Learning a Second Language in Adulthood Changes Subcortical Neural Encoding

Abstract

Second language learning has been shown to impact and reshape the central nervous system, anatomically and functionally. Most of the studies on second language learning and neuroplasticity have been focused on cortical areas, whereas the subcortical neural encoding mechanism and its relationship with L2 learning have not been examined extensively. The purpose of this study was to utilize frequency-following response (FFR) to examine if and how learning a tonal language in adulthood changes the subcortical neural encoding in hearing adults. Three groups of subjects were recruited: native speakers of Mandarin Chinese (native speakers (NS)), learners of the language (L2 learners), and those with no experience (native speakers of foreign languages (NSFL)). It is hypothesized that differences would exist in FFRs obtained from the three language experience groups. Results revealed that FFRs obtained from L2 learners were found to be more robust than the NSFL group, yet not on a par with the NS group. Such results may suggest that in human adulthood, subcortical neural encoding ability may be trainable with the acquisition of a new language and that neuroplasticity at the brainstem level can indeed be influenced by L2 learning.

Aging Affects Subcortical Pitch Information Encoding Differently in Humans With Different Language Backgrounds

“…Our results showed that the CHY group had significantly better pitch information coding, both in magnitude and accuracy, compared to the other three groups. Considering previous studies on FFRs ( Krishnan et al, 2005 ; Liu et al, 2020 ), this was not a surprising finding. More interestingly we demonstrated how different aspects of pitch information processing capacity, namely the magnitude and accuracy, may be influenced by the aging process and language experience in similar yet different ways.…”

“…Wave V latencies of click-evoked ABRs were used as a benchmark for what would be considered clinically normal electrophysiological responses (no delay in wave V latencies). ABRs also serve as a quality control method for FFR recordings; a typical practice in previous works by present author(s) and others in the field ( Wang et al, 2016 ; Liu et al, 2020 ). Wave Vs of ABR for all subjects were identified by experienced audiologists.…”

“…Identical equipment, stimuli, and experiment protocols were used in the two collaboration sites to ensure data obtained the two sites were comparable. Detailed protocols and related quality control procedures can be found in previous collaborative work ( Liu et al, 2020 ). Briefly, a monosyllabic Chinese word/yi/with high-falling tone, marked as/yi4/, was used as stimulus in this study.…”

“…The brain undergoes widespread neural specialization relating to language and cognitive processing ( Costa and Sebastián-Gallés, 2014 ; Burgaleta et al, 2016 ). At the subcortical level, pitch processing of lexical tones can be shaped by language experience in both childhood and adulthood ( Li et al, 2001 ; Wang et al, 2004 ; Liu et al, 2020 ). Previous studies between Mandarin speakers of tonal and non-tonal languages (English) have shown that pitch encoding at the subcortical level can be enhanced by experience in lexical tones, irrespective of speech or non-speech stimulus ( Krishnan et al, 2005 , 2009 ; Swaminathan et al, 2008 ).…”

“…It was suggested that the reorganization of the brainstem pitch encoding mechanisms were shaped by the corticofugal system in the early stage of language development ( Keuroghlian and Knudsen, 2007 ; Kral and Eggermont, 2007 ). It has also been shown that native speakers of non-tonal languages who learned Mandarin Chinese in their adulthood demonstrated weaker FFRs than native speakers of Mandarin Chinese, but enhanced subcortical neural pitch information encoding capacity compared to those that did not speak Mandarin Chinese at all ( Liu et al, 2020 ). This young adult cohort was well past the “critical period” of language acquisition and exhibited greater pitch information encoding.…”