Three experiments were conducted to investigate the effect of stimulus probability on perceptual encoding. In Experiment 1, the role of an abstract code as a mediator of the effect was tested. Subjects viewed and responded to stimuli in conditions which either encouraged or discouraged the use of abstract codes. Contrary to prediction, the effect of probability tended to be greater at a low level of stimulus contrast when the use of abstract codes was discouraged, and unaffected by contrast when their use was encouraged. Subjects 1 responses to a questionnaire indicated that the use of abstract codes was determined to some extent by individual strategies. It was proposed that it is unnecessary to appeal to an abstract code that differs from a name code as a mediator of the probability effect at encoding. In Experiments 2 and 3, the degree to which the probability effect at encoding may be explained by sequential expectancy effects was examined. The probability effect was greater at low contrast in Experiment 2, and the results of Experiment 3 indicated a similar trend. The magnitude of the contrast effect did not vary as a function of the preceding stimulus sequence in either experiment. Previous results have shown that the size of the contrast effect does not depend on whether or not the ensuing stimulus is preceded by a valid cue. These findings were interpreted as suggesting that the probability effect at encoding is the result of a relatively static mechanism in which expectancies do not shift over trials
