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Infants use their prefrontal cortex to learn

Researchers have always thought the prefrontal cortex (PFC) — the brain region involved in some of the highest forms of cognition and reasoning — was too underdeveloped in young children, especially infants, to participate in complex cognitive tasks. A new study suggests otherwise.

Given the task of learning simple hierarchical rules, babies appear to employ much the same circuitry as adults performing a similar task.

Researchers tracked infant brain activity by means of a Near Infrared Spectroscopy (NIRS) machine provided, along with technical support, by TechEn, Inc. NIRS safely records brain activity over the scalp and is becoming important to infant research, says study senior author Dima Amso, Associate Professor, Developmental Cognitive Neuroscience Laboratory (CLPS), Brown University. Babies wear a special headband that holds near-infrared sensors over the scalp area of interest. The sensors detect how much infrared light is absorbed by hemoglobin in the blood. The greatest blood flow reflects where brain activity is greatest.

Researchers also tracked each baby's eye blinks as recent research has found eye blinks reflect the degree of involvement of the neurotransmitter dopamine.

The findings suggest that even at the age of 8 months, a baby's prefrontal cortex (PFC) appears properly adapted to tasks important to an infant of that age, says Amso. The work is published in the Journal of Neuroscience.

"The wow factor isn't 'Look the PFC works,'" Amso said. "It's that its function is a really good fit for what these babies need to be mastering at that moment in their development."

Of course babies aren't yet equipped for writing essays or planning errands, Amso admits, but their brains are properly adapted for learning essential elements in their world and how best to organize them. The PFC is not offline. Instead it's appropriately mature for the goals of babyhood.

To make this discovery, Amso, lead author Denise Werchan PhD, fellow CLPS professor Michael Frank PhD and postdoctoral researcher Anne Collins PhD - who is now assistant professor at the University of California at Berkeley, devised a task initially developed by Collins and Frank to test PFC function in adults.

The infant version of a PFC function test was designed to parallel the circumstance of growing up
in a bilingual family.

Maybe Mom and her side of the family speak English, while Dad and his family speak Spanish.

The babies must learn that different groups of people use different words for the same things.

To scientists, this association of some people with one language and other people with another is an example of a "hierarchical rule set." The person speaking is at the higher-level that determines what language will be used. Babies must learn that Mom and her brother will say "cat" when Dad and his sister will say "gato" to refer to the same family pet.

The team wanted to determine how baby brains handle this task. They recruited 37 babies to learn a simple, abstracted version of the bilingual scenario while their brain activity and behavior were gently monitored.

On screens before them, babies were shown a face and then an image of a toy. At the same time they'd hear a particular nonsense word in a voice associated with the face, as if the depicted person - call her "person 1" - was calling the depicted toy by that word. Then they'd see a different face with a different associated voice call the same toy by a new word (i.e. as if "person 2" was speaking a different language). Over several rounds, switching back and forth, the babies were exposed to these associations of person 1 with one vocabulary and person 2 with a distinct vocabulary.

After that phase, babies were introduced to person 3 on the screen who used the same words as person 1, but also introduced a few new ones (in the bilingual family metaphor, think of person 3 as Dad's sister, if person 1 is Dad). If the babies were learning the rules, they'd associate person 3's new words with person 1, because they were speaking the same rule set or "language."

Researchers tested whether learning was occurring by next presenting the babies with persons 1 and 2 saying some of the new vocabulary of person 3. Babies who'd been learning should react differently to each instance. They should look longer at the unexpected case of person 2 using a word from the vocabulary of person 3.

In fact, that is what the babies did. On average they gazed a couple of seconds longer at the surprising situation of person 2 using an inconsistent language, than at the expected situation of person 1 speaking like person 3.

Frank and Collins have combined experimental data with computer models of brain function to suggest that when adults learn hierarchical rules, the key circuit involved is a connection between the PFC and another brain region called the striatum. That connection is mediated and reinforced by dopamine.

The results of the infrared recording and eye blink tracking both supported the hypothesis that as babies were learning they actively employed the PFC, similarly to adults. Both PFC activity - specifically in the right dorsolateral PFC - and eye blinks were significantly elevated when babies were asked to switch from one "language" to another, which is the most cognitively demanding moment for the PFC during this task.

"Once you learn these hierarchical structures, each time you need to access or use one of them you need to update the structure into working memory," Werchan adds. "When the task switches you need to update information into PFC."

Moreover, the degree of the babies' elevated PFC and eye blink activity predicted how distinctly they responded to the unexpected situation of a person speaking inconsistently with their language - a measure of how well the babies learned the rule structures.

Amso believes the findings suggest early neuro-development should be viewed differently than before. Rather than regarding young brains as immature and less functional, a better perspective may be to regard them as constantly adapting to meet key challenges they face. When healthy, the infant brain is as sophisticated as it needs to be.

Amso: "Atypical development, then, might reflect an inability to adapt to an environmental challenge, or an earlier adaptation because of a negative environment. We and others are probing these ideas as relevant to PFC development."

Recent research indicates that adults and infants spontaneously create and generalize hierarchical rule sets during incidental learning. Computational models and empirical data suggest that, in adults, this process is supported by circuits linking prefrontal cortex (PFC) with striatum and their modulation by dopamine, but the neural circuits supporting this form of learning in infants are largely unknown. We used near-infrared spectroscopy to record PFC activity in 8-month-old human infants during a simple audiovisual hierarchical-rule-learning task. Behavioral results confirmed that infants adopted hierarchical rule sets to learn and generalize spoken object–label mappings across different speaker contexts. Infants had increased activity over right dorsal lateral PFC when rule sets switched from one trial to the next, a neural marker related to updating rule sets into working memory in the adult literature. Infants' eye blink rate, a possible physiological correlate of striatal dopamine activity, also increased when rule sets switched from one trial to the next. Moreover, the increase in right dorsolateral PFC activity in conjunction with eye blink rate also predicted infants' generalization ability, providing exploratory evidence for frontostriatal involvement during learning. These findings provide evidence that PFC is involved in rudimentary hierarchical rule learning in 8-month-old infants, an ability that was previously thought to emerge later in life in concert with PFC maturation.

Hierarchical rule learning is a powerful learning mechanism that allows rules to be selected in a context-appropriate fashion and transferred or reused in novel contexts. Data from computational models and adults suggests that this learning mechanism is supported by dopamine-innervated interactions between prefrontal cortex (PFC) and striatum. Here, we provide evidence that PFC also supports hierarchical rule learning during infancy, challenging the current dogma that PFC is an underdeveloped brain system until adolescence. These results add new insights into the neurobiological mechanisms available to support learning and generalization in very early postnatal life, providing evidence that PFC and the frontostriatal circuitry are involved in organizing learning and behavior earlier in life than previously known.

Denise M. Werchan1, Anne G.E. Collins2, Michael J. Frank1, and Dima Amso1

1. Department of Cognitive, Linguistic, and Psychological Sciences, Brown University, Providence, Rhode Island , USA
2. Department of Psychology, University of California, Berkeley, California, USA

The National Science Foundation (grant: DGE-1058262) and the National Institutes of Health (Grant: R01-MH099078) funded the research
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Oct 13, 2016   Fetal Timeline   Maternal Timeline   News   News Archive   

To establish hierarchical rules for two languages, babies use similar brain circuits as adults.
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