The number zero holds a special status among numbers, indispensable for developing a comprehensive number theory. Despite its importance in mathematics, the neuronal foundation of zero in the human brain is poorly known. In new research, neuroscientists from the University Hospital Bonn, the University of Bonn and the University of Tübingen conducted single-neuron recordings in neurosurgical patients, while they made judgments involving non-symbolic number representations (dot numerosity), including the empty set, and symbolic numbers (Arabic numerals), including numeral zero. Neurons showed responsiveness to either the empty set or numeral zero, but not both.
Kutter et al. discovered that individual nerve cells in the medial temporal lobe recognize zero as a numerical value and not as a separate category ‘nothing.’
“Unlike other numbers such as one, two or three, which represent countable quantities, zero means the absence of something countable and at the same time still has a numerical value,” said Professor Florian Mormann, a neuroscientist at the University Hospital Bonn and the University of Bonn.
“In contrast to positive natural numbers, the concept of the number zero only emerged late in human history over the last two millennia.”
“This is also reflected in childhood development, as children are typically only able to understand the concept of zero and associated arithmetic rules at around the age of six.”
How the concept of zero is represented by nerve cells in the human brain has not yet been investigated.
In their new study, Professor Mormann and colleagues showed neurosurgical patients, who had had hair-thin microelectrodes inserted into their temporal lobes, numerical values from zero to nine.
The numerical values were shown as Arabic numerals on the one hand and as sets of dots on the other, including an empty set.
“Meanwhile, we were able to measure the activity of individual nerve cells and actually found neurons that signaled zero,” said Dr. Esther Kutter, a neuroscientist at the University Hospital Bonn and the University of Tübingen.
“Such neurons responded to either the Arabic numeral zero or the empty set, but not to both.”
In both cases, there was a numerical distance effect in which neurons reacted weaker, but measurably, also to the neighboring number one.
“So at the neuronal level, the concept of zero is not encoded as a separate category nothing, but as a numerical value integrated with other, countable numerical values at the lower end of the number line,” said Professor Andreas Nieder, a neuroscientist at the University of Tübingen.
“Despite this integration, the empty set is encoded differently from other numbers at the neuronal population level, especially in the case of point sets,” Professor Mormann added.
“This could explain why the recognition of the empty set also takes longer at the behavioral level than for other small numbers.”
For Arabic numerals, however, this effect was not found at either the neural or behavioral level.
“From this, we recognize the importance of symbolic representations, for example through Arabic numerals, for the integration of the number zero on the number line in the human brain,” the researchers said.
A paper on the findings appears in the journal Current Biology.
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Esther F. Kutter et al. Single-neuron representation of nonsymbolic and symbolic number zero in the human medial temporal lobe. Current Biology, published online September 24, 2024; doi: 10.1016/j.cub.2024.08.041