sss.h File Reference

Signal-Space Separation (SSS) and temporal SSS (tSSS) for MEG data. More...

#include "dsp_global.h"
#include <fiff/fiff_info.h>
#include <Eigen/Core>
#include <QVector>

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Classes
struct	UTILSLIB::SSSParams
	Implements Signal Space Separation (SSS) and temporal SSS (tSSS) for MEG data. More...
class	UTILSLIB::SSS
struct	UTILSLIB::SSS::Basis
	Precomputed SSS basis and projectors for a given sensor array. More...

Namespaces
namespace	UTILSLIB
	Shared utilities (I/O helpers, spectral analysis, layout management, warp algorithms).

Detailed Description

Signal-Space Separation (SSS) and temporal SSS (tSSS) for MEG data.

SPDX-License-Identifier: BSD-3-Clause Copyright (c) 2026 MNE-CPP Authors

Author

Christoph Dinh chris.nosp@m.toph.nosp@m..dinh.nosp@m.@mne.nosp@m.-cpp..nosp@m.org

Since

2.1.0

Date

March 2026

SSS represents the magnetic field measured by an array of MEG sensors as a series expansion in vector spherical harmonics centred on the head. Because Maxwell's equations decouple the field generated by sources inside the expansion sphere from the field generated by sources outside it, the harmonic basis splits naturally into an "internal" subspace (brain signals of interest) and an "external" subspace (environmental interference, distant artefacts). Reconstructing the data from the internal subspace alone gives a sensor-space representation that retains cortical activity while suppressing magnetic disturbances that originate outside the helmet.

tSSS adds a second pass: components that are temporally correlated between the internal and external reconstructions are attributed to near-by interference (subject movement artefacts, contaminated channels) and projected out. The correlation threshold and time-window length are the two main tSSS knobs.

References: Taulu, S. & Kajola, M., "Presentation of electromagnetic multichannel data: The signal space separation method." J. Appl. Phys. 97, 124905 (2005).

Taulu, S. & Simola, J., "Spatiotemporal signal space separation method for rejecting nearby interference in MEG measurements." Phys. Med. Biol. 51, 1759–1768 (2006).

SSS separates the MEG signal into contributions from internal sources (brain) and external sources (environmental noise) by decomposing sensor data into a basis of spherical harmonics. tSSS additionally removes internal-space components that correlate with the external subspace in sliding time windows, suppressing near-field artefacts (e.g. implants, dental work).

The spherical harmonic basis uses 4π-normalised real harmonics. Integration over each coil is approximated by a single-point model using the coil-centre position and normal direction from FiffChInfo (suitable for standard MEG; for sub-millimetre accuracy use FwdCoilSet).

Definition in file sss.h.