The Time Course of Retaining Hierarchical Representations in Visual Working Memory

It was previously shown that the features of individual items retrieved from visual working memory (VWM) are systematically biased towards the mean feature of a sample set (Brady & Alvarez, 2011), suggesting hierarchical encoding in VWM. In our work, we investigated how hierarchical representations are stored over time. Observers were shown four differently oriented triangles for 200 ms and, after 1-, 4-, or 7-second delay, they had to report either one individual orientation, or the average orientation of all triangles, rotating a probe circle. Before set presentations, observers were informed that they had to remember one particular orientation, all four individual orientations, or the average orientation. Using the mixture model (Zhang & Luck, 2008), we estimated a probability of a tested representation being in VWM and its precision, as well as a systematic bias that would indicate hierarchical encoding. We found a strong bias towards the mean in the “remember four” condition, which provides evidence for hierarchical encoding in VWM. Our main result was the absence of significant changes in retaining the elements of a hierarchical representation (the mean and individual features). This supports an idea that hierarchical representations are related to encoding, rather than storing in VWM. Both fidelity and the probability of an item being in memory decrease over time. It supports "Sudden Death" and "Gradual Decay" accounts for storing hierarchical representations.