How the BIS index trails its EEG¶
Modelling the BIS index as a lagged mix of EEG features shows a trailing average starting about 3 s after its EEG window ends: about 16 s long in half the cases (the 15 s smoothing setting) and 32–61 s in most others. Half the response weight is older than 30 s.
| Date | 2026-10-13 |
| Status | current |
| Methods | Delays and averaging windows |
| Devices | BIS monitor |
| Variables | BIS index, BIS EEG |
Data¶
All 5,853 cases with a BIS module; 5,520 fitted, 3,297 good fits (median R² 0.81).
Method¶
EEG features at 1 Hz from BIS EEG1, each stamped at the end of its 2 s window (five band powers, total power, SEF95, beta ratio, suppression). The index is a weighted sum of the features passed through a lag kernel over −10 to 120 s, fitted per case where the signal quality index is at least 50; the kernel is split into a trailing boxcar (the fast part), a slow exponential tail and a floor. Index and EEG share the monitor's own stamps, so no clock correction is needed. Checked on synthetic index values (15, 30 and 45 s windows come back as 15, 28 and 46 s) and on the suppression ratio (a known ~63 s window, read as 77 s).
Results¶
- Delay 3 s from the EEG window's end (p10–p90 3–4 s), the same in every group.
- 15 s setting: 1,578 of 3,297 good fits (48%) have a fast part of 13–17 s: about 16 s of EEG ending 3 s before the stamp. 1,304 (40%) are longer, 32–61 s (median 50 s; less certain). Five PCs never show the 15 s window.
- Most of the response is slow: the fast part holds 16% of the kernel weight; half the weight is older than 30 s, 90% older than 97 s.
| Parameter | Value | Status | From |
|---|---|---|---|
| BIS index delay | 3 s (p10–p90 3 to 4) | measured | How the BIS index trails its EEG |
| BIS index averaging window | 16 s in 48% of good fits (the 15 s setting); 32–61 s in 40%; half the weight older than 30 s | measured | How the BIS index trails its EEG |
Discussion¶
Treat the index as an average of the EEG over roughly 15–40 s ending about 3 s before its stamp, with slow weight beyond. It suits slow questions (depth of anaesthesia over minutes); anything faster needs features computed from the EEG itself, which after synchronisation sits about 42 ms from SNUADC time (17 ms with the recording's own bursts; EEG–ECG timing error after the stamp model). The 132 s propofol-to-BIS latency (Drug effects as a clock check) includes this delay. The bis_smoothing label gives each case's group.
Observations it supports¶
| Observation | Status | Summary |
|---|---|---|
| Monitor numerics describe the past, never the future | established | Every monitor numeric describes a window of 3–30 s or more that ended 1–9 s before its stamp. None is stamped before its data exist, so the stamps already respect the availability rule. |
| The BIS index is slow | established | The BIS index averages 15–60 s of EEG ending about 3 s before its stamp, with half its weight older than 30 s. It cannot time anything faster than about a minute. |