No abstract

Lateralised ERP responses were measured over posterior visual brain regions in response to visual search arrays that contained one colour singleton. In the localisation task, responses were determined by the visual hemifield where this singleton was presented. In the discrimination task, they were determined by the singletons' shape. While an N2pc component was elicited in an identical fashion in both tasks, a subsequent sustained contralateral negativity was consistently present at posterior sites in the discrimination task only. This dissociation demonstrates that these two activations reflect distinct visual processing stages. We suggest that while the N2pc reflects the ability of the visual system to both identify and localise a relevant stimulus in the scene, the late sustained activity reflects the subsequent in-depth analysis and identification of these stimuli. Keywordsvisual-spatial attention; visual cognition; visual working memory; attentional control; eventrelated brain potentials Visual attention selects relevant objects in the environment to enable their localisation and identification and to ensure that response selection is based on appropriate perceptual information. The idea that the attentional processing of visual stimuli consists of, at least partially, separable and sequential processes (such as spatial selection and stimulus identification) is central to many models of visual cognition (e.g., Treisman & Gelade, 1980;Treisman, 1996).The aim of the present study was to investigate whether the process of localising relevant items in the scene, as reflected by attentional orienting processes, and further detailed analyses of the selected items can be reflected by distinct electrophysiological responses. We recorded event-related brain potentials (ERPs) in two visual search tasks that were identical with respect to physical stimulus parameters, and only differed in the level of visual processing required to determine the correct response. We presented circular arrays of twelve coloured diamond shapes that had a corner cut off on the left or right side (see Figure

To identify electrophysiological correlates of change detection, event-related brain potentials (ERPs) were recorded in two experiments where participants monitored displays containing four faces in order to detect and report a face identity change across two successive displays. ERPs on trials where a change was present, but was not detected, differed systematically from detectedchange trials and no-change trials. These ERP modulations reflect differences in advance task preparation (affecting short-latency components) and variations in the duration of perceptual and decision-related processing (affecting participants' confidence judgements and ERP amplitudes in the P3 time range). Successful change detection was mirrored by an N2pc component at posterior electrodes contralateral to the side of a change, suggesting close links between conscious change detection and attention. These findings demonstrate that ERPs can be an important tool for dissociating different processes contributing to change blindness and change detection.

The ability to process concurrently multiple visual objects is fundamental for a coherent perception of the world. A core component of this ability is the simultaneous individuation of multiple objects. Many studies have addressed the mechanism of object individuation but it remains unknown whether the visual system mandatorily individuates all relevant elements in the visual field, or whether object indexing depends on task demands. We used a neural measure of visual selection, the N2pc component, to evaluate the flexibility of multiple object individuation. In three ERP experiments, participants saw a variable number of target elements among homogenous distracters and performed either an enumeration task (Experiment 1) or a detection task, reporting whether at least one (Experiment 2) or a specified number of target elements (Experiment 3) was present. While in the enumeration task the N2pc response increased as a function of the number of targets, no such modulation was found in Experiment 2, indicating that individuation of multiple targets is not mandatory. However, a modulation of the N2pc similar to the enumeration task was visible in Experiment 3, further highlighting that object individuation is a flexible mechanism that binds indexes to object properties and locations as needed for further object processing.