Research Background

Human sensorimotor learning and memory mechanisms have long been a research focus in the field of neuroscience. Most related studies have concentrated on the visuospatial workspace and limb movements, such as arm movements and grasping actions. These studies have helped scientists understand how sensorimotor memories are formed, stored, and retrieved. However, humans also possess a unique ability—language production and perception. Language production involves complex motor control, and auditory feedback plays a crucial role in this process. Unlike limb movements, the language motor system’s workspace is independent of the visuospatial workspace, making the study of sensorimotor memory in the speech-auditory system a unique and important research path.

Despite this, research on how the speech-auditory system stores and retrieves sensorimotor memory remains limited. Specifically, is memory in the speech-auditory system durable? Does the manner of memory acquisition (e.g., sudden or gradual introduction of altered auditory feedback) affect its retention? Is memory retrieval dependent on specific feedback conditions? These questions have yet to be fully answered. To address these issues, a research team from Yale University and McGill University designed a study to explore the durability of sensorimotor memory in the human speech-auditory system, as well as the sensitivity of its acquisition and the dependency of its retrieval, through the manipulation of auditory feedback.

Research Team and Publication Information

This study was conducted by Nishant Rao and David J. Ostry from the Yale Child Study Center at Yale University and the Department of Psychology at McGill University, respectively. The research paper was first published on December 31, 2024, in the Journal of Neurophysiology (J Neurophysiol) and was made available online on February 10, 2025.

Research Design and Methods

Research Procedure

The study was divided into two phases (Visit 1 and Visit 2), with the primary goal of evaluating the durability and retrieval conditions of newly learned speech-auditory memory in participants. The detailed research procedure is as follows:

Phase 1 (Visit 1)

1. Baseline Testing (Baseline Trials)

   • Design: Participants read pseudowords (e.g., “bep,” “dep,” “gep”) displayed on a screen while hearing their own voices through headphones.
     
   • Sample Size: A total of 30 baseline trials were conducted, interspersed with 3 noise feedback trials to evaluate participants’ vocal output in the absence of auditory feedback.
     
   • Data Processing: The first and second formant frequencies (F1 and F2) were extracted from the speech data using Praat software.
     

2. Learning Phase (Learning Trials)

   • Design: Over 210 trials, researchers either gradually or suddenly increased the first formant frequency (F1) of the participants’ heard speech by 30%.
     
   • Sample Size: 72 young adults (21 males, mean age 23.94 years) participated in the experiment.
     
   • Data Processing: The speech signal was processed in real time using Audapter and MATLAB.
     

Phase 2 (Visit 2)

1. Memory Testing (Retention Trials)
   
   • Design: Participants returned to the laboratory after 8 or 24 hours for another 210 trials, with the F1 perturbation maintained at 30%.
     
   • Data Processing: Changes in F1 and F2 were analyzed to assess memory retention.
     

Data Analysis

The study used the Burg algorithm to extract F1 and F2 frequencies from the speech data, and the data were divided into three experimental groups for statistical analysis. Multiple linear regression and analysis of variance (ANOVA) were used to evaluate memory retention under different conditions.

Main Results and Logical Relationships

1. Durability of Memory

   • Results: The study found that newly learned speech-auditory memory was retained at a rate of 70% after both 8 and 24 hours. Memory retention was unaffected by the acquisition method (abrupt or gradual).
     
   • Supporting Data: Upon reintroduction of speech feedback, participants immediately returned to a fully adapted state, indicating that the memory was not lost but dependent on specific feedback conditions.
     

2. Dependency of Memory Retrieval

   • Results: In the absence of auditory feedback, participants appeared to retain no memory of prior learning, but upon reintroduction of speech feedback, they immediately displayed full adaptation.
     
   • Supporting Data: Memory retention in noise feedback trials was significantly lower than in speech feedback trials, indicating that memory retrieval depends on the presence of speech error feedback.
     

3. Changes in Acoustic Space

   • Results: The study found that newly learned speech stabilized in a novel region of the acoustic space, not overlapping with already learned neighboring vowels.
     
   • Supporting Data: Through a control experiment, researchers found that the vowel /ɛ/ underwent significant changes in both direction and magnitude after learning.
     

Conclusion and Significance

This study demonstrates that sensorimotor memory in the human speech-auditory system is highly durable and unaffected by the manner of acquisition. Memory retrieval depends on the presence of speech error feedback, indicating that adaptive learning and habitual speech modes coexist in parallel in sensorimotor memory. Additionally, newly learned speech stabilizes in a novel region of the acoustic space, which may be related to the durability of memory.

These findings not only provide new insights into sensorimotor memory in the speech-auditory system but also open new directions for future research on speech motor learning and its applications in the nervous system. Particularly in rehabilitation studies for patients with neurodegenerative diseases or speech disorders, these findings may have significant implications.

Research Highlights

1. Durability of Memory: The study shows that newly learned speech-auditory memory is retained at a rate of 70% even after 24 hours, filling a gap in this field of research.
   
2. Dependency of Memory Retrieval: The study found that memory retrieval depends on the presence of speech error feedback, a mechanism that contrasts sharply with research on limb motor memory.
   
3. Changes in Acoustic Space: Newly learned speech stabilizes in a novel region of the acoustic space, providing a new perspective for understanding the adaptability of speech movements