The human visual system differentially represents subjectively and objectively invisible stimuli.

How unconscious processing is measured—either by asking participants what they saw (subjective) or by testing whether they can discriminate stimuli (objective)—affects conclusions about the brain's representation of unseen information. Using fMRI in 43 people viewing masked faces and houses that were either subjectively or objectively invisible, category information was found in early and higher visual cortex even for objectively invisible stimuli, though representations in anterior ventrotemporal areas were weaker. For subjectively invisible stimuli, a clear posterior-to-anterior gradient appeared, with stronger category information in ventrotemporal cortex. For objectively invisible stimuli, category information remained similar across visual areas. The measurement approach thus influences the extent of unconscious processing and the neural correlates of consciousness.

Study at a glance

Abstract

The study of unconscious processing requires a measure of conscious awareness. Awareness measures can be either subjective (based on participant's report) or objective (based on perceptual performance). The preferred awareness measure depends on the theoretical position about consciousness and may influence conclusions about the extent of unconscious processing and about the neural correlates of consciousness. We obtained functional magnetic resonance imaging (fMRI) measurements from 43 subjects while they viewed masked faces and houses that were either subjectively or objectively invisible. Even for objectively invisible (perceptually indiscriminable) stimuli, we found significant category information in both early, lower-level visual areas and in higher-level visual cortex, although representations in anterior, category-selective ventrotemporal areas were less robust. For subjectively invisible stimuli, similar to visible stimuli, there was a clear posterior-to-anterior gradient in visual cortex, with stronger category information in ventrotemporal cortex than in early visual cortex. For objectively invisible stimuli, however, category information remained virtually unchanged from early visual cortex to object- and category-selective visual areas. These results demonstrate that although both objectively and subjectively invisible stimuli are represented in visual cortex, the extent of unconscious information processing is influenced by the measurement approach. Furthermore, our data show that subjective and objective approaches are associated with different neural correlates of consciousness and thus have implications for neural theories of consciousness.