The idea that it is possible to pay attention to two or more things at the same time is an illusion: the most the human mind can do is jump very quickly from one task to another. New studies suggest that this limitation stems from the deep architecture of the brain, an organ that would need to make do with a more or less fixed supply of energy brought in by blood vessels.
“It’s life that works on the edge, almost pushing the envelope — but it works, which is what matters,” says Brazilian neuroscientist Suzana Herculano-Houzel, a researcher at Vanderbilt University (USA) and columnist for Sheet.
She and her colleagues are proposing a different model for looking at how the brain handles its energy needs. Rather than assuming that the organism is able to redirect all necessary resources to the brain tissues whenever there is a demand for it, they argue that the organ’s structure must always deal with the fact that it has little leeway—hence the need of solving one cognitive problem at a time, among other constraints intrinsic to its functioning.
The researcher details the data in favor of this hypothesis in articles in the specialized journal Frontiers in Integrative Neuroscience. Among the co-authors of the studies are Lissa Ventura-Antunes and Oisharya Moon Dasgupta, also from Vanderbilt University, and Douglas Rothman, from Yale University (USA).
Like any other organ, the brain depends on the network of blood vessels distributed throughout our body to survive. Neurons and other brain cells are supplied with oxygen and glucose (sugar) through blood capillaries, very thin “hoses” that lead to them. Blood reaches the capillaries from the main carotid artery, responsible for supplying the head and neck.
It is also known that the brain is a particularly “drinking” structure, like a car that is not at all economical when it comes to using fuel. Among humans, it can consume between 20% and 25% of the energy expended by the body throughout the day, although it corresponds only to somewhere between 2% and 3% of body weight. To a lesser extent, this disproportionate expenditure of energy by the brain is also true for other mammalian species.
An intriguing detail in this apparent waste of energy is that while blood flow can almost double in a certain region of the brain that is actively being used (say, the visual cortex, crucial for vision when one is examining the details of a photograph) , oxygen consumption increases much less in that same brain region. Such an apparent “uncoupling”, which has puzzled neuroscientists for three decades, is now explained by the blood’s ability to transport oxygen to the brain, limited by the density of capillaries in the tissue.
Furthermore, it has become increasingly clear that the brain that is apparently at rest, when the individual is awake, but not engaging in any defined cognitive task, is actually just as active, and consuming as much energy, as a more “focused” brain. . In other words, the nervous tissue would work in a completely different way from the muscles, for example, which only spend a lot of energy when the person is moving some part of the body.
Added to these data is a detailed analysis of glucose consumption in the neurons of mice and rats, carried out by Herculano-Houzel, Ventura-Antunes and Dasgupta. The trio of researchers found that while the density of neurons (that is, how many of these cells are packed in a given space) can vary hundreds of times depending on the region of the rodent’s brain, the density of capillaries that supply each region varies, at most. , four times.
For the scientists, this undermines the idea that the brain would be organized in such a way as to feed more oxygen and glucose to the regions that are more “hungry” because of the large number of neurons, since the increase in the density of nerve cells it is not accompanied by a proportional increase in blood capillaries. In fact, it seems that areas with a higher density of neurons receive proportionally less fuel than those with a lower concentration of these cells.
Taken together, all this would suggest that the most the brain can do is slightly redirect the distribution of resources to certain areas, depending on the activity performed. This is where the implications of the hypothesis for understanding the limitations of human (and other animal) attention come in.
“It is extremely difficult to do two things well at the same time. Doing them in sequence is easier”, summarizes Portuguese neuroscientist António Damásio, from the University of Southern California.
In fact, even people who seem to do well at “multitasking” in general are just switching frequently from one task to another. And several studies show that, in the vast majority of cases, the quality of tasks performed tends to drop when they are performed in parallel. There can also be long-term negative effects on the ability to concentrate.
According to Herculano-Houzel, there are indications that a kind of tug of war occurs between the different brain structures in this case, with jumps of attention from one sensory modality to another (from hearing to vision, let’s say). “What doug [Rothman] and what I propose is that there is a fundamental limitation, of energetic origin, to doing several things at the same time; Of course, other systems work over this limitation.”
If the proposal is correct, it could have implications that go beyond a more detailed understanding of how the brain works. The mechanism could help to understand why certain regions of the brain are more vulnerable to loss of function during the natural cognitive decline that occurs in aging, or in more severe forms of it, such as Alzheimer’s disease. One of the possibilities, say the researchers, is that precisely the regions with the highest density of neurons, with a proportionately smaller presence of capillaries from the beginning, are susceptible to this.
