## 4 Discussion
### Seasonality, Valence, and EEG Band-Power  
In contrast to behavioral results, we found significant group differences in EEG band-power responses to emotional pictures, and this difference interacted with emotional valence of the pictures. EEG band-power was highest for positive pictures, followed by negative pictures, and lowest for neutral pictures. A difference between the two groups with low and high seasonality was found only for responses to emotional pictures, i.e., those with negative and positive valence, but not for neutral pictures, indicating that there is an emotional effect when processing the pictures, which enhances the group difference. All the significant group effects resulted in higher EEG frequency power for participants who had low scores of seasonality. This is in line with previous reports that found a general difference in brain activation between patients with SAD and healthy controls that was independent of emotional reactivity [21; 51]. The localization of this effect is not as exact in the EEG as in fMRI, but we narrowed down the region of interest to lobes that were reported previously to be important for emotional processing [62]. We found that the difference between people with low and high seasonality scores was particularly strong over the temporal lobe when viewing negative or positive pictures. Our findings resemble earlier results that found differential hemisphere x valence interactions in EEG band-power over the temporal lobe [63; 64]. According to our data, the right hemisphere seems to play a more important role as it responds differentially in the two groups, especially during viewing of negative pictures. Less clear but consistent effects can be found for positive pictures, which are lateralized to the left hemisphere. This left-right differentiation for positive and negative stimuli is consistent to what was found in theta and alpha EEG band-power previously [63; 64]. Furthermore, this finding supports the valence hypothesis, which assumes that the left hemisphere is dominant for positive emotions while the right hemisphere is dominant for negative emotions [65]. The finding that our results are significant in the right hemisphere, while they are significant only before but not after correcting for multiple comparisons in the left hemisphere, supports the right hemisphere model of brain asymmetry in emotional processing, which states a dominant role of the right hemisphere [65]. Therefore, our findings support the claim that the valence hypothesis and the right hemisphere model of brain asymmetry in emotional processing are not mutually exclusive [66]. The enhanced group difference over the right hemisphere for negative stimuli is also in agreement with previously reported reduced lateralization of emotional processing in people at risk for depression [11]. It should also be noted that negative stimuli seem to be generally prioritized in processing [67], which might explain the enhanced difference for negative stimuli between people with and without seasonality symptoms. Another interesting dissociation in the alpha band is that group differences were detected in the early time-window (50-150 ms post-stimulus) for positive pictures but the later time-window (300-400 ms) for negative pictures. This effect is significant only before but not after correction for multiple comparisons in the gamma range, but with a similar pattern (early for positive pictures, both time-windows for negative pictures). Earlier timing of positive stimulus processing vs. later timing of negative stimulus processingcorrelates in the alpha range was reported previously [63] and our results suggest that this timing is affected by seasonality. Gamma-band reactivity to emotional stimuli, especially of negative valence, differs between patients with depression and healthy controls [68]. A common assumption is that gamma-band involvement reflects the degree to which a stimulus is consciously processed [40].