Study Reveals Complex Map of Brain’s ‘Dictionary’
Scientists exploring the human brain’s exotic terrain of soft furrows and corrugated folds said Wednesday that they have discovered where the brain handles distinct categories of words and the concepts they describe.
The finding by researchers at the University of Iowa College of Medicine and the Salk Institute for Biological Studies in La Jolla offers a provocative new insight into how the brain retrieves words to describe the world around it. The brain appears to use a kind of interactive mental thesaurus dispersed in many separate parts of the left cerebral hemisphere.
Harvard University neuropsychologist Alfonso Caramazza called the work, published today in the journal Nature, a “tremendous step forward.” UCLA linguistics expert Victoria Fromkin called it “extremely valuable.”
It is the latest in a cascade of revelations about the physical foundations of mental awareness that reveal the brain to be a more complex and adaptable creation than previously believed.
In other research made public Wednesday, scientists at the National Institute of Neurological Disorders and Stroke offered a dramatic glimpse into how the human brain can reclaim its unused sensory circuits. Their study revealed how those born blind are able to “see” Braille with their fingers through the same part of the brain--called the visual cortex--used by sighted people for normal vision.
Both teams of researchers used sophisticated imaging techniques that allow them to penetrate the veil of the skull to map the labyrinth of a normal human brain at work, capturing pictures of the biochemical shadows cast by the human thought process. In addition, the Iowa group, led by Hanna Damasio and Antonio R. Damasio, also confirmed its findings about language in a more traditional clinical study of patients whose ability to retrieve nouns was impaired by brain lesions.
The Damasio study was aimed at understanding what areas of the brain become active when someone tries to find and then say the name of a given object.
To explore what happens when the brain retrieves certain kinds of nouns, the researchers studied nine healthy adults and 127 patients with brain lesions. Both groups were asked to identify 327 items, such as photos of well-known people, animals or tools, by giving the most specific word they could think of to name the object before them.
The team quickly discovered that the parts of the brain that became active when people tried to form words about those things were not in the regions long believed to be central to language processing. Even more surprising, the areas were widely scattered.
At the same time, it appeared that additional networks in the brain were activated to help locate and retrieve the different kinds of information that add up to the meaning, construction and pronunciation of a noun.
The findings constitute physical evidence of what experts say is a remarkably complex neural system that underlies even the most common everyday speech.
The researchers said the interactive networks “do not contain in explicit form the names for all persons, animals or tools,” the group reported. “We suggest instead they hold the knowledge of how to reconstruct a certain pattern,” such as the smallest phonetic units in a language capable of conveying a distinction in meaning, such as the “m” of “mat” and the “b” of “bat” in English.
“It is not true that you have a central dictionary in the brain,” Antonio Damasio said.
“Depending on the kind of concept you are dealing with, what you have in the brain is a dictionary access arrangement. The thing that is even more intriguing is that when you come up with words that correspond to different concepts, the parts of the brain that are involved turn out to be different themselves.
“What we are suggesting is that this is extremely dynamic and linked to each individual’s experience,” he said. “Our knowledge is built on bits and pieces of many aspects of a given thing--shape, color, movement, taste--those things are not going to be laid down in one single place,” he said.
Caramazza, who works at Harvard with patients whose ability to process language is impaired, said the new study “confirms what many of us have believed: That many parts of the brain are involved in language processing. But they have shown that it is important also to deal with abstract theoretical concepts--the notion that there is lexical representation that is neither the sound or the meaning of a word.
“Damasio’s group is proposing that the brain’s dictionary is organized categorically,” he said. “That is a completely novel proposal.”
Mary Louise Kean, an expert on cognitive science at UC Irvine, called the study plausible and, in some respects, surprising.
Elizabeth Bates, who directs the Center for Research in Language at UC San Diego, said she agreed with the study’s fundamental conclusions, but emphasized that the images of the brain activity should be interpreted cautiously.
“Whenever you are looking at results of a study of some aspects of language processing, you can never be sure you are taking a picture of how the brain does this particular task--naming a picture or matching a word--or a picture of your knowledge of that word.
“What a PET scan study tells you is what areas of the brain are most active during a particular task. It does not necessarily tell you where you keep the knowledge associated with that task,” she said.
“It would be very dangerous to assume those are the same thing.
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A Mental Thesaurus
When you are struggling for just the right word, what is going on in your brain? Researchers suggest that your mind is rummaging through a kind of mental thesaurus dispersed throughout the brain’s left temporal region. Separate areas process words for distinct categories such as familiar faces, tools or animals.
BRAIN’S LEFT HEMISPHERE
Brain systems for language include neural structures to handle aspects of words, sentences and grammar.
What’s in a word?
To construct the word for an object, the brain may reassemble a collage of impressions stored in different brain regions that add up to the right noun. The concept of a cup of coffee, for example, may be the sum of taste, smell, color, touch and other sensory memories.
Source: Scientific American, Iowa University College of Medicine
