Passive exposure induces subcortical and cortical plasticity in the encoding of foreign speech sounds

The early sensitive period is crucial for receptive speech acquisition, during which the features of native language are learned mainly passively. However, it remains unclear whether the adult human brain retains sufficient plasticity for short-term passive learning of foreign phonetic features, particularly at the level of subcortical auditory encoding as reflected by the Frequency-Following Response (FFR). Here, seventeen native Finnish-speaking adults underwent six hours of passive exposure across four consecutive days to a single foreign speech sound, the vowel /a/ carrying a falling Mandarin tone. We recorded FFR during the first and last day of exposure to index subcortical plasticity and assessed its cortical correlate, the N1-like response, extracted from down-sampled FFR data. In addition, cortical change-detection responses were evaluated using event-related potentials (ERPs) elicited in passive and active oddball paradigms administered before and after the exposure to investigate whether neural changes transfer to change detection. We found that passive exposure led to increased spectral power of the FFR at the fundamental frequency of the exposed sound, accompanied by enhanced cortical N1 amplitude. The effect transferred to attentive, but not pre-attentive, detection of deviant stimuli presented among the exposed standard. Enhanced change detection was indexed by increased P3b amplitude, reflecting attentional orienting to deviance, along with faster reaction times and higher response accuracy. Together, these findings suggest that short-term passive exposure to a foreign speech feature induces learning-related neural plasticity at subcortical and cortical levels of the auditory system, highlighting the capacity of the adult human brain to adapt to a new speech environment beyond an early sensitive period.