Overview

Why do I care about the neural processes that support speech comprehension? There are several answers to that question. One is that I am both curious and a scientist, and I want to know how this fascinating organ can accomplish such a complex task biologically. Second, there is only so much we can know from observing behavior: it is well established that organisms can generate similar-looking behaviors using very different neural mechanisms. This is also true for speech comprehension, and having an understanding of these different underlying mechanisms is important for verifying theories and generating new predictions. Of course, difficulties with speech comprehension in school, or due to brain damage or disease, are extremely frustrating. The better we can understand how speech comprehension works in the healthy brain, the more hope there is for intervention or rehabilitation when it's needed.

The neural basis of speech comprehension across the lifespan

In order to carry on a simple conversation, our brains integrate a wealth of information that is anything but simple, including speech sounds, lip movements, social cues, and linguistic context. My research focuses primarily on how our brains make sense of acoustic information speech comprehension. These acoustic signals must be processed rapidly while new information continues to arrive. Although there are certainly times when we may need to carry out complicated decisions about what we hear, I focus as much as possible on natural speech understanding—that is, the type of automatic processing that occurs during everyday conversation.

One of the most interesting areas of my research concerns the degree to which individual differences in hearing or brain structure affect the types of brain networks required to understand speech. In normal aging, for example, both young and older adults rely on a core network of brain regions to process grammatically-complex spoken sentences. However, older adults show additional activity in regions outside this core network. This suggests that, as we get older, our brains are accomplishing the same task (understanding speech) in a different way, making use of a different set of cognitive (and neural) resources. This example from normal aging illustrates the broader point that our brains, and the way in which they process information, are not identical. These differences need to be considered when characterizing the neural processes that support a task.

Hearing and speech

Hearing ability is another factor which varies from person to person. As might be obvious, because speech processing typically relies on acoustic information, when this information is degraded our brains have a bit more work to do. In a recent study, we measured hearing ability in a group of adults over the age of 60, using pure tone thresholds (remember putting on headphones and raising your hand when you hear a beep? That's the one). All of the participants reported normal hearing before coming in, and the majority of them had hearing thresholds that would be considered clinically normal for their ages. Somewhat surprisingly, we found that individual hearing ability—even within these normal ranges—predicted both brain activity in auditory cortex, and gray matter volume in auditory cortex. Although there are many interesting follow-up studies to do, this research suggests that even small differences in hearing ability can impact how our brains process speech. Another good reason to protect our hearing as much as possible!

Neural plasticity

The brain is not a static organ, but one that is constantly adapting itself to our experiences. I am interest in how the brain changes to incorporate to new sources of information in the context of speech comprehension, which I have studied through perceptual learning.

Perceptual learning in the laboratory is simply an extension of the types of adjustment we do in everyday situations. For example, when listening to a lecture given by someone in an unfamiliar accent, over the course of the talk we will adjust to their speaking and it will become easier to understand. This occurs as our brain quickly learns how to map these new sounds (for example, different vowel pronounciation) onto our existing representations. With longer periods of experience—such as might happen if you move to a foreign country—this learning can be even stronger.

I study perceptual learning using speech that has been digitally manipulated in a way that makes it difficult to understand. For example, time-compressed speech involves shortening the duration of a sentence without changing the pitch. Most listeners are able to easily comprehend speech that has been shortened to 80% of its original duration, but by the time it is 50% (that is, twice as fast as usual), things start to get much more difficult. What is remarkable is that over the course of only 20 sentences or so, listeners are able to show significant improvement in their understanding. Better understanding how the brain rapidly accomplishes this adjustment will set the stage for designing, targeting, and evaluating therapeutic interventions when speech comprehension has been compromised by stroke or neurodegenerative disease.