Top-down modulation of attentional capture in vision: Electrophysiological investigations

Imagine standing in a busy airport terminal scanning the departure board for information about your flight. While you attend to the board, other sensory information such as the sight and sound of people passing by, or announcements from the PA system, will go largely unnoticed. Directing attention intentionally (or 'endogenously') to specific objects or events enables us to selectively ignore other sensory information that is irrelevant to our current intentions. Endogenous attention is manifest in everyday life - it shapes our perceptual experience and allows us to deal with a complex multisensory world. Without it the world would be a 'blooming, buzzing confusion' (William James). However, endogenous attentional control is sometimes interrupted by external events. Even while scanning the departure board, your attention will be diverted by abrupt, highly salient, or unexpected stimuli, such as a flash of light or a gunshot. Such events are said to capture attention involuntarily, or 'exogenously'. This distinction between the endogenous control and the exogenous capture of attention is fundamental for our understanding of how we interact with the external world. In many cases, we are in charge of the contents of our sensory-perceptual experience, but sometimes events in the world dictate what we become aware of. Until recently, it seemed obvious that endogenous and exogenous attention represent functionally and anatomically distinct modes of attention. However, recent evidence has suggested that attentional capture by salient visual stimuli (such as a unique red item among many uniformly green stimuli) is not really 'exogenous', but is in fact controlled by current intentions. In other words, the external world is never completely in control of our perceptual experience, which is always also determined by what is relevant to us. Although the question whether and under which conditions salient events get noticed when attention is intentionally focused elsewhere is important for theories of perception and cognition, it remains largely unresolved. This is due to the fact that previous research on this issue has used mainly behavioural performance measures, which appear insufficient to provide clear-cut answers. In the present project, we will use electrophysiological markers of attentional capture that can be obtained non-invasively from EEG scalp electrodes during experimental tasks. These markers reflect the activity of brain processes underlying attentional selection processes on a millisecond-by-millisecond basis, and will be used to discover when and how salient visual stimuli will capture observers' attention in a purely exogenous fashion. We will record such markers while participants that perform visual selection tasks where the task relevance of salient visual stimuli that are presented together with less salient background stimuli is manipulated in order to find out whether and how attentional capture is mediated by intentional control. The aim of this project is to provide new insights into how sensation and intention interact to create perceptual experience. Results will also have important implications from an applied perspective, for example with respect to the design of user interfaces and complex control systems. Efforts to increase the noticeability of important visual signals in noisy environments (such as airport terminals) may have to consider the finding that the capacity of such signals to capture attention is not simply determined by their bottom-up saliency, but also by the current intentional context.

The question whether the capture of attention by salient visual events (such as singleton stimuli in a visual search task) is triggered in a purely exogenous fashion or is contingent upon intentional task sets remains largely unresolved. We will use event-related brain potential (ERP) markers of attentional selection processes to clarify the interactions between top-down and bottom-up attention, and to resolve some of the long-standing controversies in the field. High-density EEG will be recorded non-invasively from scalp electrodes while participants perform different visual selection tasks where the task-relevance of stimulus features, dimensions, or locations is systematically manipulated. The N2pc component (a unique marker for brain processes underlying the attentional selection of visual events that can be obtained on a millisecond-by-millisecond basis) will be measured to determine the presence of attentional capture and how capture is modulated by top-down control. Other ERP components will be measured to dissociate attentional and post-selective loci for top-down modulations of behavioural capture effects, and to obtain evidence for top-down inhibitory control in visual search. We will investigate (i) the role of dimension- or feature-specific task sets, or generalised attentional modes for attentional capture; (ii) whether the efficiency of attentional target selection is modulated across trials by top-down 'dimension weights'; (iii) whether target selection in visual search is mediated in a top-down fashion by inhibitory 'visual marking'. In a more explorative mode, we will use interleaved ERP/TMS protocols recently developed in the applicant's lab to study the impact of TMS-induced disruptions of top-down control on performance and on ERP markers of attentional selectivity.