When does the Brain Respond to Information during Visual Scanning?

Aims: High stress work environments such as a flight deck (or surveillance systems) present operators with multiple instruments that have to be constantly monitored with eye-movements. Eye-tracking allows us to infer when an operator’s overt attention has been assigned to an instrument, namely when fixation begins. However, the brain could already be processing information prior to its fixation. When does the brain respond to information when the operator is free to scan the environment freely? Is it the emergence of a target stimulus? Or rather the fixation on a target stimulus? Methods: In our study, participants were required to continuously monitor four regions-of-interest (ROIs) that presented 3-letter-strings and to respond with a key-press to the appearance of a target string. This is comparable to tasks such as instrument monitoring. Allowing for self-paced visual scanning gave rise to two different conditions in our study. These two conditions differ with respect to the sequence of target appearance and target fixation: 1) There was a fixation on the target position before the target appeared. 2) The target appeared before there was a fixation on the target. In the latter condition, the event evoking a change in the EEG signal could either be the target onset or the fixation onset (i.e., start of target fixation). All ERPs from the naturalistic visual scanning scenario were compared to the ERP of a baseline condition that only had one ROI and prohibited eye-movements. Results: Our results show that ERPs that were epoched to the target onset were similar, regardless of eye-movements (black, red, blue lines). In contrast, the ERP that was epoched to the fixation onset was atypical (pink line). Conclusion: It is commonly assumed that a visual stimulus is processed when it is fixated. However, we demonstrate that visual perception can take place even prior to fixation. Experimental studies have shown how far peripheral vision is sufficient for recognizing animals in complex scenes 1. Here, we show that target onsets gave rise to brain responses even before the targets were fixated. This poses a challenge for the use of EEG/ERP in visual scanning environments. If fixation onset is not necessarily the onset of perception, they cannot always be used for epoching ERP data as has been the assumption of previous research 2,3 in natural scene viewing and reading. Future studies that seek to employ EEG/ERP measures in visual scanning work environments should take into account that the brain can respond to to-be-fixated information prior to fixation and to exercise caution in determining when this is