Scientists see consciousness as purely material

Cognition: what is awareness?

The seat of consciousness can be narrowed down further, for example through experiments in which different stimuli are presented to the two eyes. Imagine a photo of Donald Trump appears in your left eye and one of Angela Merkel in your right. It would be obvious that you perceive a strange superimposition of Trump and Merkel. But in fact you will see Trump for a few seconds, who then disappears and is replaced by Merkel, before Trump appears again. The two images alternate in an endless dance. Neuroscientists call this phenomenon binocular rivalry. Due to the ambiguous input, the brain cannot decide: is it Trump or Merkel?

What we perceive determines the »rear hot zone«

During this experiment, if you lie inside a magnetic resonance imaging scanner, which is recording your brain activity, the experimenters will see neurons moving in a wide area in the cortex. This so-called "posterior hot zone" (PHZ) extends over parts of the parietal, temporal and occipital lobes. We only perceive sensory impressions, such as the pictures of Merkel and Trump, when they are processed in this area. Information that arrives in the primary visual cortex via the optic nerve does not correspond to what we consciously see; the image that we perceive only emerges in the hot zone at the back.

If this area is stimulated with electric shocks - for example, in order to examine it more closely before the surgical removal of a nearby brain tumor or epilepsy focus - this triggers a large number of pronounced perceptions and feelings in patients. Some see flashes of light, geometric shapes or distorted faces, others report acoustic hallucinations or a feeling of familiarity or unreality, some feel the need to move a certain part of the body, and so on. It is different when the front part of the brain is excited: then almost no perceptions occur.

We owe further valuable insights to neurological patients from the first half of the 20th century. In order to remove tumors or to relieve epileptic seizures, surgeons sometimes cut large parts of the prefrontal cortex from the brain. What was particularly noticeable about those treated was how inconspicuous they remained afterwards. Many suffered from side effects such as motor disorders, tics or problems with impulse control after the operation. Most recovered quickly from the procedure, however, and lived for several years with no evidence that it had significantly changed their conscious experience. On the other hand, removing even small areas of the posterior cerebral cortex, where the hot zone is located, can severely limit the range of sensations. The patients lose the ability to recognize faces or to perceive movements, colors or spaces.

Accordingly, everything we see, hear or otherwise feel seems to be generated by regions in the posterior cortex. But how does this area differ from the rest of the cerebral cortex? The truth is, we don't know. However, and this is the exciting part, neuroscientists may now be getting closer to the answer.

In the early 2000s, Giulio Tononi from the American University of Wisconsin-Madison and Marcello Massimini, now at the University of Milan in Italy, developed a technique called "zap-and-zip" with which they can check whether someone is conscious or not Not. The “zap” of the method is a strong magnetic impulse, emanating from an insulated wire coil, which the researchers hold on the skull of their test subjects. In this way, they generate a brief electrical current in the brain tissue exposed to the magnetic field. The neurons excited to fire trigger a chain reaction: They excite or inhibit other nerve cells, and their activity spreads like a wave in the cerebral cortex. These electrical signals can be measured in the electroencephalogram (EEG) using electrodes attached to the skull. Together, the recordings from different points on the skull form a kind of video sequence of nerve cell activity.