Notes about nothing

Florian Mormann raised the possibilities of a collaboration soon after he met Andreas Nieder in 2016. Mormann is a clinical neurophysiologist at the University of Bonn and Nieder an animal physiologist at the University of Tübingen. Nieder's proposal was a joint project to study how the brain perceives zero, a topic that he himself had found unusual. Mormann had patients whose neural activity was being monitored with electrodes. They were good subjects for the study.

The invention of zero is considered one of humanity's most significant inventions. "We owe it to the Indian scholar Brahmagupta, who was the first to write about zero," says Nieder. In the centuries that followed, zero, along with the base 10 system — also developed in India — travelled to Europe through the Arab world. Nieder thinks the concept of nothingness is not an easy one for the brain to grasp. "I think the brain needs a little bit of training to understand what zero is as it does not have an empirical correspondence," he says.

So, the researchers implanted electrodes in the participants' brains and recorded single-neural activities upon being shown the number zero, and a symbolic empty set. Single-neuron activity in the brain's medial temporal lobe (MTL), which has several parts with important cognitive functions, offers new insights into how humans perceive zero. Researchers found that neurons tuned specifically to zero, both as an empty set and as the numeral, indicating the brain treats zero as a low quantity on the number line rather than as a unique concept. This pattern, seen across species, suggests an evolutionary foundation for perceiving nothingness as a quantity, which may underpin how humans understand zero symbolically.

The brain handles zero differently depending on how it is presented. When zero appears as a symbol, as in the numeral 0, it blends in with other small numbers. But when it is shown as an empty set, the brain treats it as something separate. This distinction reflects why people find it tricky to identify zero and to tell it apart from other small numbers like one. Although the MTL isn't traditionally seen as central to number processing, studies suggest it plays a role in number cognition, with parts of the temporal lobe responding specifically to numbers.

Nieder's group found that humans and other beings often confuse zero with nearby numbers. The brain tends to mix up an empty set with the number one, making it difficult to distinguish between a display with no dots and one with a single dot. This confusion occurs both cognitively and at the neural level, with neurons sometimes firing similarly in both cases. When researchers conducted experiments with crows and monkeys, they observed the same phenomenon, with these beings also struggling to differentiate between zero and one.

Nieder describes zero as among the most abstract of concepts, as it requires removing all features of objects to reveal just the number. "Understanding zero requires effort and abstract thinking," says Nieder, as it pushes our brain beyond concrete experiences. Humans use numbers not only to identify quantities but also to process information through mathematical operations like addition and subtraction, engaging cognitive functions like memory, decision-making, and attention. These functions, managed by the brain's frontal areas, are essential for cognition and set humans apart from animals. Nieder sees number theory as a unique symbolic system, critical for understanding how our brains manage complex, human-specific symbols. The brain shapes everything we can perceive, think, or feel — laying the groundwork for understanding numbers and developing number theories and mathematics. Nieder notes that since animals grasp the concept of an empty set, this ability may reflect an evolutionary inheritance from our ancestors.

"I think this is a remarkable study because it neatly bridges how the mind perceives an abstract concept and how the brain interprets it," says Prabahan Chakraborty, neurophysiologist and Research Assistant Professor at the SRM Institute of Science and Technology, Chennai. "It could open up so many questions – for instance, how about the same set of zero-coding neurons in dyslexics who have trouble interpreting numbers and letters, or in dyscalculia – where subjects have problems with math?"