Something weighing heavy on your mind? Measuring thoughts with a set of scales

Today, neuroscientists generally agree that mental processes are intimately associated with the function of the brain. However, this hasn’t always been the case. For example, Descartes’ famous distinction between res cogitans, the stuff of thought, and res extensa, the stuff of matter has persisted strongly over the 350+ years since his death. So what evidence has led to our modern understanding of the mind and brain? This is the brief story of the man who used a glorified kitchen scale to measure thoughts well over a hundred years ago in the first ever neuroimaging study.

The search to measure the mind and soul empirically is as filled with pseudoscientific woo as Gwyneth Paltrow’s bathroom cabinets. For example, the infamously poor “21 grams experiment” by Duncan MacDougall in 1907 tried to measure the decrease of mass a human undergoes at death with the notion that this may by caused by the soul leaving the body. Out of 6 patients, one of them lost 21.3g, leading to the questionable conclusion that this is the weight of the soul. Needless to say, this wouldn’t win any awards for robust experimental design nor statistics in the 21st century. Not all such attempts were so unavailing though…



Angelo Mosso was born in May 1846 into relative poverty. Having excelled through secondary school, he went on to study medicine with the help of several grants. Following his graduation from the University of Turin magna cum laude he went on to Leipzig and then Paris where he studied graphical methods to investigate the dynamics of physiological phenomena. Upon his return to Turin, he applied his newfound skills to the circulation of the brain. Unfortunately, the human brain is notoriously hard to get to without its owner being dead, or being put at severe risk of becoming dead. Luckily for Mosso, a man called Michele Bertino had a brick dropped on his head from a 40 ft bell tower. Whilst this may not sound particularly fortuitous, said brick produced a hole in Bertino’s skull which provided a literal window allowing access to his brain. Making use of a special plethysmograph he developed, Mosso recorded pulsations of Bertino’s brain under different conditions. These pulsations corresponded to changes in blood flow. He noted that cognitively demanding tasks, such as mathematical calculation, resulted in increased brain pulsation. Crucially, in such conditions he did not also see an increase in the pulse at his wrist leading to the conclusion that mental activity specifically increases blood flow to the brain. This may seem obvious to us now, but at the time was truly revolutionary.


Figure 1: (left) The plethysmograph used to measure brain pulsations. (right) The brain and wrist pulsations recorded.


Unfortunately, not everyone has a conveniently placed hole in their skulls and, even by the standard of 19th century medical ethics, creating one just to measure brain pulsations wasn’t a viable option. Mosso set out to find a way to measure blood flow in anyone’s brain and in 1882 he built what he called the “human circulation balance”. As with many incredible scientific breakthroughs, his idea was surprisingly simple. If blood is flowing to the brain during more intense thought then a carefully set up scale could theoretically measure the increase in weight of the head relative to the rest of the body. In practice, this was a bed that could pivot in the middle if the centre of mass shifted. However, there were many non-trivial issues to iron out. When someone lies down, the blood that gravity pulls down into their legs slowly redistributes throughout the body. This means that for an hour or so the weight at the head very gradually increases. His elegant solution to this problem? Wait an hour before recording; not everything needs a high tech solution. However, one thing that did need some clever engineering was the problem of participants breathing, which has a large cyclic effect on the location of the centre of mass, for which he employed a counterweighting system to dampen fluctuations.


Figure 2: The human circulation balance


Mosso had participants read materials of varying cognitive demands whilst laying on the balance. These ranged from basic newspaper articles to more complex mathematics and philosophy as well as material of emotional significance. His remarkable findings were that participants engaging in more mentally strenuous activity, like reading philosophy, resulted in the scale tipping more toward their head compared to the simpler newspaper articles. He concluded that the increase in cerebral blood flow was proportional to the complexity of, or emotion evoked by, the cognitive task. Furthermore, he was surprised to find that subjects did not react equally to the same stimulus, and speculated this might be due to differences in ‘age…and education’. In short, these were crude measurements of the mind. These remarkable findings resulted from the same mechanisms as functional magnetic resonance imaging (fMRI); the human circulation balance’s modern successor which measures changes in blood flow in different parts of the brain. It was such an exciting invention that in 1908, a French newspaper reported that the balance "would soon fully explain the physiology of the human brain". In much the same way, fMRI today has had a variety of outrageous and unsubstantiated claims made about its ability to “read the mind”. This seems to be a recurring interaction between our fascination with measuring the mind and our boundless human fallibility.


“We’re sort of fascinated by seeing thought, which seems so nonmaterial — seeing it as a material thing. I think people often feel like if they see it on an imaging scan, it’s real in a way that it isn’t real if it’s just being talked about” — RUSSEL POLDRACK

Figure 3: David Feilds’ replica of Mosso’s human circulation balance


In 2014 David Field, a psychologist at Reading University and human circulation balance enthusiast, set out to recreate Mosso’s invention despite describing it as looking like “some kind of medieval torture device. I mean it's got a big strap to kind of stop the person moving around too much”. This recreation made use of a sensitive scale placed under the head to quantify how much the scale tipped under different conditions. As a proof of concept, participants were asked to hold their breath. This causes elevated carbon dioxide levels resulting in vasodilation and a well characterised increase in blood flow to the brain. Exactly as predicted, the data revealed a gradual increase in weight at the head end of the balance during the breath hold.

They then moved on to test the ability of the balance to measure cognitive functions. In one condition subjects were played music in brief bursts with their eyes closed. In the other condition, they were played music whilst also looking at visual patterns associated with the music. While blood flow did not significantly increase during the music only condition compared to rest (likely due to an experimental design issue) it did increase in response to both music and the visual stimulus together. These findings by Field et al confirmed that Mosso had indeed built a 19th century neuroimaging machine with the potential to measure mental processes.

Mosso’s findings went on to inspire Charles Sherrington (of Nobel prize fame) and Charles S Roy who went on to confirm the relationship between neural activity and blood flow by doing some unspeakable things to dogs.. But that’s a story for another blog post…


REFERENCES Field, D. T. and Inman, L. A. (2014) ‘Weighing brain activity with the balance: a contemporary replication of Angelo Mosso’s historical experiment’, Brain. Oxford University Press, 137(2), pp. 634–639. doi: 10.1093/brain/awt352.

Sandrone, S. et al. (2014) ‘Weighing brain activity with the balance: Angelo Mosso’s original manuscripts come to light’, Brain. Oxford University Press, 137(2), pp. 621–633. doi: 10.1093/brain/awt091.

Zago, S. et al. (2009) ‘The Mosso method for recording brain pulsation: The forerunner of functional neuroimaging’, NeuroImage. Academic Press Inc., 48(4), pp. 652–656. doi: 10.1016/j.neuroimage.2009.05.062.

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