Suppression of Unattended Features Is Independent of Task Relevance.

Feature-based attention serves the separation of relevant from irrelevant features. While global amplification of attended features is coherently described as a key mechanism for feature-based attention, nature and constituting factors of neural suppressive interactions are far less clear. One aspect of global amplification is its flexible modulation by the task relevance of the to-be-attended stimulus. We examined whether suppression is similarly modulated by their respective task relevance or is mandatory for all unattended features. For this purpose, participants saw a display of randomly moving dots with 3 distinct colors and were asked to report brief events of coherent motion for a cued color. Of the 2 unattended colored clouds, one contained distracting motion events while the other was irrelevant and without such motion events throughout the experiment. We used electroencephalography-derived steady-state visual-evoked potentials to investigate early visual processing of the attended, unattended, and irrelevant color under sustained feature-based attention. The analysis revealed a biphasic process with an early amplification of the to-be-attended color followed by suppression of the to-be-ignored color relative to a pre-cue baseline. Importantly, the neural dynamics for the unattended and always irrelevant color were comparable. Suppression is thus a mandatory mechanism affecting all unattended stimuli irrespective of their task relevance.