Abstract
EEG studies suggest that the emotional content of visual stimuli is processed rapidly. In particular, the C1 component, which occurs up to 100 ms after stimulus onset and likely reflects activity in primary visual cortex V1, has been reported to be sensitive to emotional faces. However, difficulties replicating these results have been reported. We hypothesized that the nature of the task and attentional condition are key to reconcile the conflicting findings. We report three experiments of EEG activity during the C1 time range elicited by peripherally presented neutral and fearful faces under various attentional conditions: the faces were spatially attended or unattended and were either task-relevant or not. Using traditional event-related potential analysis, we found that the early activity changed depending on facial expression, attentional condition, and task. In addition, we trained classifiers to discriminate the different conditions from the EEG signals. Although the classifiers were not able to discriminate between facial expressions in any condition, they uncovered differences between spatially attended and unattended faces but solely when these were task-irrelevant. In addition, this effect was only present for neutral faces. Our study provides further indication that attention and task are key parameters when measuring early differences between emotional and neutral visual stimuli.
Highlights
To deal with the profusion and complexity of visual input, the brain has developed mechanisms to select relevant and important information
Experiment 1 explored the effect of emotional stimuli on C1 response in a nondemanding task while attention was diverted
Upright and inverted neutral and fearful faces were presented in the upper visual hemifield while the participants were asked to detect a change of colour of the fixation point
Summary
To deal with the profusion and complexity of visual input, the brain has developed mechanisms to select relevant and important information. Stimulus onset (see Pourtois, Schettino, & Vuilleumier, 2013 for review). The earliest visually evoked EEG component is the C1, which occurs within 100 ms of stimulus onset. Observation of the C1 is difficult, because its topography depends on the exact stimulus location in the visual field (Clark, Fan, & Hillyard, 1995; Jeffreys & Axford, 1972a, 1972b; Kelly, Gomez-Ramirez, & Foxe, 2008; Proverbio, Del Zotto, & Zani, 2007). Stimuli presented along the horizontal midline do generally not elicit a C1. For this reason, C1 studies present stimuli in the lower or upper visual hemifield. The sensitivity of the C1 to low-level stimulus features, such as contrast (Foxe et al, 2008), requires strict stimulus control
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