When disfluency is--and is not--a desirable difficulty: the influence of typeface clarity on metacognitive judgments and memory.

There are many instances in which perceptual disfluency leads to improved memory performance, a phenomenon often referred to as the perceptual-interference effect (e.g., Diemand-Yauman, Oppenheimer, & Vaughn (Cognition 118:111-115, 2010); Nairne (Journal of Experimental Psychology: Learning, Memory, and Cognition 14:248-255, 1988)). In some situations, however, perceptual disfluency does not affect memory (Rhodes & Castel (Journal of Experimental Psychology: General 137:615-625, 2008)), or even impairs memory (Glass, (Psychology and Aging 22:233-238, 2007)). Because of the uncertain effects of perceptual disfluency, it is important to establish when disfluency is a "desirable difficulty" (Bjork, 1994) and when it is not, and the degree to which people's judgments of learning (JOLs) reflect the consequences of processing disfluent information. In five experiments, our participants saw multiple lists of blurred and clear words and gave JOLs after each word. The JOLs were consistently higher for the perceptually fluent items in within-subjects designs, which accurately predicted the pattern of recall performance when the presentation time was short (Exps. 1a and 2a). When the final test was recognition or when the presentation time was long, however, we found no difference in recall for clear and blurred words, although JOLs continued to be higher for clear words (Exps. 2b and 3). When fluency was manipulated between subjects, neither JOLs nor recall varied between formats (Exp. 1b). This study suggests a boundary condition for the desirable difficulty of perceptual disfluency and indicates that a visual distortion, such as blurring a word, may not always induce the deeper processing necessary to create a perceptual-interference effect.