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Perceptual grouping is a multi-stage process, irreducible to a single mechanism localized anatomically or chronometrically. To understand how various grouping mechanisms interact, we combined a phenomenological report paradigm with high-density event-related potential (ERP) measurements, using a 256-channel electrode array. We varied the relative salience of competing perceptual organizations in multi-stable dot lattices and asked observers to report perceived groupings. The ability to discriminate groupings (the grouping sensitivity) was positively correlated with the amplitude of the earliest ERP peak C1 (about 60 ms after stimulus onset) over the middle occipital area. This early activity is believed to reflect spontaneous feed-forward processes preceding perceptual awareness. Grouping sensitivity was negatively correlated with the amplitude of the next peak P1 (about 110 ms), which is believed to reflect lateral and feedback interactions associated with perceptual awareness and attention. This dissociation between C1 and P1 activity implies that the recruitment of fast, spontaneous mechanisms for grouping leads to high grouping sensitivity. Observers who fail to recruit these mechanisms are trying to compensate by using later mechanisms, which depend less on stimulus properties such as proximity. [ABSTRACT FROM AUTHOR]

Abstract: Objective: Data from a previous event-related potential (ERP) study in visual-perceptual grouping [Nikolaev AR, van Leeuwen C. Flexibility in spatial and non-spatial feature grouping: an event-related potentials study. Brain Res Cogn Brain Res 2004;22:13–25] were re-analyzed to identify event-related dynamics of phase-synchronization. Methods: In 20Hz activity, uniform spreading of phase synchronization in closely spaced (∼2cm) scalp electrodes appears and disappears spontaneously. The lengths of synchronized activity intervals and how they vary as a function of stimulus presentation were compared between task and control conditions. Results: Synchronization reached a maximum in the task condition about 180ms post-stimulus onset, coinciding with the peak N180 ERP marking the deployment of task-specific attention. Synchronized intervals were longer in the task than in the control condition. Long (above 80ms) intervals occurred at a stable rate before and just after stimulus onset, but steeply decreased 200–400ms afterwards. Conclusions: Perceptual tasks lead to longer synchronized intervals in early visual areas. Attention deployment resets the ongoing synchronization. Event-related activity, besides low-frequency ERP, consists of high-frequency short and long synchronized intervals corresponding to evoked bursts and ongoing oscillations, respectively. Significance: High-density scalp recorded EEG revealed synchronization dynamics in a local, early visual area of cortex that can be interpreted as modulation of spontaneous ongoing task-related processes by attention. [Copyright &y& Elsevier]