Neural correlates of unconscious processing in functional magnetic resonance imaging: does brain activity contain more information than can be consciously reported?

Abstract

A central question of consciousness research is which cognitive processes can occur unconsciously. To investigate this, researchers typically compare participants' ability to consciously discriminate a stimulus to their unconscious processing of the same stimulus (e.g. measured via reaction time or brain activity). If participants are not significantly different from chance in the awareness (or "direct") measure while nevertheless there is a significant effect in the processing (or "indirect") measure, researchers argue that there is no conscious processing of the stimulus, while the stimulus is nevertheless somehow processed, as indicated by the processing measure. In consequence researchers conclude that the stimulus has been processed unconsciously. Using neuroimaging techniques such as functional magnetic resonance imaging (fMRI), researchers then infer which brain regions are involved in unconscious versus conscious processing. However, this methodology is based on a fundamental statistical fallacy that has likely led to an overestimation of the scope of unconscious processing, regarding both its capacity and the brain areas involved. The key problem is that sensitivities in the two measures are never directly compared. Therefore, it is not appropriate to conclude that the processing measure had higher sensitivity than the awareness measure. We reanalyzed the results from 16 fMRI studies directly comparing the sensitivities of both measures in 80 experimental conditions. Our results show that, using this sensitivity comparison method, only eight experimental conditions provide evidence for unconscious processing. These results question the validity of the interpretations commonly drawn in the field.