Chapter 15: Decoding Visual Consciousness from Human Brain Signals
John-Dylan Haynes
Despite many years of research on the neural correlates of consciousness (NCC), it is still unclear how conscious experience arises from brain activity. Many studies have treated consciousness as an all-or-nothing phenomenon—for example, by comparing conscious and unconscious processing of the same features. However, the important question how the specific contents of consciousness are encoded in the human brain has often been ignored. It is frequently assumed that the contents of consciousness are encoded in dedicated neural carriers or “core NCCs,” one for each different aspect of conscious experience. However, identifying such core NCCs is a difficult task because many regions correlate with every aspect of conscious experience. For this reason it is important to formulate empirical criteria that allow assessing whether a brain region that is involved in processing of a certain feature (say, color or motion) is also directly involved in encoding this feature in conscious experience. Now, the approach of multivariate decoding provides a novel framework for studying the relationship between consciousness and content-selective processing in more detail. It allows to directly investigate the mapping between brain states and the contents of consciousness. Most important, decoding can be used to test the important criterion of “injectivity”: A brain region can be said to encode a particular type of experiences only if it is possible to decode these experiences in a loss-free fashion from activity patterns in that region. This approach makes it possible to assess how conscious experience is encoded in the brain and how the encoding of sensory information is affected when it enters awareness.