Inner-Speech Decoding from EEG Across Three Corpora Under a Fixed Architecture and Protocol
Organizations: Computer Science Department, American International University-Bangladesh, Dhaka, Bangladesh · Department of Software Engineering, N.I. Lobachevsky National Research Nizhny Novgorod State University, Nizhny Novgorod, Russia · School of Computing, RMIT University, Melbourne, Australia
Abstract
Inner-speech decoding from electroencephalography could support non-invasive communication for people with severe motor impairment, but comparison across studies is hindered by dataset- and protocol-specific evaluation. We present a controlled cross-dataset study using one preprocessing pipeline, one within-subject five-fold cross-validation protocol, and a compact dual-attention Transformer (VC-DATNet; approximately 131,000 parameters) across three public corpora: Thinking Out Loud (10 participants), BISpeech (4), and SyncSpeech (3). VC-DATNet achieved per-participant sensor/source hybrid accuracies of 39.48% on the four-class Thinking Out Loud task, 20.60% on the eight-word BISpeech task, and 25.53% on the eight-word SyncSpeech task, for which we report the first decoding result. Sensor-space and cortical source-space representations were complementary, with each performing better for different participants; this pattern replicated across all three corpora and nine architectures. Moreover, ten decoders spanning convolutional, recurrent, temporal-convolutional, hybrid Transformer, and pretrained foundation-model families converged within a narrow accuracy range on every dataset. These findings indicate that representation choice should be individualized and suggest that performance on these public corpora is constrained more by signal quality and dataset scale than by model capacity. The released pipeline provides a reproducible benchmark for future inner-speech decoding studies
Figures & tables
| Study | Method(s) | Original evaluation | Highlights / relevance |
| CNN-based decoders | |||
| Schirrmeister et al. [ 50 ] | ShallowConvNet / DeepConvNet | BCI IV-2a, HGD | End-to-end CNNs match filter-bank pipelines |
| Lawhern et al. [ 27 ] | EEGNet (depthwise-separable conv) | BCI IV-2a, others | Compact, cross-paradigm default baseline |
| Ingolfsson et al. [ 22 ] | EEG-TCNet (temporal conv blocks) | BCI IV-2a | Low-latency convolutional decoder |
| Riyad et al. [ 48 ] | Incep-EEGNet (inception filters) | BCI IV-2a | Multi-scale temporal receptive fields |
| Mane et al. [ 38 ] | FBCNet (filter-bank + variance layer) | BCI IV-2a/2b | Explicit spectral-variance layer |
| Study | Dataset | Method | Task (classes) | Acc. (%) | Note |
| Thinking Out Loud (ds003626): the primary benchmark of this paper | |||||
| Nieto et al. 2022 [ 43 ] | ToL | Dataset release; RF / CNN baselines | 4-cls directional | 25 (chance) | De facto open ToL benchmark |
| van den Berg et al. 2021 [ 57 ] | ToL | EEGNet (2D CNN) | 4-cls directional | 29.7 | First deep-learning benchmark on ToL |
| Gasparini et al. 2022 [ 17 ] | ToL | SVM / XGBoost / LSTM / BiLSTM | 4-cls directional | 26.2–36.1 | BiLSTM (36.1) is the strongest prior ToL result |
| Bimodal Inner Speech corpus / BISpeech (ds004196) | |||||
| Wilson et al. 2022 [ 62 ] | BISpeech | SVM / RF (EEG, fMRI) | 8-word; binary | 22.2 (8-w, EEG); 62.8 (bin., EEG) | First classification study on BISpeech |
| Dataset | Year | Subj. | Ch. | Classes | Trials | Stimuli |
|---|---|---|---|---|---|---|
| Thinking Out Loud (ToL; ds003626) [ 43 ] | 2022 | 10 | 128 | 4 | 50/class ( 200/subject) | Directional words: up, down, left, and right |
| BISpeech (ds004196) [ 35 , 51 ] | 2023 | 4 | 64 | 8 | 40/word | Semantic words: four social and four numerical |
| SyncSpeech (ds006033) [ 52 ] | 2025 | 3 | 64 | 8 | 40/word | Semantic words: four social and four numerical |
| Symbol | Shape | Meaning |
| Preprocessed EEG epoch | ||
| Multi-scale temporal features | ||
| Depthwise-spatial features | ||
| Mean / log-variance pooled streams | ||
| Query / key / value tensors ( ) | ||
| per-token | Temporal gate (per-timestep) |
| Component / hyperparameter | Value |
|---|---|
| Preprocessing | |
| Sampling rate | Hz |
| Epoch length | s ( 640 samples) |
| Band-pass filter | – Hz (4th-order Butterworth) |
| Normalisation | per-trial -score |
| Architecture | |
| Dataset / task | Classes | Chance (%) | VC-DATNet (%) | Strongest prior (%) | Reference |
|---|---|---|---|---|---|
| ToL, directional | 4 | 25.0 | 39.48 (hybrid) | 36.1 (BiLSTM) | Gasparini et al. [ 17 ] |
| BISpeech, 8-word | 8 | 12.5 | 20.60 (sensor) | 14.78 (RF, EEG-only) | Wilson et al. [ 63 ] |
| BISpeech, social vs. numerical | 2 | 50.0 | 59.37 (source) | 62.81 (SVM) | Wilson et al. [ 62 ] |
| BISpeech, social 4-word | 4 | 25.0 | 38.68 (hybrid) | — (none EEG-only) | — |
| BISpeech, numerical 4-word | 4 | 25.0 | 36.78 (sensor) | — (none EEG-only) | — |
| SyncSpeech, 8-word | 8 | 12.5 | 25.53 (hybrid) | — (none published) | — |
| Representation | S01 | S02 | S03 | S04 | S05 | S06 | S07 | S08 | S09 | S10 | Mean |
|---|---|---|---|---|---|---|---|---|---|---|---|
| (a) Bench protocol: full -channel BioSemi montage, vanilla single-condition training. | |||||||||||
| Sensor-space | 30.0 | 38.8 | 45.0 | 35.0 | 32.5 | 37.0 | 37.1 | 40.0 | 40.4 | 40.0 | 37.58 |
| Source-space | 40.0 | 31.2 | 32.8 | 38.8 | 36.7 | 38.0 | 36.2 | 32.0 | 30.0 | 35.4 | 35.11 |
| Hybrid (best) | 40.0 | 38.8 | 45.0 | 38.8 | 36.7 | 38.0 | 37.1 | 40.0 | 40.4 | 40.0 | 39.48 |
| Winner | src | sens | sens | src | src | src | sens | sens | sens | sens | — |
| (b) A priori subset: inner-speech-relevant channels, cross-condition (Pron+Inner+Vis) multi-task pretrain inner-only fine-tune. | |||||||||||
| Task | Classes | Chance (%) | Sensor (%) | Source (%) | Hybrid (%) |
|---|---|---|---|---|---|
| 8-word | 8 | 12.5 | 20.60 | 17.46 | 20.60 |
| Social vs. numerical | 2 | 50.0 | 57.64 | 59.37 | 59.37 |
| Social, 4-word | 4 | 25.0 | 37.44 | 35.72 | 38.68 |
| Numerical, 4-word | 4 | 25.0 | 36.78 | 35.67 | 36.78 |
| Method | S01 | S02 | S03 | S05 | Mean |
|---|---|---|---|---|---|
| EEGNet [ 27 ] | 17.85 | 18.18 | 18.82 | 19.69 | 18.64 |
| ShallowConvNet [ 50 ] | 19.73 | 20.38 | 17.57 | 19.38 | 19.27 |
| DeepConvNet [ 50 ] | 17.87 | 17.88 | 17.87 | 17.81 | 17.86 |
| EEG-TCNet [ 22 ] | 18.80 | 16.29 | 18.50 | 17.81 | 17.85 |
| EEGPT [ 58 ] (linear probe) | 21.00 | 18.20 | 17.90 | 19.10 | 19.05 |
| TransNet [ 37 ] | 18.81 | 20.37 | 18.80 | 20.31 | 19.57 |
| Representation | Sub-01 | Sub-02 | Sub-03 | Mean |
|---|---|---|---|---|
| Sensor-space | 21.45 | 27.42 | 24.65 | 24.51 |
| Source-space | 24.52 | 26.33 | 18.22 | 23.02 |
| Hybrid (best) | 24.52 | 27.42 | 24.65 | 25.53 |
| ToL (4c) | BISpeech (8c) | SyncSpeech (8c) | ||||||||||
| Method | Family | Domain | Params | S | Sr | H | S | Sr | H | S | Sr | H |
| EEGNet (Lawhern et al.) [ 27 ] | CNN | Multi | 4.4 K | 29.7 † | — | 29.7 † | 18.40 | 17.93 | 18.64 | 21.90 | 19.74 | 22.34 |
| ShallowConvNet (Schirrmeister et al.) [ 50 ] | CNN | MI | 211.8 K | 34.70 | 32.96 | 35.21 | 19.04 | 18.40 | 19.27 | 22.53 | 23.23 | 26.41 |
| DeepConvNet (Schirrmeister et al.) [ 50 ] | CNN | MI | 345.9 K | 35.57 | 31.62 | 35.57 | 17.47 | 16.84 | 17.86 | 21.47 | 23.69 | 23.70 |
| EEGNeX (Chen et al.) [ 8 ] | CNN | Multi | 62.9 K | 35.08 | 35.44 | 36.68 | 19.10 | 18.95 | 19.66 | 26.75 | 24.20 | 26.75 |
| EEG-TCNet (Ingolfsson et al.) [ 22 ] | TCN | MI | 5.9 K | 31.28 | 30.34 | 31.99 | 17.70 | 15.19 | 17.85 | 15.23 | 14.61 | 16.81 |
| ToL ( ) | BISpeech ( ) | SyncSpeech ( ) | |||||||
| Method | Sens | Src | Tie | Sens | Src | Tie | Sens | Src | Tie |
| ShallowConvNet | 7 | 2 | 1 | 1 | 1 | 2 | 1 | 2 | 0 |
| DeepConvNet | 10 | 0 | 0 | 3 | 1 | 0 | 0 | 2 | 1 |
| EEG-TCNet | 7 | 2 | 1 | 3 | 1 | 0 | 2 | 1 | 0 |
| EEGNet | — | — | — | 3 | 1 | 0 | 2 | 1 | 0 |
| TransNet | 6 | 4 | 0 | 1 | 2 | 1 | 2 | 0 | 1 |
EEG Decoding
Electroencephalography (EEG) decoding maps brain-signal patterns to mental states, stimuli, speech, or intended actions.
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