Effects of stimulus probability and visual similarity on stimulus encoding.

Visual similarity, stimulus probability, and stimulus contrast were manipulated in two memory-scanning experiments to determine how stimulus probability affects encoding. Two hypotheses were tested: The first, a featural facilitation hypothesis, localizes the effect of stimulus probability on feature extraction; the second claims that stimulus probability has its effect on stimulus recognition. In both experiments, visual similarity was found to slow encoding, particularly under low-contrast conditions. This effect was larger for low probability stimuli than for high probability stimuli and increased as the difference in probability between two visually similar stimuli increased. These results are inconsistent with the featural facilitation hypothesis, but can be explained in terms of differential priming of internal recognition responses for members of the stimulus set, such that more probable members are more easily recognized.