The primary auditory cortex has been thought to play a role in spoken word recognition since the early 1900s. Still, numerous observations from patients with speech recognition problems, such as stroke patients, did not match well with that concept. Previous research has shown that the visual word form area, a region of the ventral visual stream, contains representations of full written words. Similar experimental evidence has yet to be presented in favor of an auditory lexicon containing spoken word representations.
Using functional magnetic resonance imaging rapid adaptation techniques, Georgetown University Medical Center neuroscientists say the brain’s auditory lexicon, a catalog of verbal language, is located in the front of the primary auditory cortex, not in the back of it- a finding that upends a century-long understanding of this area of the brain.
The Visual Word Form Area (VWFA), an area near the base of the brain’s left hemisphere, houses a lexicon for written words, and it was subsequently shown that newly learned written words are added to the VWFA. The current investigation aimed to determine whether the so-called Auditory Word Form Area (AWFA), which is situated anterior to the primary auditory cortex, has a similar vocabulary for spoken words.
26 volunteers participated in the study, directed by Srikanth Damera, MD, Ph.D., and used three rounds of functional magnetic resonance imaging (fMRI) scans to assess participants’ spoken word processing skills. Functional-MRI rapid adaptation (fMRI-RA), the method utilized in this study, is more sensitive than traditional fMRI in determining the representation of auditory words and the acquisition of new words.
Maximilian Riesenhuber, Ph.D., professor in the Department of Neuroscience at Georgetown University Medical Center and senior author of this study, said, “Since the early 1900s, scientists believed spoken word recognition took place behind the primary auditory cortex, but that model did not fit well with many observations from patients with speech recognition deficits, such as stroke patient. Our discovery of an auditory lexicon more towards the front of the brain provides a new target area to help us understand speech comprehension deficits.”
“In future studies, it will be interesting to investigate how interventions directed at the AWFA affect speech comprehension deficits in populations with different types of strokes or brain injury. We also try to understand how the written and spoken word systems interact. Beyond that, we are using the same techniques to look for auditory lexica in other brain parts, such as those responsible for speech production.”
Josef Rauschecker, Ph.D., DSc, professor in the Department of Neuroscience at Georgetown and co-author of the study, adds that many aspects of how the brain processes words, either written or verbal, remain unexplored.
“We know that when we learn to speak, we rely on our auditory system to tell us whether the sound we’ve produced accurately represents our intended word. We use that feedback to refine future attempts to say the word. However, the brain’s process for this remains poorly understood – both for young children learning to speak for the first time and for older people learning a second language.”
Journal Reference:
- Srikanth R. Damera, Lillian Chang et al. Evidence for a Spoken Word Lexicon in the Auditory Ventral Stream. Neurobiology of Language. DOI: 10.1162/nol_a_00108