Six experiments investigated the influence of frequency of occurrence of a visual stimulus (stimulus probability) on encoding processes, to investigate the mechanisms that allow cognitive processes to modify perceptual processes. Exps I and II show that frequently occurring visual letters did not facilitate encoding of visually similar letters, suggesting that stimulus probability does not directly affect the feature detectors used in encoding the letters. Exps III-VI provide evidence that stimulus probability had its effect on the availability of an abstract code that was generated by the encoding process from the visual input. Results from the experiments with letter stimuli are interpreted using a model similar to the logogen model of J. Morton (1969). Experiments with nonsense forms suggest that Ss used abstract codes in dealing with the forms only when the stimuli were constructed from a set of orthogonal features. A secondary finding was that visual quality had an effect that extended past the feature analysis stage and into a stage in which the visual input activated an abstract code. This result calls into question the common practice of interpreting the interaction of a factor with visual quality as evidence that the factor affects visual feature analysis.