Abstract
The human brain exhibits the remarkable ability to categorize speech sounds into distinct, meaningful percepts, even in challenging tasks like learning non-native speech categories in adulthood and hearing speech in noisy listening conditions. In these scenarios, there is substantial variability in perception and behavior, both across individual listeners and individual trials. While there has been extensive work characterizing stimulus-related and contextual factors that contribute to variability, recent advances in neuroscience are beginning to shed light on another potential source of variability that has not been explored in speech processing. Specifically, there are task-independent, moment-to-moment variations in neural activity in broadly-distributed cortical and subcortical networks that affect how a stimulus is perceived on a trial-by-trial basis. In this review, we discuss factors that affect speech sound learning and moment-to-moment variability in perception, particularly arousal states—neurotransmitter-dependent modulations of cortical activity. We propose that a more complete model of speech perception and learning should incorporate subcortically-mediated arousal states that alter behavior in ways that are distinct from, yet complementary to, top-down cognitive modulations. Finally, we discuss a novel neuromodulation technique, transcutaneous auricular vagus nerve stimulation (taVNS), which is particularly well-suited to investigating causal relationships between arousal mechanisms and performance in a variety of perceptual tasks. Together, these approaches provide novel testable hypotheses for explaining variability in classically challenging tasks, including non-native speech sound learning.
Highlights
The ability to perceive speech, especially under challenging conditions, reflects a remarkable set of computational processes that the human brain is well-adapted to perform.A major challenge that both expert and novice listeners face when learning a new language is substantial variability in input across different speakers, contexts, and listening environments
We propose that transcutaneous auricular vagus nerve stimulation (taVNS) holds promise as both a scientific and translational tool for understanding and manipulating arousal states that may play a major role in learning and perceptual outcomes
(2020) recently demonstrated that performance on a non-native speech sound learning could be modulated using stimulation techniques that target arousal mechanisms. We propose that these rapid fluctuations in arousal states may explain moment-to-moment variability in neural activity and behavior during speech perception, with strong implications for explaining variability in sound learning in adulthood
Summary
The ability to perceive speech, especially under challenging conditions, reflects a remarkable set of computational processes that the human brain is well-adapted to perform. The active processes of perception and comprehension are modulated by task- and goal-driven factors like attention (Heald and Nusbaum 2014; Huyck and Johnsrude 2012). Together, these factors provide listeners with the flexibility necessary to comprehend speech, including under conditions where the signal-to-noise ratio of the input is decreased (Guediche et al 2014), or in the context of challenging tasks like acquisition of an unfamiliar language (Birdsong 2018). We will discuss the challenging task of learning to discriminate and identify non-native speech sound categories in adulthood, focusing in particular on our current understanding of the neural processes that the arousal system may act upon. We propose that taVNS holds promise as both a scientific and translational tool for understanding and manipulating arousal states that may play a major role in learning and perceptual outcomes
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