Brain applies a set of rules to determine whether combinations of sounds and words used in spoken language are permissible, is the common theory in processing of spoken language by the brain. Now the work of a Massachusetts General Hospital (MGH) investigator and his team supports a different explanation – that the brain decides whether or not a combination is allowable based on words that are already known. The findings may lead to better understanding of how brain processes are disrupted in stroke patients with aphasia and also address theories about the overall operation of the brain. "Our findings have implications for the idea that the brain acts as a computer, which would mean that it uses rules – the equivalent of software commands – to manipulate information. Instead it looks like at least some of the processes that cognitive psychologists and linguists have historically attributed to the application of rules may instead emerge from the association of speech sounds with words we already know," says David Gow, PhD, of the MGH Department of Neurology.
"Recognizing words is tricky – we have different accents and different, individual vocal tracts; so the way individuals pronounce particular words always sounds a little different," he explains. "The fact that listeners almost always get those words right is really bizarre, and figuring out why that happens is an engineering problem. To address that, we borrowed a lot of ideas from other fields and people to create powerful new tools to investigate, not which parts of the brain are activated when we interpret spoken sounds, but how those areas interact."
Human beings speak more than 6,000 distinct language, and each language allows some ways to combine speech sounds into sequences but prohibits others. Although individuals are not usually conscious of these restrictions, native speakers have a strong sense of whether or not a combination is acceptable.
"Most English speakers could accept "doke" as a reasonable English word, but not "lgef," Gow explains. "When we hear a word that does not sound reasonable, we often mishear or repeat it in a way that makes it sound more acceptable. For example, the English language does not permit words that begin with the sounds "sr-," but that combination is allowed in several languages including Russian. As a result, most English speakers pronounce the Sanskrit word 'sri' – as in the name of the island nation Sri Lanka – as 'shri,' a combination of sounds found in English words like shriek and shred."
Gow's method of investigating how the human brain perceives and distinguishes among elements of spoken language combines electroencephalography (EEG), which records electrical brain activity; magnetoencephalograohy (MEG), which the measures subtle magnetic fields produced by brain activity, and magnetic resonance imaging (MRI), which reveals brain structure. Data gathered with those technologies are then analyzed using Granger causality, a method developed to determine cause-and-effect relationships among economic events, along with a Kalman filter, a procedure used to navigate missiles and spacecraft by predicting where something will be in the future. The results are "movies" of brain activity showing not only where and when activity occurs but also how signals move across the brain on a millisecond-by-millisecond level, information no other research team has produced.
In a paper published earlier this year in the online journal PLOS One, Gow and his co-author Conrad Nied, now a PhD candidate at the University of Washington, described their investigation of how the neural processes involved in the interpretation of sound combinations differ depending on whether or not a combination would be permitted in the English language. Their goal was determining which of three potential mechanisms are actually involved in the way humans "repair" nonpermissible sound combinations – the application of rules regarding sound combinations, the frequency with which particular combinations have been encountered, or whether sound combinations occur in known words.
While the results revealed complex patterns of interaction between the measured regions, the areas that had the greatest effect on regions that identify speech sounds were regions involved in the representation of words, not those responsible for rules. "We found that it's the areas of the brain involved in representing the sound of words, not sounds in isolation or abstract rules, that send back the important information. And the interesting thing is that the words you know give you the rules to follow. You want to put sounds together in a way that's easy for you to hear and to figure out what the other person is saying," explains Gow, who is a clinical instructor in Neurology at Harvard Medical School and a professor of Psychology at Salem State University.