Hyung-Bum Park (UC Riverside)
A Temporal Limit in Visual Short-term Memory Encoding

Working memory as the core of cognition sets a central bottleneck in cognition due to its well-established storage limit (i.e., the amount of information that can be represented and retained) and processing limit (e.g., the encoding process of transferring fragile perceptual representations into robust mnemonic representations in visual short-term memory). The present study developed a novel stimulus presentation protocol in a delayed estimation task to quantitatively assess the temporal limit of visual short-term memory (VSTM) encoding (i.e. how much information can be consolidated into VSTM at a given time). In this task, the to-be-remembered colors rotate, in small unnoticeable angular velocities, along a circular color space for a brief period of time. Recall for the dynamically rotating colors shifted toward the direction of angular rotation with various degrees from the color hue at the onset of the memory display. Experiment 1 established that the Zhang & Luck (2008) Standard Mixture model could well account for these recall responses across trials. Experiment 2 and 3 tested the temporal relationship between VSTM encoding of two colors using discrete (Experiment 2) and continuous (Experiment 3) color rotations. Across the two experiments, VSTM encoding of two colors could be modeled as the mixture of two von Mises (VM) functions with non-overlapping centers (m) and the uniform (U) function. This 2VM+U model reliably outperformed the standard mixture model that had only one von Mises function for the central peak of the recall error distribution, indicating a temporal lag between discrete VSTM encoding of two colors. Experiment 4 further established the validity of m as an empirical measure of VSTM encoding speed based on its significant correlation with a previously established measure of VSTM encoding speed using masked change detection task across participants. Together, these results provide preliminary evidence for the temporal limit of discrete VSTM encoding of multiple items.