Motor activation without conscious discrimination in metacontrast masking

Nine experiments investigated motor responses to geometric stimuli (a target and a distractor) preceded by masked primes. In congruent trials, the target was preceded by a smaller target-like shape and the distracter by a smaller distracter-like shape. In incongruent trials this arrangement was reversed. In neutral trials both primes were distracter-like shapes. Highly significant effects of these priming conditions on reaction time and error rate were found, although there was no conscious discrimination of primes, as assessed by a discrimination index (d' or d(s)) not different from 0. This result subsisted under conditions of time pressure, feedback, practice, and financial incentive in the discrimination task. It remained unchanged under conditions in which the reaction time and discrimination tasks were blocked, alternated, randomly mixed, or combined on each trial. It is concluded that motor activation without conscious discrimination was demonstrated.