Time course of the P300 Eye-Fixation Related Potential during the visual search for a target embedded in natural scenes
Visual Science Society, 2015
Hélène Devillez, Emmanuelle Kristensen, Nathalie Guyader, Bertrand Rivet & Anne Guérin Dugué
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The Event Related Potential P300 has been intensively studied for over forty years (Polish, 2012). It is elicited at each updating of the stimulus representation, reflecting a cascade of cognitive processes engaging attentional and memory mechanisms. This potential is a positive component, appearing 300ms after stimulus onset, with maximal amplitude in centro-parietal regions. This study is mainly concerned by the analysis of the appearance and evolution of the P300 elicited by the onset of successive fixations recorded during a visual search using an ecological experimental paradigm. This goal is achieved through the co-registration of electroencephalographic (EEG) and eye-tracking (ET) signals. EEG and ET signals were recorded for thirty-four observers when exploring natural scenes during 5 sec to answer if a target object was or not present. To solve the main difficulty due to overlaps between the observed Eye Fixation Related Potentials (EFRP) elicited by consecutive fixations, we have used a new methodology based on the xDawn algorithm (Rivet, et al.; 2009). From each trial, a temporal window of 2s centered on the first fixation inside the Region of Interest (target). This duration was chosen to include five consecutive fixations (two before the first entrance, and two after). The fixations were tagged depending to their rank and position (inside/outside) the ROI. Neural activities were modelled as a cumulative response of specific latent responses elicited by each fixation with the corresponding onset. After xDawn estimation, the topographic maps have shown a potential identified as the P300 component elicited by the visual input through the consecutive fixations landed on the ROI. This result confirms previous studies but has been obtained for the first time on ecological paradigm thanks to the co-registration of EEG and ET signals and a new methodology to denoise potentials from overlaps of potentials elicited by consecutive fixations.
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