Learning-induced neural plasticity associated with acquisition of a difficult second-language phonetic contrast

Abstract

Adult native Japanese speakers have difficulty perceiving the English /rl/ phonetic contrast even after years of exposure. However, after extensive perceptual identification training long lasting improvement in identification performance that generalizes to novel stimuli can be attained. In this fMRI study we investigate localized changes in brain activity associated with one-month of extensive feedback-based perceptual identification training by native Japanese speakers learning the English /rl/ phonetic contrast. Before and after training separate functional brain imaging sessions were conducted for identification of the English /rl/ contrast (difficult for Japanese speakers), /bg/ contrast (easy), and /bv/ contrast (difficult). Neural plasticity, denoted by exclusive enhancement in brain activity for the /rl/ contrast (not present for the /bv/ and /bg/ conditions), does not only occur in brain regions involved with acoustic–phonetic processing (superior temporal areas, supramarginal gyrus, planum temporale) but also in additional bilateral cortical (Broca’s area, premotor cortex, orosensory cortex) and subcortical regions (cerebellum, basal ganglia, substantia nigra) involved with speech production as well as with formation of perceptual–motor mappings. The results support the hypothesis that learning of an auditory–articulatory mapping improves identification performance by allowing a perception to be made in reference to potential action. [Work supported by TAO, Japan.]