Opponent model for line interval discrimination: interval and vernier performance compared.

Vernier and spatial interval thresholds were determined as a function of contrast, using thin bars with a Gaussian luminance profile. The previously reported contrast dependence of vernier acuity was confirmed, but contrast had a much smaller effect upon interval acuity. A subsidiary experiment showed that the relative contrast of the two bars in an interval discrimination could be randomly perturbed over trials without substantially reducing acuity. A model of interval discrimination is developed, including an opponent stage between hypothetical spatial coincidence detectors.