New findings by researchers from the University of Maryland and KU Leuven, Belgium, show that the accuracy of tests that measure a person’s comprehension of what they’ve heard, can be significantly improved through the use of some of the methods and EEG technology already used in many hearing tests. Their work also suggests that this approach could potentially be adapted for the development of new “smart” hearing devices that would be able to continuously adapt to the needs of individual users in specific and changing environments.
There is a known difference between hearing someone talk and understanding what they are saying. If an individual is talking from another room, it can be very easy to hear that they are saying something, yet difficult to fully understand what they are saying. This is even true for people who use hearing aids, as they often say the devices allow them to hear, but not always to understand.
This difference is reflected in current testing as well. Hearing tests are well-established and reliable, but those designed to measure understanding are not nearly as reliable. Hearing tests are typically performed in one of two ways. A person puts on headphones and indicates to the test operator whenever they hear a tone. Or a cap made up of a set of electroencephalogram (EEG) electrodes can be placed on the person’s head, which will directly and automatically record brain waves while they listen to the sounds. Hearing tests given to babies shortly after they are born often use this method. Electrodes placed on the baby's head measure whether brain waves occur in response to sound exposure.
The big advantage of electroencephalogram-based hearing testing is that the EEG is an objective measure, and the person undergoing the test does not have to do anything for it to work. In fact, babies often sleep through the test.
Tests used to measure how well speech is understood are more problematic. Typically, the test requires a person to identify speech by repeating back to the tester what they think they have just heard. Because this test requires the person’s participation, it is not always effective. For example, the person’s auditory prosthesis (i.e. a hearing aid or cochlear implant) may not fit correctly or the person may have impaired cognitive function, such as a memory problem. Perhaps they do not fully understand the language being spoken or the person may not be motivated or attentive to the task.
In a new paper published in the Journal of the Association for Research in Otolaryngology, “Speech Intelligibility Predicted from Neural Entrainment of the Speech Envelope,” Professor Tom Francart, KU Leuven, Belgium, Professor Jonathan Simon, University of Maryland Department of Electrical Engineering and colleagues showed EEG-based testing could also be used to measure speech understanding. The new objective method they developed is accurate and does not depend on the person’s active participation.
“This means the test will work regardless of the listener’s state of mind,” Simon explains. “We don’t want a test that would fail just because they stopped paying attention.”
Instead of presenting the person with tonal sounds while they are wearing the electrodes (as in the hearing test), the researchers had them listen to a sample of speech.
“With 64 electrodes, we measure the brain waves when someone listens to a sentence,” says Francart. “We filter out the brainwaves that aren’t linked to the speech sound as much as possible. We compare the remaining signal with the original sentence.”
The way the brain processes speech can be inferred from the correlation between these two signals. If there is sufficient similarity, it means that the person not only has heard something, but also has properly understood the message, Francart says.
The new test promises to be much better at determining speech intelligibility than the current “behaviorally” measured test. It is both automatic and objective, and it can provide more valuable information. This could lead to better diagnoses in patients with speech comprehension issues.
It will be especially useful for people struggling with cognitive issues or those who cannot provide feedback. In addition, the test could help people who have just received a hearing aid and may need it adjusted. Someday, such testing could help to automatically fit hearing aids.
“At the moment, hearing aids ensure that sounds are audible, but not necessarily intelligible,” Francart says. “With built-in electrodes, the device could measure how well the speech is understood. Then it could determine whether adjustments are needed, for example, for something like background noise.”
According to Simon, “Adjustments could be made automatically, based on how successfully the brain is able to turn the processed speech sound into an understandable speech sound.”
This research was funded by the European Research Council (GA 637424), the Research Foundation—Flanders (FWO) and KU Leuven; and by the National Institutes of Health (R01-DC-014085).
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March 27, 2018