Varied stimulus quality and stimulus probability in 2 reaction time (RT) experiments with college students (n = 24). In Exp I Ss were run in a Sternberg paradigm with a fixed-set procedure, and stimulus quality was manipulated by varying the contrast between the stimulus and its background. Both stimulus probability and stimulus quality affected RT, and the interaction of these 2 variables was highly significant. In Exp II stimulus probability and quality were varied as in Exp I, but S's task was to name the stimulus. Again, stimulus probability and quality affected RT, although both effects were smaller than in Exp I. The interaction of stimulus probability and quality was also significant. Results of both experiments indicate that stimulus probability affects the stimulus encoding stage; this was interpreted as evidence against recent self-terminating search models of the memory scanning process. (PsycINFO Database Record (c) 2006 APA, all rights reserved)