Feature-based attention is independent of object appearance.

How attention interacts with low-level visual representations to give rise to perception remains a central yet controversial question in neuroscience. While several previous studies suggest that the units of attentional selection are individual objects, other evidence points instead toward lower-level features, such as an attended color or direction of motion. We used both human fMRI and psychophysics to investigate the relationship between object-based and feature-based attention. Specifically, we focused on whether feature-based attention is modulated by object appearance, comparing three conditions: (a) features appearing as one object; (b) features appearing as two separate but identical objects; (c) features appearing as two different objects. Stimuli were two random-dot fields presented bilaterally to central fixation, and object appearance was induced by the presence of one or two boxes around the fields. In the fMRI experiment, participants performed a luminance discrimination task on one side, and ignored the other side, where we probed for enhanced activity when either it was perceived as belonging to a same object, or shared features with the task side. In the psychophysical experiments, participants performed luminance discrimination on both sides with overlapping red and green dots, now attending to either the same features (red/red or green/green) or different features (red/green or green/red) on both sides. Results show that feature-based attentional enhancement exists in all three conditions, i.e., regardless whether features appear as one object, two identical objects, or two different objects. Our findings indicate that feature-based attention differs from object-based attention in that it is not dependent upon object appearance. Thus feature-based attention may be mediated by earlier cortical processes independent of perceiving visual features into well-formed objects.