Problems in using d' measures to assess subjective awareness

An accurate method of assessing the awareness to primes is crucial to investigations of subliminal perception. In a recent paper, Vermeiren and Cleeremans (2012) contend that traditional measures of prime detection potentially overestimate awareness to the prime. This can lead to wrongly classifying effects as subliminal, since even when primes are detected at chance level, it could be due to factors other than visibility. Here, I address this and point to another fundamental issue that is inherent to d' calculations, and has to be considered when using signal detection methods to assess awareness. In subliminal perception studies, unconscious processing of the stimuli is assumed when d' is not significantly different from zero. However, this is a null finding that leaves open the possibility of there being differences that the statistical test was not sensitive enough to detect. Hence, reported subliminal effects, especially small effects, could have occurred as a consequence of visible trials even when d' did not significantly differ from chance. Therefore, additional measures such as bootstrapping, which could complement d' in ensuring that the effects were not a consequence of a small number of trials, should be utilized in future studies.