Effects of contrast and spatial frequency on vernier acuity.

We have examined vernier acuity using sinusoidal luminance gratings. Vernier thresholds were affected by both grating contrast and spatial frequency. With fixed (50%) contrast gratings, vernier thresholds reached minimum values of approximately 10 sec of arc at spatial frequencies between 6 and 16 c/deg. Vernier thresholds for all spatial frequencies are related to contrast by a power law with exponents of approximately -0.8. Thresholds approach half a grating period (180 deg phase shift) as grating contrast approaches detection thresholds. We discuss our results in relation to three models for vernier detection. Most of our data are consistent with the predictions of Wilson's [(1986) Vision Res. 26, 453-469] model. Detection of vernier off-sets at low spatial frequencies may depend on detection of the horizontal border formed between the two halves of the grating.