Summary: Researchers identified the brain’s verbal language catalog, or auditory lexicon, in an unexpected location: the front of the primary auditory cortex, not its back.
This discovery defies a century-long assumption about brain organization and could significantly influence strategies for recovery and rehabilitation post-brain injury. Involving 26 volunteers and functional magnetic resonance imaging, the study investigates the Auditory Word Form Area’s role in spoken word processing.
The findings open new pathways to understand and potentially remediate speech comprehension deficits, especially following strokes or brain injuries.
The auditory lexicon, cataloging verbal language, has been located in the front of the primary auditory cortex, contrary to long-held beliefs.
The study used a functional-MRI rapid adaptation technique, more sensitive than conventional fMRI, to analyze spoken word processing and new word learning.
The findings could affect rehabilitation strategies following brain injuries and contribute to a better understanding of speech comprehension deficits.
Source: Georgetown University Medical Center
Georgetown University Medical Center neuroscientists say the brain’s auditory lexicon, a catalog of verbal language, is actually located in the front of the primary auditory cortex, not in back of it — a finding that upends a century-long understanding of this area of the brain.
The new understanding matters because it may impact recovery and rehabilitation following a brain injury such as a stroke.
The findings appear in Neurobiology of Language on July 5, 2023.
Riesenhuber’s lab showed the existence of a lexicon for written words at the base of the brain’s left hemisphere in a region known as the Visual Word Form Area (VWFA) and subsequently determined that newly learned written words are added to the VWFA.
The present study sought to test whether a similar lexicon existed for spoken words in the so-called Auditory Word Form Area (AWFA), located anterior to primary auditory cortex.
“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 patients,” says Maximilian Riesenhuber, PhD, professor in the Department of Neuroscience at Georgetown University Medical Center and senior author of this study.
“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 the study, led by Srikanth Damera, MD, PhD, 26 volunteers went through three rounds of functional magnetic resonance imaging (fMRI) scans to examine their spoken word processing abilities.
The technique used in this study was called functional-MRI rapid adaptation (fMRI-RA), which is more sensitive than conventional fMRI in assessing representation of auditory words as well as the learning of new words.
“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,” says Riesenhuber.
“We are also trying 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 parts of the brain, such as those responsible for speech production.”
Josef Rauschecker, PhD, 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,” he said.
“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, but also for older people learning a second language.”
In addition to Riesenhuber , Damera and Rauschecker, the other authors at Georgetown University are Lillian Chang, Plamen Nikolov, James Mattei, Suneel Banerjee, Patrick H. Cox and Xiong Jiang. Laurie S. Glezer is at San Diego State University, CA.
Funding: This work was supported by funds from the National Science Foundation grant BCS-1756313. Additional support was provided by NIH grant 1S10OD023561. This work also used resources provided through the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant ACI-1548562.
About this linguistics and neuroscience research news
Evidence for a Spoken Word Lexicon in the Auditory Ventral Stream
The existence of a neural representation for whole words (i.e., a lexicon) is a common feature of many models of speech processing.
Prior studies have provided evidence for a visual lexicon containing representations of whole written words in an area of the ventral visual stream known as the visual word form area.
Similar experimental support for an auditory lexicon containing representations of spoken words has yet to be shown.
Using functional magnetic resonance imageing rapid adaptation techniques, we provide evidence for an auditory lexicon in the auditory word form area in the human left anterior superior temporal gyrus that contains representations highly selective for individual spoken words.
Furthermore, we show that familiarization with novel auditory words sharpens the selectivity of their representations in the auditory word form area.
These findings reveal strong parallels in how the brain represents written and spoken words, showing convergent processing strategies across modalities in the visual and auditory ventral streams.