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Our brains internally represent the outside world through the coordinated activity of billions of neurons. A single neuron responds unreliably: Even when the same stimulus is shown repeatedly, its activity varies from trial to trial. Neuron populations can compensate for this variability by representing the same information across many neurons, a strategy known as population coding.

However, neurons do not always fluctuate independently. When many neuronal signals rise and fall together, their shared variability—known as noise correlation—can overlap with the pattern of activity that carries information about a stimulus and may eventually cap the information the population can convey. This possibility puzzled an international team of researchers from Kyoto University, Harvard University and the University of California, Los Angeles. The paper is published in the journal Science Advances.

"We face a fundamental question: Why does the brain have so many neurons if shared fluctuations impose a ceiling on information?" asks S. Amin Moosavi of UCLA.

The researchers reanalyzed recordings of approximately 18,000 to 21,000 neurons from each mouse's primary visual cortex as it viewed subtly different visual stimuli. They then examined how much information about those differences could be read from increasingly large groups of neurons.

By repeatedly drawing random subpopulations of different sizes and separating the noise in each into distinct activity patterns, the team found two power laws that retained the same form after adjusting for population size: one describing the distribution of noise strengths and the other describing how each noise pattern aligned with the signal.

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