test(eeg): add deterministic synthetic Raw fixture (5 ch, 256 Hz, 10 s)

tests/fixtures/build_eeg_fixture.py

@@ -0,0 +1,54 @@
+"""Generate a deterministic synthetic MNE Raw fixture for EEG pipeline tests.
+
+The fixture is committed to the repo alongside this script so test runs are
+reproducible without re-running the script. Re-run only if the contract changes.
+
+Channels: 4 EEG (Cz, Pz, O1, O2) + 1 EOG (EOG061).
+Sampling rate: 256 Hz. Duration: 10 s.
+Synthetic content: a 10 Hz alpha sine on each EEG channel, plus a 1.5 Hz EOG
+"blink" injected on EOG061 and bleed-through on the frontal-most EEG channel
+(Cz) so ICA has something to detect.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import mne
+import numpy as np
+
+
+def build() -> Path:
+    rng = np.random.default_rng(seed=42)
+    sfreq = 256.0
+    duration_s = 10.0
+    n_samples = int(sfreq * duration_s)
+    t = np.arange(n_samples) / sfreq
+
+    # Base alpha (10 Hz) + small white noise on every EEG channel.
+    eeg_alpha = np.sin(2 * np.pi * 10.0 * t)
+    eeg_noise = rng.standard_normal((4, n_samples)) * 1e-6
+    eeg = (eeg_alpha[None, :] * 1e-5) + eeg_noise
+
+    # EOG blink: low-frequency square-ish pulse train at ~1.5 Hz.
+    eog_pulse = (np.sin(2 * np.pi * 1.5 * t) > 0.95).astype(float) * 1e-4
+
+    # Bleed EOG into Cz (channel 0) so ICA finds an EOG-correlated component.
+    eeg[0] += 0.3 * eog_pulse
+
+    data = np.vstack([eeg, eog_pulse[None, :]])  # shape: (5, n_samples)
+
+    info = mne.create_info(
+        ch_names=["Cz", "Pz", "O1", "O2", "EOG061"],
+        sfreq=sfreq,
+        ch_types=["eeg", "eeg", "eeg", "eeg", "eog"],
+    )
+    raw = mne.io.RawArray(data, info, verbose="ERROR")
+
+    out = Path(__file__).parent / "eeg_sample.fif"
+    raw.save(out, overwrite=True, verbose="ERROR")
+    return out
+
+
+if __name__ == "__main__":
+    p = build()
+    print(f"Wrote {p}")