21.7 THIS IS HOW WE DO IT: Does thinking make your head heavier?

We sometimes speak of thoughts that “weigh heavy” on our mind, or brain. Brain activity—particularly in response to cognitively complex tasks or emotionally engaging stimuli—can seem costly when compared with times when our brain is at rest. But is there a literal, measurable cost to brain activity?

In 1884, an Italian researcher named Angelo Mosso pondered these questions and proposed a testable hypothesis:

In order to supply the necessary fuel and oxygen for increased brain activity, more blood is pumped to the brain, causing the head to become heavier.

Recall, after all, that all body parts don’t always get all the blood they can take—as we saw in our discussion of “food coma” in Section 21.6. After a meal, there is an increase in the amount of blood pumped to the capillaries surrounding the digestive tract. Does the same thing occur in response to increased brain activity?

Mosso’s hypothesis is a reasonable one. But, of course, for a hypothesis to be useful, we must be able to test it. And toward this end, he invented an apparatus for measuring an increase in the weight of a human head, called a “human circulation balance,” and reported some preliminary observations. Due to some confounding variables and the limitations of his apparatus, however, it was difficult to draw definitive conclusions.

In 2014, a student and her professor decided to recreate Mosso’s balance and to rigorously test his hypothesis by carefully evaluating the weight of brain activity.

The human circulation balance is essentially a long wooden board, balanced like a see-saw. (Mosso’s daughter said that he called it the “metal cradle” and “the machine to weigh the soul.”) A subject lies on the board, and a scale is placed under the end where the subject’s head is, to register any downward force. Small weights are added or removed from the other end of the board (where the subject’s feet are) until the scale registers just the slightest downward force—approximately 2 newtons. (A newton is a standard unit of force; on the earth’s surface, 1 newton is the equivalent of about 0.22 pounds.) This indicates that the balance is slightly tipped toward the head end. The scale can then register any additional downward force due to increased blood flow to the head.

The researchers recruited 14 subjects and used an experimental design in which the subject would lie still on the balance and be exposed to a stimulus for 2 seconds, followed by 23 seconds of rest. During the stimulus period, the subject was exposed to (1) music alone, or (2) music and a visual display of colorful geometric shapes synchronized to the music, or (3) no stimulation, as a control. The researchers recorded the force on the scale during the stimulation (or control) period. Any increase in head weight was attributed to increased mental activity due to increased blood flow. For each subject, the experiment was repeated 22 times.

Some fine-tuning of the apparatus was necessary to eliminate the impact of breathing and heartbeats on the force measured, so that the researchers could actually detect a force due to additional blood pumped to the head during mental activity. And the experimental protocol was designed to minimize any extraneous stimuli: subjects rested on the balance for 1 hour before measurements were made, and they all wore headphones and kept their eyes shut during the experiment (except during the visual stimulation trials, in which they looked at a display on a computer screen above them).

Yes! Listening to music only, the force increased by 0.61% relative to when no stimulus was used. And music and a visual display together produced a 1.21% increase. Although these amounts are very small—representing only about 0.005 newtons—the differences among the three stimuli were consistent across the subjects and were statistically significant.

The greater blood flow in response to both auditory and visual stimulation is not surprising, given that such stimuli target two distinct parts of the brain. Mosso also experimented with having subjects read complex math texts and a letter from an upset creditor (for which, he wrote, “the balance fell all at once”). Would you expect the addition of exposure to odors to have any effect?

With a clearly stated, testable hypothesis and a simple experimental setup—straightforward enough to have been devised 130 years ago—it is possible to generate interesting observations and draw conclusive results. And brain activity does, indeed, cause increased blood flow to your head (though perhaps not enough for you to notice).

Sophisticated brain imaging technologies used today, including MRI and PET scans—which can detect movement of blood and metabolic activity within the brain—have also documented greater blood flow in response to neural activity. For this reason, Mosso’s hypotheses and experimental strategy can be considered important precursors to today’s ideas about brain activity and the functioning of the circulatory system.