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But phylogeny can only tell scientists so much. The team wondered if the differences might be down to the animals’ neurophysiology. But they weren’t sure which aspect of the brain to measure.
In the past, researchers have often used an animal’s total brain volume as a proxy for cognitive ability. Basically, bigger is better. But when Bryer and Koopman extracted the data, they found a weak correlation between brain size and quantitative sensitivity.They turned to a relatively new indicator – cortical neurons density– It tells scientists how many neurons are in the cerebral cortex. (The cortex is the outer layer of mammalian brain tissue involved in complex cognition.)
Closer to home: To quickly count the number of neurons per milligram of brain, the researchers had to liquefy it. (“She calls it ‘brain soup,'” Cantron said of Vanderbilt University neuroscientist Suzana Herculano-Houzel, who developed the method. “It’s literally melting it in a chemical. .”) In this case, the researchers used Herculano’s dataset — Houzel’s lab — to extract published neuronal density data for 12 species. Here, correlations were clear: neuronal density had the greatest impact on quantitative sensitivity of all metrics tested, including traits such as family range size and social group size. Since neuronal density is largely limited by the species’ genes, the team sees this as additional evidence that evolution plays a huge role.
The magic of neuron density is that it has an effect on cognition, but it has nothing to do with brain size. For some mammals, larger brains may have larger neurons and therefore lower densities. But this is by no means a general rule. It’s just its own thing. Smaller neurons, with smaller branches, can be clustered more closely together, giving the brain a finer-grained sense of the world. “Think about the number of pixels in a camera: the more pixels, the higher the resolution,” said Herculano-Houzel, who was not involved in the study.
As the field of cognitive science moves away from old assumptions about evolution, new discoveries are valuable, she said. Historically, scientists have explained interspecies differences in cognition in terms of differences in body size, brain size, or the questionable notion that humans and primates are more evolved than other animals. “There is no way in nature to build a brain and a body around it,” Herculano-Houzel said. “There is no ideal brain. No better one brain. “
Brosnan agrees that the Carnegie Mellon team’s findings disprove the old assumption that primates are cognitively “better” than birds or other vertebrates. “In fact, if you look closely, even within smaller taxa, there is quite a bit of variability,” she said. For example, gorillas were mediocre on this task despite being gorillas. For Brosnan, this suggests a need to study the cognitive abilities of less traditional species, such as reptiles. “What we’ve seen shows they’re really smart,” she said. “We just need to learn more about them.”
Still, humans are the best performers when it comes to estimating numbers. We can do this with about 10% accuracy. Cantlon suspects that neural processes are remarkably similar in all species, but humans can do this with a higher degree of sensitivity. It’s a skill that may lead us to be able to count – and may lead to our symbolic representation of numbers and letters.
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