Understanding Multilingual Medical ASR Adaptation Through Layer-Wise Analysis
Authors: Souranil Kahali, Rituparna Bose, Abner Hernandez, Tomas Arias-Vergara, Andreas Maier, Ning Ma, Paula Andrea Perez-Toro
Organizations: Pattern Recognition Lab, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany · School of Computer Science, University of Sheffield, Sheffield, UK · Chair for AI in Healthcare and Medicine, Technical University of Munich (TUM), Munich, Germany
Medical automatic speech recognition (MedASR) requires adaptation to specialised terminology, limited annotated clinical data, and multilingual use cases. Although large-scale pretrained ASR models such as Whisper achieve strong generalisation, their behaviour after medical and multilingual adaptation remains insufficiently understood beyond word error rate (WER). This paper investigates how multilingual medical adaptation reshapes the internal representations of Whisper models through layer-wise encoder analysis. We compare zero-shot decoding, English-only fine-tuning, German-only diagnostic fine-tuning, two-stage EN->EN+DE continuation, and direct EN+DE fine-tuning across Whisper model sizes. Fine-tuning substantially improves MedASR performance, but the best model depends on the adaptation setting: Whisper-Medium gives the lowest English WER (7.72%) and the lowest combined EN+DE WER under direct EN+DE training (26.30%); German-only Whisper-Large-v3 gives the lowest German WER (44.96%), but as a within-corpus diagnostic on 86 single-speaker training utterances rather than robust generalisation. Layer-wise analysis of the two-stage Whisper-Small trajectory shows that English medical fine-tuning produces the dominant encoder shift, whereas multilingual continuation largely preserves the adapted representation space. Domain and language information remain highly recoverable across layers, while linearly recoverable error-predictive cues weaken as WER improves.
Figures & tables
Fig. 1: Overview of the multilingual MedASR adaptation and analysis pipeline. English and German medical speech are used for monolingual fine-tuning, two-stage EN→EN+DE continuation, and direct EN+DE fine-tuning. Adapted models are evaluated with WER, and Whisper-Small encoder representations are analysed through drift and probing tasks.
Model
Adapt
English ↓
German ↓
Combined ↓
Zero-shot
Whisper B
None
21.30
70.42
44.65
Whisper S
None
17.57
62.98
39.15
Whisper M
None
16.77
71.99 †
43.02
Whisper L
None
16.50
56.51
35.52
Monolingual (Mono) medical fine-tuning
TABLE I: ASR WER (%) on English and German medical speech test sets. Base (B), Small (S), Medium (M) and Large (L) Whisper versions where adapted.
Type
Utterances
EN symptom term
REF: stomach pain and bloating ; ZS: stomach pain and rotting ; S2: stomach pain and bloating .
EN clinical term
REF: could I have a concussion ; ZS: could I have a competition ; S2: could I have a concussion .
REF: die Prostater drüben; ZS/S2: Brotzeit verdrücken/verdrüben; DE-Large: die Prostata drüben.
TABLE II: Representative transcription errors. ZS-Small / S2-Small = Whisper-Small zero-shot / two-stage EN → EN+DE; DE-Large = German-only Whisper-Large-v3. Italics mark the contested term.
Fig. 2: Bootstrap-averaged representation drift across encoder layers (L0–L12) over five seeds; shading denotes ± 1 std. Cosine similarity is shown on the left and Linear CKA on the right. Pretrained → EN-FT produces the largest upper-layer shift, whereas EN-FT → ML-FT remains close to the adapted encoder. ML-FT EN vs. DE shows high cosine similarity but low Linear CKA, indicating centroid alignment with language-sensitive geometry.
Drift pair
Cosine range
Lin. CKA range
Pretrained → EN-FT
0.906–1.000
0.946–1.000
EN-FT → ML-FT
0.989–1.000
0.988–1.000
ML-FT: EN vs. DE
0.980–0.994
0.040–0.107
TABLE III: Representation drift summary over five bootstrap seeds. Values report the per-layer range across L0–L12.
Fig. 3: Layer-wise probe macro-F1 across encoder layers (L0–L12) for the three probing tasks, bootstrap-aggregated over five seeds (shading: ± 1 std). (a) Domain probe (medical English vs. LibriSpeech) and (b) Language probe (English medical vs. German medical) remain near ceiling across all checkpoints and layers, so peak-layer claims are not meaningful here. (c) Error-predictability probe uses rank-based low-/high-WER tertile labels. The pretrained encoder carries the strongest linearly recoverable error signal at the best layer; this signal weakens after English fine-tuning and multilingual continuation while decoder WER improves on the same 100 clips (19.87% → 13.61% → 12.48%).
Checkpoint
Domain F1
Lang. F1
WER F1
Dec. WER
min
min
best layer
(%)
Pretrained
≥0.984
≥0.990
0.721±0.028 (L2)
19.87
EN-FT
≥0.984
≥0.993
0.619±0.039 (L6)
13.61
ML-FT
≥0.985
≥0.993
0.556±0.033 (L11)
12.48
Dec. WER is computed on the same 100 English utterances used to label the WER probe (greedy decoding) and is not layer-wise.
TABLE IV: Probe summary over five bootstrap seeds. Domain and Language report the minimum macro-F1 across layers; WER F1 reports the best-layer low-/high-WER probe result under a rank-based tertile split ( n=33 per class).
Although Whisper models benefit from large-scale multilingual pre-training, their performance on Burmese medical speech remains limited. This work presents a Burmese medical speech recognition framework built on a high-quality 28-hour corpus recorded and validated by native speakers. We fine-tune Whisper models using full fine-tuning (FFT) and parameter-efficient fine-tuning (PEFT) with LoRA. To evaluate robustness, we apply waveform- and spectrogram-level data augmentation under controlled noise and simulated room acoustics. While augmentation reduces performance on clean speech, it significantly improves robustness in noisy and reverberant environments across FFT and PEFT settings. Our best-performing system, fully fine-tuned myMediWhisper-Medium without augmentation, achieves a state-of-the-art Word Error Rate (WER) of 23.44%, outperforming much larger general-domain fine-tuned models. Dataset and other resources can be found at the Huggingface repository: https://huggingface.co/datasets/LULab/mediTalk-mm-rdy.
Ye Kyaw Thu, Ye Bhone Lin, Thura Aung +6
National Electronics and Computer Technology Center (NECTEC), Thailand · Language Understanding Laboratory, Myanmar · King Mongkut’s University of Technology Thonburi, Thailand +1
This article concerns comparative studies on the Automatic Speech Recognition (ASR) model incorporated with the Large Language Model (LLM) used for medical interviews. The proposed solution is tested on polish language benchmarks and dataset with medical interviews. The latest ASR technologies are based on convolutional neural networks (CNNs), recurrent neural networks (RNNs) and Transformers. Most of them work as end-to-end solutions. The presented approach in the case of the Whisper model shows a two-stage solution with End-To-End ASR and LLM working together in a pipeline. The ASR output is an input for LLM. The LLM is a component by which the output from ASR is corrected and improved. Comparative studies for automatic recognition of the Polish language between modern End-To-End deep learning architectures and the ASR hybrid model were performed. The medical interview tests were performed with two state-of-the-art ASR models: OpenAI Whisper incorporated with LLM and Scribe ElevenLabs. Additionally, the results were compared with five more end-to-end models (QuartzNet, FastConformer, Wav2Vec 2.0 XLSR and ESPnet Model Zoo) on Mozilla Common Voice and VoxPopuli databases. Tests were conducted for clean audio signal, signal with bandwidth limitation, and degraded. The tested models were evaluated on the basis of Word Error Rate (WER) and Character Error Rate (CER). The results show that the Whisper model performs by far the best among the open-source models. ElevenLabs Scribe model, on the other hand, performs best for Polish on both general benchmark and medical data.
Marcin Pietroń, Szymon Piórkowski, Kamil Faber +6
Institute of Electronics AGH, Krakow · Coraz Zdrowiej, Krakow · Department of Computer Science AGH, Krakow +2
Automatic Speech recognition (ASR) struggles with Pathological speech due to persistent speaker-specific mismatch. In clinical and accessibility scenarios, only limited target-speaker data is typically available, making full model retraining impractical. We present a component-level analysis of LoRA adaptation in Whisper, investigating how adapter placement interacts with the model size, training efficiency and utterance-duration in single-speaker settings. Our experimental comparative study shows that while scaling reduces errors, it does not eliminate mismatch. Furthermore, the full attention LoRa provides the most reliable accuracy, and the best time--accuracy trade-off. Improvements vary across utterance length, with medium-length recordings benefiting the most, while short and long utterances remain challenging. These findings highlight the importance of adaptation placement and duration-dependent evaluation, under challenging, data-scarce personalization scenarios.
Giuseppe Giaimis, Sergei Katkov, Muhammad Azfar Yaqub +1