The steady-state visual evoked potential (SSVEP), a neural signature of attentional resource allocation, is enhanced for affective compared to neutral visual scenes. Recently, it has been demonstrated that modulation of early visual cortex associated with viewing of unpleasant scenes presented in a rapid succession relies on emotional content extraction from each individual image shown in a rapid serial visual presentation (RSVP) at 6 Hz (~167 ms per image). Against expectations, the SSVEP was reduced when viewing visual streams of unpleasant compared to neutral scenes. Here, we investigated to what extent that finding was limited to the 6 Hz rate and whether it generalizes to pleasant pictures. We recorded SSVEPs elicited by RSVP of neutral and emotional scenes presented at 3, 4, 6.67, and 8.57 Hz rates. We demonstrated that SSVEP amplitudes were enhanced for unpleasant compared to neutral images with a presentation rate of 3, 4, and 8.57 Hz. By contrast, SSVEP decreased for both pleasant and unpleasant relative to neutral RSVP streams shown at 6.67 Hz. Our findings suggest that a linear superposition of ERPs evoked by each individual image in an RSVP may lead to SSVEP amplitude patterns that increase or decrease the power at the driving frequency, which in turn might produce the observed differential emotional amplitude modulations. The results provide new methodological considerations for investigating temporal dynamics of early visual cortex modulation during sustained perception of affective scenes with SSVEP using RSVP paradigms.

Emotionally arousing stimuli are known to rapidly draw the brain's processing resources, even when they are task-irrelevant. The steady-state visual evoked potential (SSVEP) response, a neural response to a flickering stimulus which effectively allows measurement of the processing resources devoted to that stimulus, has been used to examine this process of attentional shifting. Previous studies have used a task in which participants detected periods of coherent motion in flickering random dot kinematograms (RDKs) which generate an SSVEP, and found that task-irrelevant emotional stimuli rapidly withdraw attentional resources from the task-relevant RDKs. However, it is not clear whether the changes in the SSVEP response are conditional on higher-level extraction of emotional cues as indexed by well-known event-related potential (ERPs) components, or if affective bias in competition for visual attention resources could be a consequence of an inherent, relatively time-invariant shifting process. In the present study, we used two different types of emotional distractors -IAPS pictures and facial expressions -for which emotional cue extraction occurs at different speeds, being typically earlier for faces (at ~170 ms, as indexed by the N170) than for IAPS images (220-230 ms, Early Posterior Negativity, EPN). We found that attentional resources were withdrawn from the foreground task towards task-irrelevant emotional background images from the International Affective Picture System (IAPS) following the extraction of emotional cues as indexed by visual ERP components. We also found that emotional modulation of attentional resources as measured by the SSVEP occurred earlier for faces (around 180 ms) than for IAPS pictures (around 400 ms). This is consistent with lowlevel attentional resources being re-allocated after emotional cue extraction rather than being linked to a time-fixed shifting process.

The steady-state visual evoked potential (SSVEP), a neurophysiological marker of attentional resource allocation with its generators in early visual cortex, exhibits enhanced amplitude for emotional compared to neutral complex pictures. Emotional cue extraction for complex images is linked to the N1-EPN complex with a peak latency of ∼140-160 ms. We tested whether neural facilitation in early visual cortex with affective pictures requires emotional cue extraction of individual images, even when a stream of images of the same valence category is presented. Images were shown at either 6 Hz (167 ms, allowing for extraction) or 15 Hz (67 ms per image, causing disruption of processing by the following image). Results showed SSVEP amplitude enhancement for emotional compared to neutral images at a presentation rate of 6 Hz but no differences at 15 Hz. This was not due to featural differences between the two valence categories. Results strongly suggest that individual images need to be displayed for sufficient time allowing for emotional cue extraction to drive affective neural modulation in early visual cortex.

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