At first, zero caused confusion. “Its ability to represent ‘nothing’ and enable complex mathematical operations challenged deeply ingrained theological and philosophical ideas,” Nieder said. Particularly due to the influence of the church, philosophers and theologians associated “nothing” with chaos and disorder and were disinclined to accept it. Many even feared it, considering it “the devil’s number,” Barnett said.

But soon, merchants recognized that zero was valuable for business. By the 15th century, it had become widespread in commerce, finance and mathematics across Europe, but never shed its aura of mystery. “If you multiply a number by zero, it disappears. That was really hard to understand,” Semenza said. That feeling persists today: Although zero is now pervasive and seemingly simple, math students and mathematicians alike continue to wrestle with it.

Zero is the “eccentric uncle in the family of numbers,” Nieder said. To use zero in calculations, mathematicians had to establish all sorts of rules. You can’t divide any other number by zero, but you can divide zero by any other number. A nonzero number to the power of zero gives you one; zero to a power of a nonzero number gives you zero, but zero to a power of zero gives you a calculator error — and a headache.

Yet “the idea of zero, or something that plays the role of a zero, somehow appears all over math,” said Neil Barton, a philosopher of math at the National University of Singapore. Without it, modern mathematics wouldn’t exist — you wouldn’t be able to solve a function, do calculus or distinguish between 1 and 1 million.

However you look at it, zero is unique. For researchers interested in how the brain handles numbers, zero was “the most fascinating number of them all,” Nieder said. He suspected that if zero is special in history and math, then the brain must process it specially, too.

Nieder’s group has previously shown that some neurons in the brain have favorite numbers. Some favor 3 and will fire more rapidly when presented with three apples, for example, than they would with two or four — and much more than with five or seven. The more rapidly neurons fire, the more interested they are in a specific number. This is true not only in humans but also in other animals. While nonhuman animals don’t understand numbers when they’re represented as digits — an entirely human construct — they can estimate quantities, also known as numerosities. Researchers previously found that monkeys and crows have neurons that are specifically tuned to the numerosity zero.

But until very recently no one had ever probed for the neural basis of zero in humans.

**“Just Another Number”**

Barnett was interested in absence before he was interested in zero. The majority of consciousness and perception science over the last century has focused on what happens in the brain when we detect something in the environment. “But this ignores the whole other side of things,” he said, “which is that you can often have experiences of something not being there.” For example, if you go to grab your keys and they aren’t where you left them on the hallway table, you experience absence.

Previously, researchers assumed that absence was represented in the brain by neurons not firing. But recent studies have shown that the brain encodes absence with unique neural patterns. To push this work further, Barnett turned to zero, “a concept of absence that all of us are familiar with,” he said.

A few years ago, Barnett began his absence studies with the number zero. He recruited 24 participants to perform tasks related to zero as they sat in a magnetoencephalography scanner, which “looks like these old-school hairdresser” machines, he said. As neurons fire, they generate voltages, which in turn create a magnetic field that can be detected by the machine. By analyzing the magnetic fields, researchers can learn how populations of neurons respond when prompted to think about specific topics, such as zero.

Barnett and his adviser at University College London, Stephen Fleming, were looking for evidence of the numerical distance effect, a phenomenon that occurs when the brain processes nonzero numbers. Basically, the brain can more easily distinguish between two numbers if they are far apart on the number line than if they are close together. So it confuses 6 and 7 more often than 6 and 9. The researchers figured that if the brain processes zero similarly to other numbers, it should also show the numerical distance effect.

Indeed, in results published in *Current Biology* in August, Barnett and Fleming concluded that the brain treats zero, both as a digit and as a quantity, in much the same way that it treats the other numbers. “Zero is represented on this neural number line,” Barnett said.

Nieder, for his part, has been obsessed with zero and absence for the better part of a decade. In 2016, he proposed that the neurological mechanisms that encode absence may be shared with those that encode zero. He hypothesized that zero must have evolved from more fundamental representations of perceptual absence. First, the brain had to understand the absence of a stimulus, like a light being off; only then could it recognize “nothing” as a category akin to “something,” but representing everything that isn’t something. Finally, it had to turn “nothing” into a quantitative concept. By understanding how the brain encodes zero, he believed, we might be able to understand how the brain deals with absence.

Since 2015, Nieder has collaborated with Florian Mormann, the head of the cognitive and clinical neurophysiology group at the University of Bonn, who treats epilepsy patients. For their treatment, these patients have electrodes implanted in their brains, which Mormann can use to collect neuroscience research data with patient consent.

Mormann recorded brain activity from single neurons as the patients performed number-related tasks. In the analysis, he and Nieder first focused on nonzero numbers and found that the brain processes large numbers differently than smaller ones. Then they went back to their data to focus only on zero.

As they reported in *Current Biology* in September, Nieder and Mormann found the same thing Barnett had, that zero exhibited a numerical distance effect in the brain. That means that, to the brain, zero is just another number, Nieder said.

Or is it?

“When one looks closely,” he said, “zero is still an outlier among the other numbers.”

**What Makes Zero Special**

Nieder’s team found some differences between the way the brain represents zero and the way it handles other numbers. For one thing, more neurons had zero as their preferred number than any other small number. Because there are more neurons that code for zero, the brain can represent the empty set with more accuracy than it can represent other small quantities, they found.

“Zero is encoded together with other small numbers, but it is represented more distinctly compared to them,” Nieder said. “It is simply an eccentric outlier and represented as such in the brain.”

However, they didn’t find any difference in accuracy in the way the brain represented the digit version of zero — 0 — compared to the other digits. “All number symbols are abstractions, functioning as formal symbols within a symbolic system,” Nieder said. “As such, zero is treated like any other formal number.”