Thoughts
© woseogjeber/iStockphotoThe human brain creates complex thoughts by flexibly combining the meanings of individual words.
Pink elephants serenading the world's tallest woman might be an improbable situation, but our brain is able to comprehend this thought.

Humans can generate an infinite set of ideas from a finite set of words, but how the brain accomplishes this feat remains unclear.

Now, a new study by US scientists suggests the human mind flexibly combines the meanings of individual words to compose structured thoughts.

Writing in the Proceedings of the National Academy of Sciences today, they reveal our brain works like a computer in processing meaning and identify key regions where this takes place.

"Given the vast number of sentences we can understand and produce, it would be implausible for the brain to allocate individual neurons to represent each possible sentence meaning," writes first author graduate student Steven Frankland.

"Instead, it is likely that the brain employs a system for flexibly combining representations of simpler meanings to compose more complex meanings."

Co-author for the study Frankland and Professor Joshua Greene, both in the Department of Psychology at Harvard University, devised two experiments to identify regions in the brain that encode for meaning where the structure of the sentence is critical; and then how the brain represents this meaning.

In the first experiment, 18 participants undergoing a functional MRI scan read simple sentences that could be conveyed in either the active or passive voice.

These sentences could also be arranged to have mirror meanings: "the dog bit the man" or "the man bit the dog".

Through this they were able to identify that the left mid-superior temporal cortex in the brain plays a key role in decoding of sentence meaning and predicting the required response based on this information.

In the second experiment, four nouns -- man, girl, dog, cat -- and five verbs (chased, blocked, approached, bumped and scratched) were used in various combinations to create sentences.

For example the 34 participants would read sentences while undergoing fMRI such as "the dog chased the man" and "the girl was scratched by the cat".

An analysis of the scans from this experiment show one distinct area of the left mid-superior temporal cortex is responsible for processing "Who did it?" (the agent) while a neighbouring separate region encoded for "To whom it was done?" (the patient).

The researchers say the findings support long-held theories that the brain acts like a computer processing data.

"The way computers and computer programming works is there is this distinction between variables and the values of variables," says Greene.

He likened it to using a program to convert Fahrenheit temperature to its Celsius equivalent where the variable is the temperature and it undergoes an algebraic equation for the conversion.

"The idea is you don't have a separate program for every Fahrenheit temperature," he says.

Greene says the nonsensical sentence about pink elephants serenading the world's tallest woman highlights the human brain's capacity.

"You can hear this sentence for the first time and you can understand it," he says.

"What we don't understand is how you get neurons to do that [because] neurons are good at learning over time and building up associations over time.

"The way the brain solves this problem is by having populations of neurons representing the agent (who did it). It is the role that becomes the variable.

"So you have this bit of the cortex representing who is doing something (agent) and next to it is another bit representing who it is being done to (patient)."

Greene says these regions then act like computer data registers with "slots" for the agent and "slots" for the patient values.

"So it is not about the words and the order in which they appear. It is about the underlying meaning."

However Frankland warns against suggestions that the left mid-superior temporal cortex is the "unique locus of complex thought".

"On the contrary, we suspect that the lmSTC is merely part of a distributed neural system responsible for accessing and combining representations housed elsewhere in the cortex," he writes.