Task-modulated, feature-selective responses in early visual, parietal and frontal cortices during visual working memory maintenance

摘要

Recent fMRI studies have revealed that the parietal and frontal cortices, as well as the early visual areas, convey memory-related, feature-selective information during visual working memory maintenance (Ester & Serences; Yu & Shim, VSS 2014). However, it remains unclear if such feature-tuning responses are modulated by task demand. In the current study, using fMRI and a forward encoding model (Brouwer & Heeger, 2009; 2011), we examined the effect of task-relevance on the voxel-based, feature-selective tuning responses over memory delay. On each trial, two gratings with different colors and orientations were presented sequentially. Participants remembered color, orientation, or both color and orientation of the cued grating. The stimuli and procedure remained identical across conditions, and only the to-be-remembered feature dimension(s) differed. After a long delay period, participants were required to reproduce the remembered feature as accurately as possible on an orientation/color wheel. We successfully reconstructed color-selective as well as orientation-selective population tuning responses in early visual areas (V1-V4v), IPS and FEF during the retention period. These results indicate that non-spatial surface features, such as color, can be retained over delay even in parietal and frontal areas. Crucially, the two features showed distinct task-modulated effects during working memory: orientation tuning responses were significantly degraded when orientation was not remembered, whereas color tuning responses remained similar regardless of whether color was remembered or not. This result is consistent with behavioral memory precision measures, which showed that color memory is more immune to task demands than orientation memory. Therefore, our results suggest that the quality of the reconstructed feature-tuning responses in early visual, parietal, and frontal areas during memory delay can reflect the strength of each feature representation in multi-feature objects in working memory, even when that feature is task-irrelevant.