Safe and sensible baseline correction of pupil-size data
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Abstract
Measurement of pupil size (pupillometry) has recently gained renewed interest from psychologists, but there is little agreement on how pupil-size data is best analyzed. Here we focus on one aspect of pupillometric analyses: baseline correction, that is, analyzing changes in pupil size relative to a baseline period. Baseline correction is useful in experiments that investigate the effect of some experimental manipulation on pupil size. In such experiments, baseline correction improves statistical power by taking into account random fluctuations in pupil size over time. However, we show that baseline correction can also distort data if unrealistically small pupil sizes are recorded during the baseline period, which can easily occur due to eye blinks, data loss, or other distortions. Divisive baseline correction (corrected pupil size = pupil size / baseline) is affected more strongly by such distortions than subtractive baseline correction (corrected pupil size = pupil size - baseline). We make four recommendations for safe and sensible baseline correction of pupil-size data: 1) use subtractive baseline correction; 2) visually compare your corrected and uncorrected data; 3) be wary of pupil-size effects that emerge faster than the latency of the pupillary response allows (within ±220 ms after the manipulation that induces the effect); and 4) remove trials on which baseline pupil size is unrealistically small (indicative of blinks and other distortions).
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2017. Safe and sensible baseline correction of pupil-size data. PeerJ Preprints 5:e2725v1 https://doi.org/10.7287/peerj.preprints.2725v1Author comment
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Competing Interests
The authors declare that they have no competing interests.
Author Contributions
Sebastiaan Mathôt conceived and designed the experiments, performed the experiments, analyzed the data, wrote the paper, prepared figures and/or tables, reviewed drafts of the paper.
Jasper Fabius wrote the paper, reviewed drafts of the paper.
Elle van Heusden performed the experiments, wrote the paper, reviewed drafts of the paper.
Stefan Van der Stigchel wrote the paper, reviewed drafts of the paper.
Human Ethics
The following information was supplied relating to ethical approvals (i.e., approving body and any reference numbers):
We have re-used data that was previously collected for a different purpose as part of another study. This other study was approved by the ethics committee of Utrecht University, and all participants had signed informed consent before participating. See https://doi.org/10.7287/peerj.preprints.1478.
Data Deposition
The following information was supplied regarding data availability:
Funding
The research leading to these results has received funding from: the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement n° 622738 awarded to Sebastiaan Mathôt. This work is part of the research programme VENI with project number 451-16-023, which is financed by the Netherlands Organisation for Scientific Research (NWO). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
