Ultrasound imaging in second language research: Systematic review and thematic analysis

Several publications have explored second language (L2) articulation through lingual ultrasound imaging technology. This systematic review and thematic analysis collate and evaluate these studies, focusing on methodologies, experimental setups, and findings. The review includes 31 works: 23 on ultrasound biofeedback and 8 on characterizing L2 articulation. English is the predominant language studied (82 % as L1 or L2), with participants mainly young adults (2–60 participants per study). The 23 ultrasound biofeedback studies showed significant variation in session numbers and length, including 16 PICO studies (i.e. study design with participants, intervention, controls/comparison group, outcome) where ultrasound biofeedback was compared to auditory feedback and/or control conditions. Data analysis of biofeedback studies often included acoustic or perceptual assessments in addition or instead of ultrasound data analysis. Analysis of results indicate that ultrasound biofeedback is effective for improving L2 articulation. However, the PICO studies revealed that while ultrasound biofeedback may offer certain advantages, these findings remain preliminary and warrant further investigation. Learner characteristics and target selection may affect biofeedback efficacy. Ultrasound also proved valuable for characterizing L2 articulation by showing articulatory and coarticulatory patterns, particularly in English sounds like /ɹ/, /l/, and various vowels. L2 characterization studies frequently used dynamic speech movement analysis. Moving forward, researchers are encouraged to use dynamic movement analysis also in biofeedback studies to deepen understanding of articulation processes. Expanding linguistic and demographic diversity in future research is essential to capturing language heterogeneity.