Feature-based attention is also object-based

Abstract

Feature-based attention has been revealed as a global enhancement of attended visual features throughout the visual cortex. Object-based attention was shown as better performance when concurrently discriminating two features of the same object compared to two features of different objects. We used high field strength (4T) functional MRI to investigate whether feature-based attention is also object-based, i.e., does cortical enhancement of attended features result from subjects treating two features as a single same object? The stimuli were two drifting Gabor patches presented bilaterally to the central fixation cross. Subjects performed orientation discrimination using a two-interval forced-choice paradigm on one side and ignored the stimulus on the other side. The ignored stimulus was always vertical (V) and drifting slowly (S) and the attended stimulus either was identical(VS) or horizontal (H) and drifting faster (F). We compared visual cortical enhancement of the ignored stimulus when subjects attended on the other side to either the identical (VS) or a different (HF) stimulus in two conditions: either the Gabor stimuli on both sides appeared to belong to a same object (with both Gabors simply displayed on a normal gray background), or as two separate objects (each Gabor displayed in a grey box appearing on top of a textured background, with cast shadows effects around the boxes). Results showed that in the single-object condition all three subjects consistently had significant enhancement of the ignored stimulus (SPM T-values T1=4.43; T2=4.24; T3=3.93 with P(uncorrect)<0.001, TR=3s, Voxel size=3×3×3mm^3) ) in area MT+ , which confirmed previous observations of feature-based attentional modulation. But this enhancement disappeared (no significant enhancement) when stimuli appeared as two different objects. These results indicate that feature-based attentional modulation is also object-based, i.e., only occurs when between features that belong to a same object.