Early visual processing allows for selective behavior, shifts of attention, and conscious visual experience in spite of masking.

Object-substitution masking (OSM) occurs when a briefly displayed target in a search array is surrounded by a mask, which remains onscreen after the target has disappeared. It has been suggested that OSM results from a specific interference with reentrant visual processing, while the initial feedforward processing is left intact. Here, we tested the prediction that the fastest saccadic responses towards a masked target, supposedly triggered before the onset of reentrant processing, are not impaired by OSM. Indeed, saccades faster than 350ms "escaped" the influence of the mask. Notably, participants' judgements of subjective awareness indicated that stimulus processing during this early stage is not entirely devoid of conscious awareness. Furthermore, the N2pc event-related potential component indicated shifts of spatial attention towards the masked targets on trials with correct fast saccades, suggesting that both target detection and spatial attention can be based on the computations accomplished during the initial feedforward sweep.

Probing the dynamics of perisaccadic vision with EEG.

Eye movements (saccades) are a necessary component of vision - with a sequence of saccades, we can explore the entire visual scene quickly. The role of saccades is beyond mechanical, in fact they are closely tied to mechanisms of visual cognition, in particular to spatial attention. Two physiological processes, pre-saccadic shifts of attention and saccadic suppression illustrate the interplay of saccades and visual perception: while the attentional shift enhances visual sensitivity to the future saccade target, saccadic suppression inhibits it for non-target objects. Both processes are transient and active already before the saccade onset. So far, these processes have been studied primarily behaviorally in human subjects or via invasive electrophysiological recordings in monkeys. Despite considerable advances, a physiological characterization of these two mechanisms is still lacking in the human literature. Here, we will present a new methodology for investigating perisaccadic events in human subjects. Using electroencephalography (EEG) co-registered with eye tracking (ET), we sampled the complete time-course of perisaccadic activity and identified two ERP components - pre-saccadic voltage enhancement and post-saccadic voltage reduction - that likely relate to shifts of spatial attention and saccadic suppression.

A pool of pairs of related objects (POPORO) for investigating visual semantic integration: behavioral and electrophysiological validation.

Semantic processing of verbal and visual stimuli has been investigated in semantic violation or semantic priming paradigms in which a stimulus is either related or unrelated to a previously established semantic context. A hallmark of semantic priming is the N400 event-related potential (ERP)--a deflection of the ERP that is more negative for semantically unrelated target stimuli. The majority of studies investigating the N400 and semantic integration have used verbal material (words or sentences), and standardized stimulus sets with norms for semantic relatedness have been published for verbal but not for visual material. However, semantic processing of visual objects (as opposed to words) is an important issue in research on visual cognition. In this study, we present a set of 800 pairs of semantically related and unrelated visual objects. The images were rated for semantic relatedness by a sample of 132 participants. Furthermore, we analyzed low-level image properties and matched the two semantic categories according to these features. An ERP study confirmed the suitability of this image set for evoking a robust N400 effect of semantic integration. Additionally, using a general linear modeling approach of single-trial data, we also demonstrate that low-level visual image properties and semantic relatedness are in fact only minimally overlapping. The image set is available for download from the authors' website. We expect that the image set will facilitate studies investigating mechanisms of semantic and contextual processing of visual stimuli.