Foveal metacontrast: I. Criterion content and practice effects.

Strong metacontrast was found during experiments involving foveal presentation of stimuli. Follow-up experiments indicate that foveal metacontrast can be accounted for in some conditions by a practice effect and/or a criterion-content change, which occurs with practice and involves a biphasic brightness response to the target. Consideration of these factors may help resolve some of the many apparent disparities in the metacontrast literature. The relevance of these factors to metacontrast theories is discussed.