Neuroplasticity of speech-in-noise processing in older adults assessed by functional near-infrared spectroscopy (fNIRS)

Abstract

Functional near-infrared spectroscopy (fNIRS) is a non-invasive optical neuroimaging technique that is portable and acoustically silent, has become a promising tool for evaluating auditory brain functions in hearing-vulnerable individuals. This study, for the first time, used fNIRS to evaluate neuroplasticity of speech processing in older adults. Ten older adults, most of whom had moderate-to-mild hearing loss, participated in a 4-week speech-in-noise training. Their speech-in-noise performances and fNIRS brain responses to speech (auditory sentences in noise), nonspeech (spectrally-rotated speech) and visual (flashing chequerboards) stimuli were evaluated pre- (T0) and post-training (immediately post-training, T1; and after a 4-week retention, T2). Behaviourally, speech-in-noise performances were improved after retention (T2 vs. T0) but not immediately post-training (T1 vs. T0). Neurally, brain responses to speech vs. nonspeech in the left frontal cortex decreased significantly post-training (both T1 and T2 vs. T0), reflecting possible alleviation of listening efforts. Furthermore, functional connectivity was significantly enhanced between temporal, parietal and frontal lobes, mainly after retention (T2 vs. T0), corresponding to the significant behavioural improvements. Finally, connectivity was significantly decreased between auditory and higher-level non-auditory (parietal and frontal) cortices in response to visual stimuli post-training (T1 vs. T0), indicating decreased cross-modal takeover of speech-related regions during visual processing. The results thus showed that neuroplasticity can be observed before behavioural changes. To our knowledge, this is the first fNIRS study to evaluate speech-based auditory neuroplasticity in older adults. It thus provides important implications for auditory neuroscience research by illustrating the promises of detecting auditory neuroplasticity using fNIRS in hearing-vulnerable individuals.