Fig. 3.

Principle of the experiment. The red and blue curves describe hypothetical psychometric functions: They give an observer's expected performance as a function of contrast in the crowded and uncrowded conditions, as in Fig. 2. Suppose we always give the choice between a fixed, uncrowded stimulus with contrast x_U and another, crowded stimulus with contrast x_C. How can we set x_C to make the observer feel more confident about the crowded stimulus? According to the cue-monitoring hypothesis, two cues to uncertainty are available: contrast and crowdedness. If the observer picks crowdedness as a cue, then he or she will always prefer the uncrowded stimulus, no matter what the value of x_C. If the observer uses contrast as a cue, then he or she should prefer the crowded stimulus as soon as . Alternatively, if confidence follows performance, what the observer should do is choose the crowded stimulus as soon as it yields a higher expected performance: here, any point beyond . As shown above the plot, for any , the predictions of the performance-based hypothesis differ from those of either of the two cue-based models.