More Than One Way to Build a Thought

Post by Lincoln Tracy 

What's the science?

Have you ever wondered how you understand a sentence that you’ve never read before? Our brains rely on the principle of compositionality, or how we understand the meanings and order of words within a sentence. Even simple sentences require us to understand each word individually, as well as the order the words are presented in. While several studies over the last 20 years have used functional neuroimaging techniques to study how the brain develops strategies to represent the meanings of different parts of a sentence, less is known about how the brain combines individual meanings of words to store the meaning of a complete sentence. This is particularly true for storing the meaning of sentences, such as who did what to whom. This week in Cerebral Cortex, Frankland and Greene use functional magnetic resonance imaging and encoding models to describe how three brain regions contribute to the representation of relational combinations within a sentence.

How did they do it?

First, the authors created a series of 240 simple and unique sentences from a pool of six animal words and eight verbs. Each sentence described one animal interacting with another animal in some way – e.g., “the hawk surprised the moose”. Second, they recruited 55 English speaking individuals (31 females, mean age of 23 years) from the Cambridge (Massachusetts) area. Each participant underwent a functional magnetic resonance imaging scan where they were presented with each of the 240 sentences, one at a time. On a third of the trials, participants were asked a comprehension question after reading the sentence to test whether they understood and were able to remember the sentence. Once all the scans had been completed, the authors looked at the entire brain to identify regions that encode complex and structured semantic information that was similar across sentences. They also examined activity specifically in the left-mid superior temporal cortex and the hippocampus, two other brain regions known to be associated with representing relationships within an event. Specifically, they were interested in determining what kind of information was stored in each of these regions as we try to understand a sentence.

What did they find?

The authors found that information stored in a part of the brain in the anterior-medial prefrontal cortex can be used to interpret new sentences. That is, this section of the prefrontal cortex uses narrow roles that are re-used across sentences – e.g., the same representation is used for the sentence “the cow approached the goose” and “the cow approached the hawk”. Second, they found that the left-mid superior temporal cortex carries more basic information about the unique structure of new sentences — in contrast to the role of the prefrontal cortex. Third, they found that the hippocampus more commonly treated sentences that contained the same parts as dissimilar; another different role compared to the prefrontal cortex. Taken together, these three regions seem to play different roles in encoding how an event within a sentence is composed. However, the roles of all three regions are important and necessary.

What's the impact?

This study identified that while regions within the frontal and temporal lobes of our brain both help us understand a sentence by carrying information about who did what to whom, these regions differ in the level of detail they store. This study also identified that a part of the anterior-medial prefrontal cortex stores information that can be used when trying to understand new sentences. These findings suggest that the medial prefrontal cortex plays an important role in reusing existing knowledge to store information about new combinations of words in sentences, but that the left-mid superior temporal cortex uses a more arbitrary scheme for storing information. Further research is required to understand how the brain adaptively organizes and utilizes these different systems to provide a unified understanding of novel and complex sentences.

Frankland et al. Two Ways to Build A Thought: Distinct Forms of Compositional Semantic Representation Across Brain Regions. Cerebral Cortex (2020). Access the original scientific publication here.