The question of the brain's capacity usually brings up remarks that the human brain contains about 100 billion neurons. If each one has, say, 1,000 or more connections to other neurons, this produces some 100 trillion connections in which our memory can be held. These connections are between synapses, which change in strength and size when activated. These changes are a critical part of the memory code. In fact, synaptic strength is analogous to the 1s and 0s that computers use to encode information.
But, here's the thing: unlike the binary code of computers, there are more than two sizes available to synapses. On the basis of the not-very-precise tools researchers had available, they had come up with three sizes: small, medium and large. They also had calculated that the difference between the smallest and largest was a factor of 60.
Here is where the new work comes in, because new techniques have enabled researchers to now see that synapses have far more options open to them. Synapses can, it seems, vary by as little as 8%, creating a possible 26 different sizes available, which corresponds to storing 4.7 bits of information at each synapse, as opposed to one or two.
Despite the precision that this 8% speaks to, hippocampal synapses are notoriously unreliable, with signals typically activating the next neuron only 10-20% of the time. But this seeming unreliability is a feature not a bug. It means a single spike isn't going to do the job; what's needed is a stable change in synaptic strength, which comes from repeated and averaged inputs. Synapses are constantly adjusting, averaging out their success and failure rates over time.
The researchers calculate that, for the smallest synapses, about 1,500 events cause a change in their size/ability (20 minutes), while for the largest synapses, only a couple hundred signaling events (1 to 2 minutes) cause a change. In other words, every 2 to 20 minutes, your synapses are going up or down to the next size, in response to the signals they're receiving.
Based on this new information, the new estimate is that the brain can hold at least a petabyte of information, about as much as the World Wide Web currently holds. This is ten times more than previously estimated.
At the moment, only hippocampal neurons have been investigated. More work is needed to determine whether the same is true across the brain.
In the meantime, the work has given us a better notion of how memories are encoded in the brain, increased the potential capacity of the human brain, and offers a new way of thinking about information networks that may enable engineers to build better, more energy-efficient, computers.
Full text at http://elifesciences.org/content/4/e10778v2
(2016). Nanoconnectomic upper bound on the variability of synaptic plasticity.
eLife. 4, e10778.