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Bruno Dubuc, Patrick Robert, Denis Paquet, and Al Daigen

Tuesday, 9 May 2023
“Dark neurons” and “sparse coding”, two related concepts?

By analogy with the dark matter and dark energy of the cosmos, some neuroscientists use the term dark neurons to refer to those neurons that very rarely generate action potentials (not to be confused with the use of the same expression to mean neurons that for unknown reasons stain very intensely in histological sections). Mark Humphries is a neurobiologist who has closely studied dark neurons in the first of these two senses and discussed them in his book The Spike and in various articles and interviews. Humphries was the first to recognize the importance of spontaneous neuronal activity in the brain, in particular in certain structures such as the thalamus. But he has also raised many questions about the role and the possible significance of these dark neurons, which were first detected when calcium-based optical imaging techniques became available. Before that, in traditional electrophysiology, investigators found neurons by slowly inserting an electrode into nerve tissue until a visual readback or audible signal indicated the emission of action potentials near the electrode’s tip. Obviously, this method could detect only those neurons that were active. But once investigators began to use newer, optical methods that let them visualize all types of neurons in the cortex, they quickly realized that most of these neurons did not emit nerve impulses spontaneously. And even when experimental animals were exposed to various kind of stimuli, very few neurons actually reacted to them. What, then, might be the purpose of all these neurons that produced less than one impulse per minute?

Some scientists have speculated that the behaviour of these neurons that are so seldom active may have some connection with the concept of sparse coding, which is receiving growing attention. The idea is that the memory patterns in the brain are formed not of large assemblies of neurons, as has traditionally been thought, but rather of relatively small numbers of neurons distributed sparsely throughout it. Both mechanisms would have advantages and disadvantages for encoding information, and the debate in the field of computational neuroscience continues.

According to a sparse-coding hypothesis, these dark neurons may not always be silent and may become more active when exposed to a wider range of stimuli under natural conditions that are more varied than in a laboratory, or when an individual is retrieving a memory.

Alternatively, according to a population hypothesis, even though these neurons fire very rarely, there are so many of them that collectively they may generate enormous numbers of action potentials and have large effects.

To sum up: we are still very far from understanding what the vast majority of the neurons in the human brain actually do, just as we are far from understanding what the dark matter and dark energy in the cosmos do. But it is pretty interesting that scientists have estimated that at least half of the brain’s activity may be generated by only about 10% if its neurons. The cliché that we use only 10% of our brains comes to mind, but in this case we’re talking not about some urban legend, but rather about some well supported scientific findings.

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