Saturday, February 4, 2017

Brain: Is Size Everything?

Properties other than size also make human brains different.

There is no doubt that humans experience a unique sense of consciousness and form of cognition. While other animals like chimpanzees, dolphins, dogs, elephants, and crows are amazingly smart in general and have highly developed capabilities of their own, they don't touch our abilities for planning, remembering, focusing, and at the top of the list- language, both spoken and thought. It remains hard to characterize these differences, because we are still learning so much about the cognition of other animals, and because even with introspection, language, science, and the rest of our inquisitive armamentarium, our own mental processes remain opaque to a large degree.

However, there are clear differences, and how can they be expained? We turn out not to be the only animals with large brains. Sperm whales have brains about nine times as heavy as ours. Even some dolpins have bigger brains than we do. Monkeys typically have smaller brains than cows, despite much higher intelligence. Mice have brains that are, per body weight, almost ten times as large as ours. So size is not, by itself, the issue. There is more going on.

Biggest does not equal smartest. At least that is what we think.

On the other hand, over the last few million years, size has clearly meant something, as our brains have grown at a very rapid clip. So it appears that within a given lineage, size increases may serve as the easiest way to change cognitive capacity, and can serve as a proxy for intelligence. But it is far more hazardous to make comparisons between different lineages, since their architectures and thus capabilities may be very different. So who knows- maybe octopuses are smarter than we are, despite having smaller brains, and several of them.

But bigger is better, among our closer relatives.

For example, thanks to our particular architecture, we pack more neurons into a gram of brain than do whales, so we end up with as many or more brain cells as elephants and whales. And what is more, our brain being more compact gives those neurons a distinct advantage in speed / connectivity. There has been a good deal of work on the genetic level to look for genes and other genetic features that show accelerated evolution in the human lineage- quantitative work that can show with high confidence that some gene variation or regulatory site is novel and significant in humans. But linking that data to the human phenotype has been a challenge, as is true generally with human genetics. The best route has typically been to find other variations in the same area that lead to disease or other pathology, which can give strong clues about the overall function. Or providing mice with the human version of the gene, though the chances of seeing something informative, let alone amazing(!), by this route are rather slim.

Getting back to brain function, a recent paper discussed new work in the field, particularly on the properties of neurons themselves, which might help explain some of our mental distinctiveness. This was all done on brains from recently-living humans, which are understandably hard to get and hard to work with, in a brain slice+electrodes system. One finding is that we have a unique class of "super-neurons"- cells which fire so strongly that a single one can set off responses to the next neuron and thus to larger cortical circuits. This is not seen in other species (per their claim) and is unusual because in typical brain tissue / circuits, it takes converging firings from several or many upstream cells to bump a neuron into action- which is, after all, the whole point of information integration.

The efficiency possibilities are clear. If a percept can happen from the firing of a single famous face cell, (though these are likely to be part of a neural network, rather than regimented as one cell per face), then we need fewer of them to carry memories. The cells and synapses discussed here actually target inhibitory neurons, but the logic remains the same- that if single cells can control large-scale network activities, you need fewer of them, though their tuning and activity then are of paramount importance.

Two neurons meet... The intensely spiking pyramidal neuron (red) which firs first, and the post-synaptic, receiving cell  (blue) are portrayed (C) as they were stained and micrographed in the tissue. The layers of the cortical sheet are given roman numerals. Synapses between them are numbered in D. Panel B shows an averaged stimulus -> response graph of the two cells, showing that the receiving basket cell (bc) quite frequently fires (74% of the time) when the sending cell fires, despite their very sparse synapses. The lower graph (26%) shows the other events, when pyramidal cell firing evokes only a grudging sigh in the receiving cell. In other species, this is all one would see in such single-cell stimulus / encounters. Panel A shows that the receiving cell not only fires once, but several times per upstream spike.
"Although the ratio of triggering poly- versus monosynaptic postsynaptic potentials was 0.01 in the rat and 1.73 in the human in our hands, it should be emphasized that the human patients were treated differently during anesthesia and surgery, and the excitability of human neurons might be different in the external solution also used for rat experiments."
"However, the human neocortical neurons also exhibit specializations only reported in our species. One such feature is the capacity of excitatory principal cells to elicit firing in local inhibitory interneurons with a single action potential via very strong excitatory synapses. It has been suggested that this feature has specifically evolved to enhance coordinated firing of neuronal ensembles in higher brain functions."

Additionally, learning happens very quickly among these super-neurons, so that they do not regularly overwhelm their targets. After ten minutes of stimulation, the downstream cell had already learned to ignore the stimulus. So while most processing takes place in the usual integrative network pathways to come up with usefully transformed information, there seem to be cases when directness and efficiency rose in importance, in the human lineage, and thus led to the development of these super-neurons. This kind of study adds a cell-biology level to the much-better characterized, but as yet tenuously connected phenotypic and genetic levels of differences that make humans distinctive from their ancestors and fellow-beings on the planet.

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