Why are our brains better than computers?

“1 petabyte!” Sound familiar? Believe it or not, that’s our brain’s storage capacity. The number represents a “real bombshell in the field of neuroscience," according to Terry Sejnowski, principal co-author of the study published in eLife. Indeed, the figure is 10 times greater than previous estimates. In practical terms, it's 1,000 times more than most current hard drives. And despite its capacity, our brain requires less energy than a computer. But how is our brain able to function on full power with so little energy?

Measuring the amount of information our brain can store seems like a difficult task. But a team from the Salk Institute in San Diego has managed to do just that. By analyzing the brain tissue of rats, the team was able to create a 3D reproduction of a tiny fraction of the hippocampus, an area of the brain involved in memory. Our memories and thoughts are the result of electrical and chemical activity patterns in our brain. The key event in this activity occurs when the axon (the head) of the neuron sends information to the dendrite (the tail) of another neuron, which receives it on one of its many dendritic spines. It is the dendritic spines that will, through this area of contact (the synapse), allow the transmission of a nervous (electrical) signal via neurotransmitters. Each neuron can have thousands of synapses with thousands of other neurons; the strength of each synapse also varies.

Using their 3D micro-volume (a cube of 6 microns square), Terry Sejnowski and his team were able to study the synapses in great detail, and were able to observe an unusual activity in 10% of the synaptic connections: an axon can be connected twice or more to the same dendrite. Simply put, this means that a neuron can send several messages to a second neuron. But that’s only the first finding of this research… Tom Bartol, one of the members of the team, used an advanced microscope and sophisticated algorithms to optimize the reconstruction of the neural connections at a nanomolecular level in order to see if the synapses of a single neuron varied in size.

Synaptic strength can be estimated based on its volume: the quantity of neurotransmitters, and the surface area. The volume can vary by a factor of 60, depending on the quantity of information that is transmitted. This is what scientist refer to as synaptic strength, which can be converted into bytes (the same unit used to measure memory in computers). Normally, synapses are classified into 3 categories: small, medium, and large. But the authors of this study observed 26 sizes! Thus, information storage is much more efficient than previously thought. In comparison to computers, 26 synapse sizes are roughly equivalent to 4.7 bytes of information. Previously, scientists estimated only 1 to 2 bytes of information were contained in each synapse. And that’s how we come to this famous figure of 1 petabyte for the entire brain!

The researchers were also able to demonstrate the variability and flexibility of synaptic strengths. Every 2 to 20 minutes, our synapses increase or decrease in size according to the signals they receive, allowing them to transport more information and increase memory. About 1,500 transmissions cause the volume to be reduced in 20 minutes and only a couple hundred signals lead to an increase in volume in 2 minutes. This brain capacity, coupled with the fact that, during adulthood, the brain generates only 20 Watts of continuous output (about the equivalent of a low energy lightbulb), explains why it can store so much precise information while consuming little energy.

After the “green” brain, when can we expect to see a low-energy computer?
Source: Thomas M Bartol Jr, Cailey Bromer, Justin Kinney, Michael A Chirillo, Jennifer NBourne, Kristen M Harris, Terrence J Sejnowski. Nanoconnectomic upper bound on the variability of synaptic plasticity. In eLife, nov. 2015.

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