What separates human intelligence from that of other animals? According to new research published in the journal Cell, it’s not simply that we have more neurons – it’s how those neurons work.
"In human neurons, there is more electrical compartmentalization," co-author Mark Harnett, assistant professor of Brain and Cognitive Sciences at MIT, told MIT News, "and that allows these units to be a little bit more independent, potentially leading to increased computational capabilities of single neurons.”
It comes down to these things called dendrites – the structures branching out from the neuron that are responsible for conducting information (in the form of electrical signals) from one cell to another. They are sort of like an organic equivalent to the transistors in a computer. Vast networks of neurons are constantly communicating with one another and it is this that controls our thoughts and behavior.
However, as electrical signals move along the dendrites and away from the cell body, they get weaker.
To find out how exactly the length of dendrites affects their electrical properties, researchers compared the electrical activity of those in humans to those in rats using fingernail-sized sections of the anterior temporal lobe, which were taken from epilepsy patients who already had to have it removed during their surgery.
Human dendrites are much longer than rat dendrites because the human cortex is so much thicker. Whereas the cortex comprises just 30 percent of brain volume in rats, it encompasses about 75 percent of human brain volume.
Aside from this (fairly significant) structural difference, the overall organization of the brain is remarkably similar with six layers of neurons. Those in the fifth layer contain dendrites that can extend all the way to the first.
The researchers used a process called patch-clamp electrophysiology to observe how the electrical signals traveled along the dendrites in the samples. (They had been stored in a solution to keep the tissue alive for up to two days.) Because the human dendrites need to be longer in length to reach layer one from layer five, the team found they also transmitted a weaker signal compared to rat dendrites.
They also discovered a difference in the density of ion channels that controls the current flow between the rat and human samples. While the number of ion channels remained the same, the density was lower in the human samples, which Harnett says could help explain some of the differences in electrical activity.
Now, scientists have to work out what effect these differences in electrical activity has on human intelligence, though Harnett suspects they allow individual neurons to perform more complex computations because more regions of the dendrite can influence the strength of incoming signals.