Time matters: Feature-specific prioritization follows feature integration in visual object processing

Objects represent a fundamental selection unit of visual attention. However, at odds with the integrated competition account, our recent study demonstrated that attentional facilitation of constituent features does not spread automatically within an object, but instead depends on the specific task relevance of each feature. Here, we employed a novel experimental design, allowing simultaneous electrophysiological measurements of the allocation of attention to two distinct features (rotation and color) within one object (a square) during both trial-wise and block-wise cued shifts of attention. This was possible through the presentation of a square that evokes two distinct steady-state visual evoked potentials (SSVEPs) for its rotation and its color changes, respectively. Given the continuous oscillatory nature of SSVEPs, we were able to investigate the time course of neural activity in the early visual cortex of the human brain when subjects attended to one of the two features, compared to when the whole object was attended. This approach enabled us to uncover feature-based mechanisms of attention within one object, as well as their interaction with object-based mechanisms. Both behavioral and electrophysiological results indicate a biphasic process composed of an early transient integration of the constituent object features, followed by sustained mechanisms of feature selection with amplification of the to-be-attended feature, followed temporally by suppression of the to-be-ignored feature.