Emotional stimuli draw attention and give rise to electroencephalography (EEG) markers specifically related to affective processing. The aim of this study was to examine the eye movement behavior and oscillatory responses in the EEG when the subjects were viewing emotional and neutral images. Co-registration of eye movements and EEG data was used in order to present the emotional stimuli in a more naturalistic and complex setting.
Studies on brain oscillatory activity have identified several frequency bands in which activity varies as a response to emotional stimulus content. Oscillatory activity in the theta band has been shown to vary in response to images, faces and music with emotional content in several studies, and was therefore chosen as the EEG component of interest in this study. Eye movements were analyzed in order to see whether emotional images attract attention faster and for a longer duration than neutral images, and whether the first fixation in a trial usually lands on an emotional image, proving that emotional content does indeed attract attention parafoveally. The eye movement variables used in the study were target entry time, number of fixations and dwell time (i.e., the sum of fixation durations on the target region).
The EEG and eye movements of 19 subjects were recorded while they were freely viewing the stimulus images. The images were presented so that subjects saw four images at once in each corner of the computer screen. One of the images was a target image, which was either negative, positive or neutral in emotional content. The other three images were neutral. The low-level visual features of the images were controlled for. Information about where the subject’s gaze was directed at each time point was used to time-lock the EEG analysis to those time points when the subject was looking at a target image.
The EEG analysis demonstrated that activity in the theta frequency-band synchronized more in response to emotional than neutral images across the scalp. The observed effect occurred during ~100-600 ms after the first transitional saccade to the target image, and was more prominent for negative than positive images. This result confirms the role of theta oscillations in emotional processing also in a more naturalistic setting (i.e., during free viewing). The analysis of eye movement revealed that emotional images received more fixations and they were looked at for longer than neutral images. Emotional (especially negative) images were fixated on faster than neutral images, which confirms prior findings suggesting that subjects are able to identify the emotional content of an image parafoveally. The results from this study also validate the use co-registration of eye movements and EEG during a free viewing paradigm to study visually induced brain responses.
Acknowledgements: Teemu Peltonen
This study was funded by Helsingin Sanomat Foundation