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Chimpanzees, studies show, can be curious. But unlike children as young as three-to-five years of age, they do not seek explanations for causes of phenomena. Put simply, they do not ask Why? Small children, on the other hand, seem to understand quite early that effects are inextricably linked to causes, and their curiosity assigns value to competing tasks, based on the potential of those tasks to maximize learning. In other words, curiosity is an effective engine of discovery.

Recent cognitive and neuroimaging studies support a view, however, that what we call “curiosity" may actually encompass a family of different psychological states that are powered by distinct circuits in the brain. For instance, the curiosity that is triggered by surprising or ambiguous stimuli (perceptual curiosity, in the language of psychologist Daniel Berlyne) is associated with an aversive feeling of deprivation. That is, when we encounter phenomena that appear to be incompatible with our previous knowledge or when we feel that there is a gap created by uncertainty, we are driven to seek new insights that will reduce the unpleasant sensation. This information-gap scenario had in fact been suggested in the early 1990s as a general theory of curiosity by economist and psychologist George Loewenstein.

On the other hand, we usually experience the type of curiosity that drives all scientific research, artistic endeavors, and exploratory behavior (the true love for knowledge, referred to by Berlyne as epistemic curiosity), as a pleasurable state, one in which we anticipate reward.

Two seminal experiments used functional magnetic resonance iImaging (fMRI) to examine the neural underpinnings of curiosity—which parts of the brain are activated when perceptual or epistemic curiosity are induced and relieved. In one, led by neuroscientist Marieke Jepma and her collaborators, perceptual curiosity was triggered through ambiguous visual input: The researchers showed participants blurry images of common objects. In the second, researchers Min Jeong Kang, Colin Camerer,  and their colleagues generated epistemic curiosity by asking their subjects trivia questions.

In Jepma’s study, the researchers found that perceptual curiosity activated brain regions that are sensitive to conflict and unpleasant arousal (the anterior insula and the anterior cingulate cortex). In Kang’s project, which involved epistemic curiosity, on the other hand, the brain areas that were significantly energized (the left caudate and the lateral prefrontal cortex) were those known to be associated with anticipation of reward. The relief of perceptual curiosity was found to activate the reward circuitry, and in both experiments the relief was also accompanied by enhanced incidental memory.

These experiments (combined with supporting results from cognitive studies) suggest that perceptual curiosity is primarily associated with an unpleasant condition; it resembles scratching an intellectual itch. In contrast, epistemic curiosity, the drive for the acquisition of knowledge, provides an intrinsic motivation, an activity undertaken for its own sake, and it is pleasurable. In fact, I would go so far as to argue that had we known these facts about perceptual and epistemic curiosity from the start, we might have chosen different words to describe the two psychological states, rather than using the same word—curiosity—for both.

In a related experiment, neuroscientists Matthias Gruber, Bernard Gelman, and Charan Ranganath discovered that the activation of the brain in the case of epistemic curiosity precisely followed the pathways that transmit dopamine signals. Since dopamine is a neurotransmitter that plays a pivotal role in the brain’s reward circuit, the findings confirmed that epistemic curiosity taps into the reward system. These and other experiments also revealed that the satisfaction of curiosity (of any type) enhances memory and learning (even of incidental information). As Gruber put it: “Curiosity may put the brain in a state that allows it to learn and return any kind of information, like a vortex that sucks in what you are motivated to learn and also everything around it.”

What is it, however, that gives humans this ability to turn every phenomenon into meaningful Why and How questions? After all, it is curiosity (of all types) and the desire to get to the bottom of things that gave birth to the early spiritual quests and to scientific exploration. Similarly, the associated aptitude to describe and articulate both what exists in the real world and abstract constructs in the world of our imagination has spawned the arts and literature. These unique capabilities are probably a consequence of the number of neurons in our brains, in particular in the cerebral cortex and the striatum.

You may find it surprising to hear that until about a decade ago the number of neurons in the brain was not known with a high degree of precision. This situation changed when Brazilian researcher Suzana Herculano-Houzel and her team introduced a new method for counting neurons. They basically dissolved brains into a homogeneous soup, which allowed them to count neurons in a sample of the liquid. They found that an average human brain contains about 86 billion neurons, compared to an orangutan, who has only about a third of this number, and a rat, which has only 189 million.

Why do humans have more neurons in their cerebral cortex than any other animal? One part of the answer is known: Because we are primates. It turns out that not all brains are built by the same scaling rules. Primates manage to cram more neurons into smaller masses. In primates, a brain that is ten times heavier also has ten times more neurons. In rodents, by comparison, a ten times larger number of neurons requires a fifty times larger mass! This more efficient packing has given humans their first advantage, at least over nonprimate species. That still does not explain why humans have more neurons than, say, gorilla. Nor does it elucidate why the number of neurons in humans doubled in the million and a half years or so that separate Homo habilis (the “handy man”) and Homo sapiens.

Most researchers agree that, to answer these questions, energy considerations have to come into the picture. The operation of the human brain is very energetically expensive; it consumes about 20-25 percent of the energy budget of the entire body, as opposed to at most about 10 percent for other species. This is entirely due to the large number of neurons.

Basically, researchers have shown that even if primates spend the maximum time foraging for food that their physiology allows (nine to ten hours a day), they cannot afford both a large body and a large brain. A primate weighing 165 pounds can  support only about 30 billion neurons. Archaic humans therefore had to find ways to significantly enhance the effectiveness of their caloric intake.

Opinions vary as to the relative merits of a number of potential energy-saving adaptations and on the precise order in which those might have occurred, but several converging factors probably enforced each other. They probably included: a diet rich in meat; shortened guts, which saved digestive energy; cooking, which reduced time spent own chewing; and upright walking, which required less energy than moving on feet and knuckles.

Curiosity probably played a crucial role in this mix, through a feedback loop. In other words, by being curious about new kinds of food, about creating new tools, and about the use of fire for cooking, humans probably enhanced their own ability to be curious.

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