Vision & Brain Signal Processing (ViBS)

Coordinator :

Gaze-EEG co-registration is the joint the electroencephalography (EEG) and eye-tracking (ET) acquisition. This co-registration technique is an effective way of delving into cognitive processes. It also allows for more ecological experimental protocols, which are not constrained by "external" markers, such as the onset of a word or image, and in which participants can perform fixations at their own pace. This type of joint acquisition therefore has a strong added value for the understanding of the time course of neural activity during cognitive tasks requiring a high degree of accuracy in the synchronization of timings. To do so, we have developed a synchronization method between EEG and gaze signals. Synchronization through common triggers is widely used in all existing methods, because it is very simple and effective. However, this solution has been found to fail in certain practical situations, namely for the spurious detection of triggers and/or when the timestamps of triggers sampled by each acquisition device are not jointly distributed linearly for the entire duration of an experiment. We propose two additional mechanisms, the "Longest Common Subsequence" algorithm and a piecewise linear regression, in order to overcome the limitations of the classical method of synchronizing common triggers. The resulted method is implemented in C++ as a DOS application which is very fast, robust and fully automatic. All is available in the Zenodo repository (https://doi.org/10.5281/zenodo.5554071) with examples on real data.

Let us illustrate the results on a dataset with two pauses during recording. After triggers matching, we observe the temporal shifts between their timestamp in each modality (figures (a), (b)). After synchronization, we observe (figure (c)) that the distribution of the alignment error between the two modalities is well realigned, and that its support is restricted to ±2 samples due to the computational rounding.

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After synchronization, EEG signals, triggered by eye events, are called Eye Fixation or Saccade Related Potentials (EFRP / ESRP). However, there are two major concerns with estimating visually-evoked potentials (EFRP or ESRP): (1) the overlap with the potential elicited at stimulus onset and the overlap with the visually-evoked potentials with each other and (2) the influence of eye movement on the EEG amplitude.  For these estimations, numerous deconvolution methods had been developed. Among these, the most used are based on the Generalized Linear Model associated with spline regression to take into account confounding variables and/or to study the impact of a co-variable of interest on the amplitude of the EEG signal. See section “Intra-saccadic motion perception” for an application example.

Coordinator:

Anne Guérin-Dugué (PR, ViBS)

Involved researchers:

Emmanuelle Kristensen (IR, ViBS), B. Rivet (MCF, ViBS)

Coordinator :

Anne Guérin-Dugué (PR, ViBS)

Involved researchers:

E. Kristensen (IR, ViBS), Gaëlle Nicolas (Phd, ViBS)

In collaboration with:

Eric Castet (DR, LPC, AMU, Marseille), Michel Dojat (DR, GIN, UGA, Grenoble)

Perceiving is essentially an active decision process, making sense of uncertain sensory signals. Indeed, the information about our environment is often noisy or ambiguous at the sensory level, and it must be processed and disambiguated to yield a coherent perception of the world in which we evolve. For instance, in vision we only have access to two-dimensional (2D) projections of the surrounding 3D environment, which leads to an intrinsic ambiguity for perception.

We aim at gathering behavioural and physiological evidence about the three key functions in the closed-loop processing of human visual perception— expectation, exploration, and exploitation, in order to better understand and
model their interaction.

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The main interest is of this project is to find (electro-)physiological markers of evidence accumulation that could feed a drift-diffusion model underlying active vision. The resulting drift-diffusion process feeds a Markov decision process, and we use ambiguous visual stimuli in order to force alternating perceptual decisions within subjects. In order to support our hypothesese, we collect electro-encephaolgraphic (EEG), gaze, and behavioural data and interventional transcranial magnetic stimulation (TMS) at the IRMaGe platform using stimuli built with the psychopy software. Data is analysed with in-house algorithms, leveraging on the possibilites of the MNE software for EEG analysis.

Coordinator :

Ronald Phlypo

This research project aims at developing new tools and making theoretical advances in the field of applied Riemannian geometry. For instance:

• visualization and manipulation of data in the Riemannian manifold of symmetric positive definite matrices (Congedo and Barachant, 2014)
• an algorithm for estimating the geometric mean of a set of positive-definite matrices using approximate joint diagonalization (Congedo et al., 2015).
• a fast algorithm for estimating power means of positive-definite matrices (Congedo et al., 2017).
• the solution to the Procrustes problem for positive-definite matrices (Bhatia and Congedo, 2019).
• finding approximate joint diagonalizers using Riemannian optimization (Alyani et al., 2017; Bouchard et al., 2018; Bouchard et al., 2020)

Coordinator :

Coordinator :