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arxiv: 2605.24525 · v1 · pith:LL3F4PJ4new · submitted 2026-05-23 · 📡 eess.IV · eess.SP

A Signal Extraction Approach for Remote Heart Rate Variability Assessment Using Proxy Measure in a Driving Simulator

Pith reviewed 2026-06-30 12:15 UTC · model grok-4.3

classification 📡 eess.IV eess.SP
keywords remote photoplethysmographyheart rate variabilitydriving simulatorsignal extractionmotion artifactssuperpixel regionspulse rateECG validation
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The pith

Remote photoplethysmography extracts pulse rate and heart rate variability from facial videos in a driving simulator with errors low enough to match electrocardiography statistics.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper tests remote photoplethysmography algorithms including 2SR, CHROM, POS, and PCA on superpixel regions of the face to monitor heart activity without contact during baseline and simulated driving tasks. It adds two peak enhancement techniques based on the Lp norm and fractional-order derivatives, plus a signal-to-noise ratio check on 20-second segments to handle motion. Results from 29 participants show pulse rate errors as low as 1.92 bpm and heart rate variability measures that preserve the same statistical patterns as simultaneous ECG recordings. This setup matters for developing non-contact ways to track driver physiology in controlled moving environments where electrodes are impractical.

Core claim

In recordings from 29 participants, the 2SR algorithm combined with fractional-order derivative enhancement and 20 superpixel regions yields a mean absolute error of 1.92 bpm for pulse rate against ECG, while the best configurations reach 0.061 s MAE for SDNN and 0.081 s for RMSSD; inter-beat interval detection reaches an F1 score of 0.93, and all rPPG-derived parameters reproduce the statistical structure of the reference ECG across both baseline and driving conditions, with CHROM recommended for HRV and Lp norm at p around 6-7 as an effective enhancer.

What carries the argument

Application of spatial subspace rotation or chrominance methods to 10 or 20 superpixel facial regions, sharpened by Lp-norm or fractional-order derivative peak enhancement, followed by signal-to-noise ratio filtering of 20-second segments to reduce motion artifacts.

If this is right

  • 2SR with FOD and 20 superpixels gives the lowest pulse rate error, while CHROM with Lp norm performs best for HRV parameters.
  • Optimal settings cluster around p=6-7 for the Lp norm and fractional orders of 1.0-1.4.
  • All tested configurations reproduce the reference ECG statistical structure for both baseline and driving conditions.
  • FOD requires caution because it can introduce slow changes in the rPPG waveform shape.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same pipeline could support fatigue or stress monitoring in real vehicles if motion patterns remain comparable to the simulator.
  • Extending the SNR quality filter to variable-length segments might reduce data loss in longer recordings.
  • Combining the recommended CHROM configuration with other facial or vehicle sensors could test robustness beyond isolated video input.

Load-bearing premise

The signal-to-noise ratio assessment on 20-second segments removes motion artifacts without discarding valid physiological signals or creating selection bias in the retained data.

What would settle it

A new dataset of driving simulator recordings where rPPG-derived SDNN and RMSSD values show statistically different distributions from simultaneous ECG values after identical processing would falsify the reproduction of statistical structure.

Figures

Figures reproduced from arXiv: 2605.24525 by {\DJ}or{\dj}e D. Ne\v{s}kovi\'c, Nadica Miljkovi\'c.

Figure 1
Figure 1. Figure 1: Block diagram of proposed method for rPPG performance. Abbreviations are: Red Green Blue (RGB), Pulse Rate [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Display of rPPG signal extraction after applying the CHROM, PCA (the first Principal Component [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: The impact of different p values on the application of the Lp norm on rPPG signal with reference ECG presented in graphs. For p = 1, the Lp norm actually does not change the extracted rPPG signal. The second metric (SNR 0.7–3 Hz) integrates PSD over the full physiological HR band of 0.7 Hz – 3 Hz, with a ±0.1 Hz notch at 2 Hz excluded from both signal and noise calculations to suppress a recurring non-phys… view at source ↗
Figure 4
Figure 4. Figure 4: The impact of different values of the Fractional Order Derivative (FOD) for the application of GL FOD for peak enhancement in rPPG signal with presented reference ECG. The value of FOD of 1 is equal to the application of standardized Pan-Tompkins algorithm. Also, the extracted rPPG without peak enhancement is used. A notable change in peak location is noticed due to the slow changes of rPPG signal, making … view at source ↗
Figure 5
Figure 5. Figure 5: Example of regions that proved to be the most useful for extracting the rPPG signal in one subject when 20 SP are [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Comparison of SNR values (useful part of PSD defined within a band range from 0.7 Hz to 3 Hz) across two peak [PITH_FULL_IMAGE:figures/full_fig_p012_6.png] view at source ↗
Figure 11
Figure 11. Figure 11: Example of a 20 s long ECG segment containing a pathological beat (confirmed by a cardiologist) and the [PITH_FULL_IMAGE:figures/full_fig_p021_11.png] view at source ↗
read the original abstract

This study evaluates remote Photopletismography (rPPG) algorithms, Spatial Subspace Rotation (2SR), Chrominance-based method (CHROM), Plane-Orthogonal-to-Skin (POS), and Principal Component Analysis (PCA), applied to selected superpixel-based facial regions (with target counts of 10 and 20 regions) for monitoring in a driving simulator. Two novel peak enhancement approaches, based on the Lp norm and Fractional-Order Derivative (FOD), are introduced to enable robust Heart Rate Variability (HRV) estimation. A signal-to-noise ratio-based quality assessment of 20 s segments serves as a data cleaning mechanism to mitigate motion artifacts inherent to dynamic recording conditions. In a sample of 29 participants recorded during baseline and driving simulation conditions, Pulse Rate (PR) is calculated with clinically acceptable accuracy across configurations (validated against simultaneous Electrocardiography (ECG) recordings), achieving the lowest Mean Absolute Error (MAE) of 1.92 bpm (sd = 1.72) using 2SR with FOD and 20 superpixel regions. The best-case MAE reached 0.061 s for Standard Deviation of Normal-to-Normal intervals (SDNN) and 0.081 s for Root Mean Square of Successive Differences (RMSSD), with inter-beat interval detection yielding an F1 score of 0.93. Optimal parameters clustered around p = 6-7 for Lp norm and fractional orders of 1.0-1.4. All rPPG-derived parameters reproduced the statistical structure of the reference ECG across conditions and configurations. Caution is advised when using FOD due to slow changes in the rPPG waveform. Overall, 2SR is recommended for PR, while CHROM for HRV estimation, using Lp norm with 20 superpixels, providing clear methodological guidance for rPPG monitoring in driving simulators

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper evaluates rPPG algorithms (2SR, CHROM, POS, PCA) on superpixel facial regions (10 or 20) for remote PR and HRV monitoring in a driving simulator. It introduces Lp-norm and fractional-order derivative (FOD) peak enhancement, applies SNR-based quality assessment to 20 s segments for motion-artifact mitigation, and validates against simultaneous ECG in 29 participants. It reports lowest MAE of 1.92 bpm (sd=1.72) for PR using 2SR+FOD+20 regions, best-case MAEs of 0.061 s (SDNN) and 0.081 s (RMSSD), F1=0.93 for IBI detection, reproduction of ECG statistical structure, optimal parameters p=6-7 and fractional order 1.0-1.4, and recommends 2SR for PR and CHROM for HRV with Lp norm and 20 superpixels.

Significance. If the central claims hold, the work supplies concrete, directly validated performance numbers for rPPG under realistic motion, together with explicit parameter ranges and configuration recommendations that could guide practical deployment in simulators or vehicles. The simultaneous ECG reference and reporting of both PR and HRV metrics (SDNN, RMSSD) plus F1 scores constitute a clear strength.

major comments (2)
  1. [Abstract / Methods (SNR quality assessment)] Abstract (data-cleaning paragraph) and Methods (SNR quality assessment): the manuscript states that an SNR-based gate on 20 s segments is used to remove motion artifacts but supplies no quantitative check that the retained segments preserve the original distribution of HRV statistics. No Kolmogorov-Smirnov test, paired comparison of SDNN/RMSSD before vs. after filtering, or per-condition discard fraction is reported. Because the headline claim that “all rPPG-derived parameters reproduced the statistical structure of the reference ECG” is made on the filtered data only, this omission is load-bearing for the validity of the reproduction result.
  2. [Results (parameter optimization)] Results (parameter-optimization paragraph): the reported optimal ranges (p = 6-7, fractional order 1.0-1.4) are presented without stating whether they were obtained by participant-wise cross-validation, nested validation, or post-hoc selection on the pooled data. If the latter, the quoted “best-case” MAEs may be optimistically biased and the recommendation of specific configurations requires re-evaluation.
minor comments (2)
  1. [Abstract] Abstract: the phrase “clinically acceptable accuracy” is used without citing the specific clinical tolerance thresholds (e.g., <5 bpm or <10 % error) against which the 1.92 bpm MAE is judged.
  2. [Methods] Notation: Lp norm order p and fractional order are introduced without an explicit equation or reference to the precise definition employed (e.g., the fractional derivative operator).

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments, which highlight important aspects of methodological transparency. We address each major comment below and will revise the manuscript to incorporate the requested clarifications and additional analyses.

read point-by-point responses
  1. Referee: [Abstract / Methods (SNR quality assessment)] Abstract (data-cleaning paragraph) and Methods (SNR quality assessment): the manuscript states that an SNR-based gate on 20 s segments is used to remove motion artifacts but supplies no quantitative check that the retained segments preserve the original distribution of HRV statistics. No Kolmogorov-Smirnov test, paired comparison of SDNN/RMSSD before vs. after filtering, or per-condition discard fraction is reported. Because the headline claim that “all rPPG-derived parameters reproduced the statistical structure of the reference ECG” is made on the filtered data only, this omission is load-bearing for the validity of the reproduction result.

    Authors: We agree that quantitative verification of the SNR filtering's impact on HRV distributions is necessary to support the reproduction claim. In the revised manuscript, we will add a Kolmogorov-Smirnov test comparing SDNN and RMSSD distributions before versus after the SNR gate, paired comparisons where appropriate, and the per-condition fraction of discarded segments. These additions will be reported in the Methods and Results sections. revision: yes

  2. Referee: [Results (parameter optimization)] Results (parameter-optimization paragraph): the reported optimal ranges (p = 6-7, fractional order 1.0-1.4) are presented without stating whether they were obtained by participant-wise cross-validation, nested validation, or post-hoc selection on the pooled data. If the latter, the quoted “best-case” MAEs may be optimistically biased and the recommendation of specific configurations requires re-evaluation.

    Authors: The reported ranges were identified via grid search on the pooled dataset. We acknowledge this can introduce optimistic bias in the best-case MAEs. In the revision, we will explicitly describe the optimization procedure, note the potential for bias as a limitation, and add a participant-wise cross-validation analysis to re-evaluate and confirm the robustness of the recommended configurations (p=6-7, fractional order 1.0-1.4). revision: yes

Circularity Check

0 steps flagged

No significant circularity; results are direct empirical comparisons to ECG reference

full rationale

The paper's central claims consist of MAE, SDNN/RMSSD errors, and F1 scores obtained by comparing rPPG-derived quantities (from 2SR/CHROM/POS/PCA with Lp-norm or FOD peak enhancement) against simultaneous ECG recordings on the same 20 s segments. These metrics are computed externally and do not reduce, by any equation in the paper, to quantities defined by the paper's own fitted parameters or preprocessing choices. The SNR-based segment filter is a data-cleaning step whose effect on the retained distribution is not itself used to define the reported accuracies. No self-citation chains, uniqueness theorems, or ansatzes imported from prior author work appear in the provided text; the derivation chain is standard signal processing followed by independent validation against an external reference.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The work is an empirical validation study that relies on standard domain assumptions from rPPG literature rather than new mathematical derivations; free parameters are the tuned values for the introduced enhancement methods.

free parameters (2)
  • Lp norm order p = 6-7
    Optimal parameters clustered around p = 6-7 for the Lp norm peak enhancement method.
  • fractional order = 1.0-1.4
    Optimal fractional orders of 1.0-1.4 for the FOD peak enhancement method.
axioms (2)
  • domain assumption Facial video color changes can serve as a proxy for cardiac pulse despite head motion in a driving simulator
    Central premise enabling all rPPG comparisons to ECG in dynamic conditions.
  • domain assumption SNR-based segment selection removes motion artifacts without systematically biasing HRV statistics
    Invoked as the data cleaning mechanism to mitigate motion artifacts.

pith-pipeline@v0.9.1-grok · 5899 in / 1619 out tokens · 31593 ms · 2026-06-30T12:15:47.003301+00:00 · methodology

discussion (0)

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Reference graph

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