Dynamics of efficient ensemble coding

Ensemble coding creates compressed representations of a stimulus array. When discriminating the ensemble average against a reference, however, items in the ensemble with feature values closer to the reference are typically weighed stronger. We have recently shown that this "robust averaging" behavior can be explained as a form of efficient coding, where the sensory encoding precision is dynamically adapted to efficiently represent ensemble stimuli according to their overall distribution relative to a trial-by-trial varying reference. However, the specific mechanisms underlying such dynamic, efficient ensemble coding remain unknown. Here, we demonstrate that the relative timing between the presentation of the reference and the stimulus ensemble strongly affects efficient ensemble coding. We systematically probed participants decision behavior for varying time intervals between the presentation of the ensemble and the reference. We found that efficient ensemble coding was only clearly established when reference and ensemble were simultaneously presented. It was much weaker when the ensemble preceded the reference, and largely absent when the ensemble followed the reference. As captured by our model, reduced efficient ensemble coding thereby coincided with decreased decision accuracy in those asynchronous conditions. Our results indicate that any temporal offset between the ensemble and reference stimuli substantially disrupts the dynamic and efficient allocation of coding resource. This suggests that efficient ensemble coding is not the result of a preparatory attentional process nor due to evidence selection at the decision stage. Rather, it arises from a fast interaction between the simultaneously evoked, sensory representations of reference and ensemble stimuli.