 0.06 to 0.25. Our data showed that as stimulus contrast rises, sampling efficiency for vernier acuity remains nearly constant (~30%) but internal noise grows rapidly.

© 1989 Optical Society of America