Mechanisms of target localization in visual change detection: an interplay of gating and filtering.

Localizing visual information requires drawing on retinotopic activation maps located in visual cortex that are modulated both by stimulus saliency and a top-down bias toward relevant inputs. The present study investigated the role of this spatial stimulus representation for target localization by means of event-related potentials (ERPs) of the electroencephalogram (EEG). In different blocks, participants were instructed to localize a luminance change or simply detect its presence in a fast sequence further containing a randomly occurring orientation change distractor. Both in the localization and detection task, the lowest behavioral performances were shown when target and distractor were presented contralateral to each other (spatial conflict condition). On ERP level, higher posterior asymmetries in favor of the relevant luminance change in the N1 and N2 time windows were observed for the localization compared to the detection task and suggested an increased top-down bias on spatial target representation in visual areas when the response to a relevant stimulus had a spatial component. The increased top-down bias included both a stronger gating of relevant information in the N1 time window for unilateral feature changes and a subsequent increased filtering process with an onset that was delayed as a function of the distraction caused by irrelevant signals in the display. These mechanisms were impaired when participants failed to localize the luminance change, indicating that the top-down amplification of spatial target representation in visual areas relative to the irrelevant surrounding is a substantial component of localizing visual feature changes.