Abstract
The spatial extent of visual attention, which facilitates perceptual performances, has been classified into spatial and object-based attention. Object-based attention refers to a type of attention limited within an object that encloses a cued location, which contrasts with spatial attention that spreads around the attentional focus. Whether object-based attention is enabled by the deformation of an attentional “spotlight” or by the prioritization of locations to which the attentional spotlight is directed is a matter of much debate. The present study addresses this issue by employing an EEG (electro-encephalogram) technique called steady-state visual evoked potentials (SSVEP) during a psychophysical experiment. The SSVEP amplitude is modulated by visual attention. During the EEG recording, we asked participants to perform a rapid serial visual presentation task designed to exhibit object-based attention, and a simple detection task to measure the spatial spreading/deployment of attention. The visual stimuli flickered at different frequencies at different spatial locations, which enabled us to tag stimulus locations with temporal-frequency components in the EEG data. We found an effect of object-based attention on SSVEP amplitudes and behavioral performances. Additionally, the absence of event related potential changes at the cued location, triggered by random and frequent presentations of detection-task stimuli throughout a trial, suggests that object-based attention may be based on a steady state mechanism, i.e., spatial spreading, rather than a dynamic one, such as prioritizing shifts of attention to locations within the cued object.
