Similarity in feature space dictates the efficiency of attentional selection during ensemble processing

Humans can rapidly and accurately extract statistical information about features of the visualenvironment, an ability referred to as ensemble perception. However, little is known about howensemble estimates are affected when task-irrelevant and distracting feature information ispresent. Here we tested how effectively feature-based attention –when tuned to a specific color–can select a single item set out of two intermixed ensembles of colored lines. Participants wereinstructed to report the average orientation of a target-colored item set, while ignoring a seconddifferently-colored set. To assess how representational overlap between the two sets impactscolor-based selection, we systematically varied the orientation similarity between the relevantand irrelevant items. Our results showed that participants’ orientation reports were reliablybiased towards the irrelevant items, but interestingly, these biases were only observed when theitem sets overlapped in orientation space. In a second experiment, using a visual mask todisrupt access to color information at different time points, we found that these biases werestronger when less time was available to process the stimuli. Together, these results suggestthat ensemble representations are rapidly formed based on all available information in therelevant feature dimension, regardless of task relevance, and that selective attention weightsand separates these ensemble representations at a relatively later processing stage. Thisselection appears highly effective when the underlying population activity generated by the twosets is separable along the to-be-estimated feature dimension, but is dampened when relevantand irrelevant ensemble representations overlap in feature space.