Conditioning and blocking of the McCollough effect.

The cliassical conditioning explanation of the McCollough effect was extended by investigating blocking and allied phenomena predicted by the Rescorla-Wagner (1972) equations. The subjects were first exposed to colored gratings or moving spirals and then were tested after induction with a compound of both types of stimuli. The effects appeared to be additive, and the stronger stimuli overshadowed the weaker stimuli during compound training. Blocking of the McCollough effect, or of the spiral, was observed after prior training on the other stimulus. A supplementary procedure suggested that it was possible to unblock either effect by increasing color saturation and contrast. The results show close agreement with the predictions of conditioning theory.