Feature-based enhancement of visual stimuli at task-irrelevant locations

Abstract

The visual system selects inputs for extended processing on the basis of their features or locations. To date, considerable research has investigated the mechanisms underlying both featural and spatial selection, yet the extent to which these processes interact remains unknown. Here, we tested whether feature-based attention acts independently of spatial location by examining whether a featural set applies at locations that are completely task-irrelevant (i.e., outside the focus of spatial attention). In blocks of trials, observers monitored central vision for target letters in a cued color (red or green) that appeared within multi-letter arrays. Within each array, the target was defined either by a unique color (unique feature search) or by a combination of color and shape (conjunction search). The letter arrays were surrounded by colored checkerboards in peripheral vision that were always task-irrelevant and could therefore be ignored throughout the experiment. We used ‘frequency tagging’ in which flickering stimuli produce cortical oscillations at the flicker frequency, detectable at the scalp via electroencephalography (EEG). This approach allowed us to distinguish neural responses evoked by the task-relevant central stream from activity related to the task-irrelevant peripheral checkerboards. The checkerboard stimuli contained interleaved elements of three different colors: those that matched the target color, those that matched the distractor color, and those of a third, neutral color. During unique feature search, the magnitude of checkerboard-evoked brain oscillations was independent of the search color. During conjunction search, however, we found that checkerboard elements matching the target color evoked enhanced cortical oscillations relative to those associated with distractor or neutral colors. These results provide strong evidence that feature-based attention applies globally across the visual field, irrespective of the current focus of spatial attention.