The latest research has overturned our consistent view of the brain: When we are resting or in a daze, the brain does not stop running, and some mysterious "background" neural activity always exists.
The default mode of the brain
● Neuroscientists have long believed that when we rest, the neural circuits in the brain are closed.
●However, brain imaging experiments show that there is some continuous "background activity" in the brain.
● This default mode prepares people for reactions to future events.
●Errors in neural connections between brain regions related to the default mode may cause a range of diseases from Alzheimer's disease to depression.
Written by Marcus E. Raichle
Translator Feng Zejun
You are napping on a rocking chair outside the house, with a magazine on your lap. Suddenly, a fly stops on your arm, and you pick up the magazine to shoot it. What happened in your brain after the flies stopped on you? What about before stopping? For a long time, neuroscientists have believed that when people are resting, the neural circuits in the brain are basically closed. In this sense, the neural activity at this time is "random noise", like a snowflake-like pattern displayed on a TV that has not received a signal. When the fly stops on your arm, the brain regains consciousness and is ready to perform the "slap fly mission." But recent neuroimaging studies have revealed a completely different fact: When people lie down and rest, the brain is not idle, and many important neural activities are still going on.
It has been confirmed that when our brain is resting-such as sitting in a chair in a daze, lying in a bed to sleep, or receiving anesthesia, the various brain regions are still constantly transmitting information. This kind of uninterrupted information transmission is called the default mode of the brain, and it consumes 20 times the energy that we consume when we slapping flies or consciously responding to other external stimuli. In fact, most of the events we consciously do, such as eating and speaking, are deviations from the baseline neural activity in the brain's default mode.
The key to understanding the default mode of the brain is to find the previously unknown brain system-the default mode network (DMN). In the process of organizing neural activities, the role of the default mode neural network is still being studied, but we know that the brain is forming memories and organizing other nervous systems that need to prepare for future events (such as Feeling that the fly stops on the arm, it flaps subconsciously. This action requires the brain's motor system to be ready at any time), which may be the way preset by the default mode neural network. In synchronizing the behavior of brain regions, the default mode neural network may also play an important role-making each brain region like a runner, in a reasonable "ready" state at the moment the starting gun is fired. If the default mode neural network is indeed preparing for the conscious activity of the brain, then studying the behavior of this network may allow scientists to find some clues to reveal the nature of conscious experience. In addition, neuroscientists also speculate that the destruction of the default mode neural network may cause confusion and a series of complex brain diseases from Alzheimer's disease to depression.
Looking for dark energy
"The brain is always active" is not a new idea. Hans Berger, the inventor of the electroencephalogram (waveform that records the electrical activity of the brain), is a proponent of this view. In 1929, in a series of groundbreaking papers, based on the uninterrupted brain waves detected by the instrument, he speculated that "the central nervous system is always in a fairly active state, not just when people are awake."
But Berg's view of how the brain works has not attracted the attention of the scientific community, even after non-invasive imaging techniques have become a routine method in neuroscience laboratories. In the late 1970s, positron-emission tomography (PET) came out, which can measure the glucose metabolism rate, blood flow, and oxygen uptake in the brain. To a certain extent, these indicators can reflect the brain’s nerves. Activity level: In 1992, functional magnetic resonance imaging (fMRI) was born, which can help scientists achieve the same goal by measuring oxygen consumption in the brain. Although these techniques are not limited to measuring brain activity, most experimental designs inadvertently leave the impression that most brain areas are usually "quiet" and will not be active until a specific task is required. stand up.
Generally speaking, when doing imaging experiments, neuroscientists will find ways to determine which brain areas produce a specific perception or are related to a certain behavior. The best way to find these brain regions is to directly compare the differences in brain activity between the two related states. If researchers want to know which brain regions are more important for the behavior of “reading words”, they will compare the subjects’ performance when reading words aloud (experimental group) and silently looking at the same set of words (control group). What are the differences in brain imaging. In order to accurately find this difference, researchers must eliminate the brain imaging map of the subject who reads the word from the brain imaging map of the subject who reads the word. After processing, the neuronal activity in the brain area that is still active on the imaging map may be necessary for the act of reading words. In this case, the basic brain activity, that is, the "background" neural activity that always exists, will be eliminated. The experimental results obtained in this way make it easy to think that the brain's "switch" is turned on only when performing a specific task, and at other times, the brain is in an inactive state.
What happens in the brain when people are resting or in a daze? In the past few years, we and other research groups have become very interested in this issue, because many studies have suggested that in this state, there is a certain degree of background activity in the brain.
Just by visually observing the brain imaging map, you can find evidence of the existence of brain background activity: whether from the control group or the test group, the brain imaging map always shows that multiple brain areas are in a very busy state. Because there is background "noise", it is almost impossible to find the difference between the two types of brain imaging images by observing the original image with the naked eye. To accomplish this task, we can only use a computer to perform sophisticated image analysis.
Further analysis found that when performing specific tasks, the increase in brain energy consumption will not exceed 5% of the basic neural activity. In the neural circuit, most of the neural activities have nothing to do with external events, and the energy consumed by these activities accounts for 60% to 80% of the total energy consumed by the brain. Therefore, we learn from astronomers and call these fixed neural activities the dark energy of the brain—invisible dark energy occupies the vast majority of the material energy in the universe.
Another reason we speculate that dark energy may exist in the brain is that research has found that only very little sensory information can actually reach the central processing area of the brain. When visual information is transmitted from the eyes to the visual cortex, the signal strength will be greatly attenuated.
There are countless information around us. About tens of billions of bits of information reach the retina every second, but there are only 1 million visual output neural connections connected to it. The information transmitted from the retina to the brain is only 6 million bits per second. The visual cortex has only 10,000 bits of information.
After further processing, visual information can enter the brain area responsible for producing conscious perception. Surprisingly, the information that ultimately forms conscious perception is less than 100 bits per second. If these are all the information that the brain can use, such a small amount of information is obviously unlikely to form perception, so the fixed brain neural activity must have played a role in this process.
The number of synapses also suggests that dark energy in the brain may exist. Synapses are the connection points between neurons. In the visual cortex, the number of synapses responsible for transmitting visual information is less than 10% of all synapses. Therefore, most synapses must be used to establish connections between neurons in the visual cortex.
Marcus E. Rachel is Professor of Radiology and Neurology at Washington University St. Louis School of Medicine. For many years, his research team has been studying human brain functions through positron emission tomography and functional magnetic resonance imaging. In 1992, he was elected to the American Academy of Medicine and became a member of the American Academy of Sciences four years later.
Translator of this article:
Feng Zejun, Ph.D. in neurobiology from Fudan University in Shanghai, mainly engaged in brain function related research, long-term translation of scientific and technological articles, and has published many translations in "Global Science" magazine.