0. Abstract
Background Traditionally, EEG/MEG data are high-pass filtered and baseline-corrected to remove slow drifts. Minor deleterious effects of high-pass filtering in traditional time-series analysis have been well-documented, including temporal displacements. However, its effects on time-resolved multivariate pattern classification analyses (MVPA) are largely unknown.
New Method To prevent potential displacement effects, we extend an alternative method of removing slow drift noise – robust detrending – with a procedure in which we mask out all cortical events from each trial. We refer to this method as trial-masked robust detrending.
Results In both real and simulated EEG data of a working memory experiment, we show that both high-pass filtering and standard robust detrending create artifacts that result in the displacement of multivariate patterns into activity silent periods, particularly apparent in temporal generalization analyses, and especially in combination with baseline correction. We show that trial-masked robust detrending is free from such displacements.
Comparison with Existing Method(s) Temporal displacement may emerge even with modest filter cut-off settings such as 0.05 Hz, and even in regular robust detrending. However, trial-masked robust detrending results in artifact-free decoding without displacements. Baseline correction may unwittingly obfuscate spurious decoding effects and displace them to the rest of the trial.
Conclusions Decoding analyses benefits from trial-masked robust detrending, without the unwanted side effects introduced by filtering or regular robust detrending. However, for sufficiently clean data sets and sufficiently strong signals, no filtering or detrending at all may work adequately. Implications for other types of data are discussed, followed by a number of recommendations.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
↵* Author note: CNLO and JJF share senior authorship. JvD and CNLO designed the experiment. JvD and JJF designed and conducted the simulations and analyses. CNLO, JvD and JJF wrote the paper. This work was supported by ERC-CoG-2013-615423 grant from the European Research Council, and NWO Vici grant 453-16-002 both awarded to CNLO.
Minor revision, some minor changing of wording, added some references, added a figure.