In recent years, neuroscientists have noticed a remarkable pattern in the way neurons fire in brain samples. This activity seems to occur in avalanches which vary in size with a distribution that is scale invariant.
Scale invariance is a somewhat counterintuitive phenomenon. It means that the scale at which you examine data makes no difference to the distribution you observe. In other words, the distribution looks exactly the same whether you look at it close up or from far away.
Scientists have seen this kind of behaviour, called criticality, in all kinds of systems: the size of earthquakes, forest fires, epidemics and so on–all have the same kind of distribution.
It occurs in systems that are delicately balanced between inactivity, where the changes are always small, to a state of overactivity where any change tends to be runaway.
But there is a problem with this way of thinking, say Tiago Ribeiro at the Federal University of Pernambuco in Brazil and several pals. While neuroscientists have observed criticality in brain slices and in sleeping and awake animals, nobody has seen it in animals that are free to behave in any way they want.
This raises doubts about the relevance of criticality to brain function, say Ribeiro and co. Perhaps it only occurs in these artificial situations.
To settle the matter, Ribeiro have carried out the first measurements of neuronal avalanche distribution in 14 rats carrying out certain tasks and throughout their full sleep-wake cycle.
Ribeiro say their results show clear evidence of criticality throughout this cycle (although the same rats show a different pattern when anaesthetised).
During the experiments, the team introduced the rats to a new object and measured the pattern of firing before and afterwards. “Exposure to novel objects is a procedure known to increase firing rates, induce plasticity factors and promote dendritic sprouting in the cerebral cortex and hippocampus, leading to memory formation and learning of object identity,” they say.
One idea is that this process of learning generates dramatic changes not just in firing rates but also in the distribution of avalanches that this creates. However, Ribeiro and co found no such change in the before and after signals.
Instead, brains seem to be optimized for the encoding of memory patterns across all natural states. “Indeed, the results are compatible with the hypothesis that individual memories are encoded by specific spike avalanches,” they say.
That’s an interesting result that will help to build consensus that the brain operates in a remarkable, delicately balanced state.
There’s good reason to imagine why nature might choose such a state for the brain. Such systems have a richer variety of behaviour than others and would be better able to process data over a wider range of circumstances. “In theory, criticality provides many desirable features for the behaving brain, optimizing computational capabilities, information transmission, sensitivity to sensory stimuli and size of memory repertoires,” say Ribeiro and co.
Understanding exactly how and why this criticality occurs will be an important focus of activity in the years to come.
Ref: arxiv.org/abs/1101.2434: Spike Avalanches Exhibit Universal Dynamics Across The Sleep-Wake Cycle
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