Estimating Learning Effects: A Short-Time Fourier Transform Regression Model for MEG Source Localization

TL;DR

This paper introduces STFT-R, a novel source localization method for MEG data that incorporates trial-by-trial learning effects and structured sparsity, improving interpretability and accuracy in identifying brain regions involved in perceptual learning.

Contribution

The paper develops STFT-R, an extension of the short-time Fourier transform approach that models learning effects and emphasizes regions of interest using hierarchical L21 penalties.

Findings

STFT-R outperforms traditional methods in simulated data source reconstruction.

STFT-R provides more interpretable insights into time-frequency components related to learning.

The method successfully identifies brain regions associated with perceptual learning in real MEG data.

Abstract

Magnetoencephalography (MEG) has a high temporal resolution well-suited for studying perceptual learning. However, to identify where learning happens in the brain, one needs to ap- ply source localization techniques to project MEG sensor data into brain space. Previous source localization methods, such as the short-time Fourier transform (STFT) method by Gramfort et al.([Gramfort et al., 2013]) produced intriguing results, but they were not designed to incor- porate trial-by-trial learning effects. Here we modify the approach in [Gramfort et al., 2013] to produce an STFT-based source localization method (STFT-R) that includes an additional regression of the STFT components on covariates such as the behavioral learning curve. We also exploit a hierarchical L 21 penalty to induce structured sparsity of STFT components and to emphasize signals from regions of interest (ROIs) that are selected according to prior knowl- edge. In reconstructing the ROI source signals from simulated data, STFT-R achieved smaller errors than a two-step method using the popular minimum-norm estimate (MNE), and in a real-world human learning experiment, STFT-R yielded more interpretable results about what time-frequency components of the ROI signals were correlated with learning.