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Face Selective Neural Activity: Comparisons Between Fixed and Free Viewing

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Abstract

Event Related Potentials (ERPs) are widely used to study category-selective EEG responses to visual stimuli, such as the face-selective N170 component. Typically, this is done by flashing stimuli at the point of static gaze fixation. While allowing for good experimental control, these paradigms ignore the dynamic role of eye-movements in natural vision. Fixation-related potentials (FRPs), obtained using simultaneous EEG and eye-tracking, overcome this limitation. Various studies have used FRPs to study processes such as lexical processing, target detection and attention allocation. The goal of this study was to carefully compare face-sensitive activity time-locked to an abrupt stimulus onset at fixation, with that time-locked to a self-generated fixation on a stimulus. Twelve participants participated in three experimental conditions: Free-viewing (FRPs), Cued-viewing (FRPs) and Control (ERPs). We used a multiple regression approach to disentangle overlapping activity components. Our results show that the N170 face-effect is evident for the first fixation on a stimulus, whether it follows a self-generated saccade or stimulus appearance at fixation point. The N170 face-effect has similar topography across viewing conditions, but there were major differences within each stimulus category. We ascribe these differences to an overlap of the fixation-related lambda response and the N170. We tested the plausibility of this account using dipole simulations. Finally, the N170 exhibits category-specific adaptation in free viewing. This study establishes the comparability of the free-viewing N170 face-effect with the classic event-related effect, while highlighting the importance of accounting for eye-movement related effects.

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dipoles used in the model. b Position (loc) and orientation (ori) of the modeled dipoles in Talairach coordinates. c The dynamics of each dipole. P1\lambda dipoles peak at 90 ms, N1 dipoles peak at 170 ms

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Notes

  1. In our design, categorical information of the stimulus was hindered in parafoveal view (see "Materials and Methods" section).

  2. Saccade detection was qualitatively verified by comparison to the Engbert et al. paradigm (Engbert & Kliegl, 2003). Note that the fixations analyzed in this study follow macro- rather than micro-saccades.

  3. Note we used a very large time window to preclude possible long term effects of fixation onset but this might not be necessary and adds a large number of predictors to the model.

  4. Memory problems occurred when we used the default MATLAB solver for the regression analysis since our design matrix was extremely large. An alternative to solving the regression in Python is to use the LSMR solver for MATLAB as used in the Unfold toolbox (Ehinger & Dimigen, 2019).

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Acknowledgements

This work is supported by grant 1902/14 from the Israel Science Foundation to L.Y.D. L.Y.D is supported by the Jack H. Skirball research fund. We thank Dr. Edden Gerber for his MATLAB implementation of the pre-processing procedure. We thank Chen Gueta, Namoi Revel and Chen Berkman with their help in the data collection, Oded Wertheimer for programming, Gal Chen for help with the statistical analysis and all the lab members for their valuable comments.

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Correspondence to Carmel R. Auerbach-Asch.

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The authors declare no conflict of interests.

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All procedures complied with the ethical regulations of the Hebrew University of Jerusalem for experiments involving human subjects and was approved by the institutional review board and subjects signed an informed consent prior to their participation. The study was funded by a grant from the Israel Science foundation to LYD.

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Auerbach-Asch, C.R., Bein, O. & Deouell, L.Y. Face Selective Neural Activity: Comparisons Between Fixed and Free Viewing. Brain Topogr 33, 336–354 (2020). https://doi.org/10.1007/s10548-020-00764-7

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  • DOI: https://doi.org/10.1007/s10548-020-00764-7

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