Cognitive influences on perceptual processing.

These experiments investigate the influence of frequency of occurrence of a visual stimulus (stimulus probability) on encoding processes, in an attempt to discover what sorts of mechanisms allow cognitive processes to modify perceptual processes. Experiments 1 and 2 show that frequently occurring visual letters do not facilitate encoding of visually similar letters. This implies that stimulus probability does not directly affect the feature detectors used in encoding the letters. Four more experiments provide evidence that stimulus probability has its effect on the availability of an abstract code that is generated by the encoding process from the visual input. Results from the experiments with letter stimuli could be interpreted using a model similar to the logogen model of Morton. Experiments with nonsense forms suggest that subjects use abstract codes in dealing with the forms only when the stimuli are constructed from a set of orthogonal features. A secondary finding was that visual quality has an effect that extends past the feature analysis stage and into a stage in which the visual input activities 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.