“The Signatures of Conscious Thought” is the fourth chapter of “Consciousness and the Brain: Deciphering How the Brain Codes our Thoughts” is an outstanding book by the French neuroscientist Stanislas Dehaene who is the Chair of Experimental Psychology at the College of France. This is the fifth consecutive post on this outstanding book. In this chapter Dr. Dehaene discusses four reliable signatures of consciousness—physiological markers that index whether the participant experienced a conscious percept.
The first signature is a sudden ignition of parietal and prefrontal circuits that is caused by a conscious stimulus (remember that the participant indicates whether the stimulus is conscious).
The second signature is found in the EEG in which conscious access is accompanied by a slow wave called the P3 wave, which emerges as late as one-third of a second after the stimulus.
The third signature is the result of conscious ignition that also triggers a late and sudden burst of high frequency oscillations.
The fourth signature consists of many regions exchanging bidirectional messages over long distances in the cortes, which form a global brain web.
The conscious brain can perceive only a single chunk at a time. Working memory rehearses these chunks to keep the active so they can be further processed. The processing of a second chunk can be delayed if it occurs prior to the processing of the first chunk. This is known as the psychological refractory period.
We can process a stimulus before we become consciously aware of the stimulus. For example, if we place a hand on a hot stove, we’ll take it off the stove before we consciously perceive the pain caused by the hot stove.
Consciousness lives in loops of reverberating neuronal activity, circulating in the web of our cortical connections, causing our conscious experience.
fMRI and scalp recording of brain potentials catch just a glimpse of the underlying brain activity. Explorations of the third and fourth signatures require electrodes being placed directly inside the brain. Such implantations of electrodes are indicated for certain epileptic patients, so science can capitalize on victims of this unfortunate malady. I hope it provides some satisfaction to these patients that the data that is derived from these electrodes is greatly advancing science.
Subliminal stimuli can propagate deeply into the cortex, but this brain activity is strongly amplified when the threshold for awareness is crossed, thus yielding reliable and valid signatures of consciousness.