Our senses constantly receive information from the world around us, but we consciously perceive only a small portion of it. Nonetheless, even stimuli that are not consciously perceived are registered in our brain and influence our behavior. This is known as unconscious perception.
Researchers disagree about how brain activity differs during conscious and unconscious perception. Some think that both consciously and unconsciously perceived objects are processed in the same way in the brain, but that the brain is more active during conscious perception. Others think that different neurons process the information in different types of perception.
Dr. Moti Salti from BGU and Soroka University Medical Center and his colleagues have now investigated this issue. While recording participants’ brain activity, a line was briefly presented in one of eight different possible locations on a screen. The line was masked so it would be consciously perceived in roughly half of the presentations. Participants had to report the location of the line, and then say whether they had seen it or had merely guessed its location. Even when they reported that they were guessing, participants identified the location of the line better than by chance, indicating unconscious perception on “guess” trials. This enabled Salti to compare how the brain encodes consciously perceived and unconsciously perceived stimuli.
Unlike previous studies in which the brain activity associated with “seen” and “unseen” stimuli was compared, Salti used a different approach to extract the neural activity underlying consciousness. A classifying algorithm was trained on a subset of the data to recognize from the recorded brain activity where on the screen a line had appeared. Applying this algorithm to the remaining data revealed the dynamics of stimulus encoding. Consciously and unconsciously perceived stimuli are encoded by the same neural responses for about a quarter of a second. From this point on, consciously perceived stimuli benefit from a series of additional brain processes, each restricted in time. For unconsciously perceived stimuli, this chain of processing breaks and a slow decay of encoding is observed.
Salti and his colleagues therefore conclude that conscious perception is represented differently than unconscious perception in the brain, and produces more extensive and structured brain activity. Future work will focus on understanding these differences in neural coding and their contribution to the interplay between conscious and unconscious perception.
“In this paper we alter the approach towards unveiling the Neural Correlates of Consciousness,” says Salti, “Instead of comparing general activity associated with conscious and unconscious percepts, we use a “mind reading” technique and track the neural coding of actual contents that were consciously and unconsciously perceived.”
Salti’s co-authors include: Prof. Stanislas Dehaene, Prof. Lauri Parkonnen, Dr. Simo Monto, Dr. Jean-Remy King and Dr. Lucie Charles.
