Speech sounds elicit comparable neural responses and stimulate the same region in the brain of humans, macaques and guinea pigs, according to a study that may help pave the way for better understanding and diagnosis of auditory processing deficits.
A multidisciplinary group of University of Pittsburgh researchers reported in the journal eNeuro that the brain's responses to sound -- called frequency-following responses, or FFRs -- can be recorded from small electrodes placed onto a person's scalp.
They are used by clinicians to quickly assess a child's hearing capacity and flag a slew of potential speech and language disorders, such as dyslexia and autism. But the method has a major drawback - it is lacking specificity.
"These tests can only tell us that something is off - but we don't know what that 'off' is," said Bharath Chandrasekaran, Professor at Pitt's School of Health and Rehabilitation Sciences.
"Understanding the source and mechanism of FFR generation would allow for the development of specific markers of speech impairments, which would be instrumental in improving clinical diagnostics of auditory processing deficits," he added.
The FFRs are also used to identify any issues in auditory processing, or the way that the brain interprets sounds coming from the environment, especially speech. The closer the FFR profile resembles the profile of the sound source, the stronger the brain's auditory processing capacity. In contrast, the more different the two profiles are, the higher the chances of diagnosing an auditory deficit.
Until recently, scientists thought that FFRs arise deep inside the brain stem -- the brain's innermost structures close to the base of the skull -- and ripple outward, eventually reaching the surface of the brain and the scalp.
Pitt researchers proved the long-standing theory wrong as they found that FFRs are generated not only at the brain stem, but also in the auditory cortex of the brain -- the region responsible for the processing of sounds located right around the temple, only a few centimeters away from the surface of the skull -- and that the pattern of FFR generation is similar across mammals.
In response to four different tones of the Mandarin syllable "yi", the brains of English-speaking individuals who aren't familiar with Mandarin Chinese generated similar FFRs as macaque monkeys and guinea pigs, both of which have very similar hearing range and sensitivity to humans.
A better understanding of the way auditory deficits manifest in the brain can fill a critical gap in the development of fast, accurate and non-invasive diagnostics, the researchers said.