How do people born blind represent rainbows, or the color red, that they cannot perceive? How do we all represent abstract concepts, like freedom or justice?
People are capable of conceiving complex ideas and abstract concepts that do not have sensory referents, things to which these concepts point to in the world. The question of how such concepts are represented in the mind has been discussed by philosophers for centuries. In the 18th century, rationalists and empiricists debated about the role of the senses in generating knowledge, and it continues unabated.
A key phenomenon that sheds light on this question is a unique and rare clinical profile, in which some people who suffer from semantic dementia find it difficult to speak about concrete objects and name them but can speak about abstract concepts without difficulty. This means that we store and process these concepts separately in our brains, because they can be separately damaged. But how?
In a new study published in Nature Communications, a team of researchers from Harvard University, Ella Striem-Amit and Alfonso Caramazza and Beijing Normal University, Xiaoying Wang and Yanchao Bi, used the absence of sensory visual experience in people born blind to address this question, and study how abstract semantic knowledge is represented.
The researchers used functional magnetic resonance imaging (fMRI) to scan the brains of people born blind and mapped the areas of their brains which responded to names of concepts they cannot perceive. These included names of colors (red) and weather phenomena that we perceive only though vision (like rainbows). They also included names of places that are too large for a blind person to experience as a whole complex scene at once, truly not being able to see the forest for the trees. Because these concepts are impossible to perceive by the blind, they learn them through language, much like other abstract concepts as “virtue”.
The researchers found that concepts with perceptible referents in the blind and the sighted and abstract concepts activate the same areas in the two groups, in distinct regions for the two types of concepts. Importantly, concepts which do not have perceptible referents in the blind (red, rainbow, forest) selectively activated a different part of the brain from that in the sighted. The brain area that responds to these concepts in the blind is in the anterior temporal lobe, which also responds to abstract concepts in both groups.
Knowledge of concepts gained exclusively through language, the way the blind learn about colors and rainbows, is represented in the same area as abstract concepts. These findings show that the organization of conceptual knowledge in the brain is determined by the type of content that is represented and by whether it is acquired through sensory experience or through language.
The results raise an intriguing question: Does the fact that concepts like rainbow are represented in a different brain area from that in the sighted imply that these concepts are different in the two groups? That’s the hard question.
This project was funded by European Union’s Horizon 2020 Research and Innovation Programme under Marie Sklodowska-Curie Grant Agreement 654837