Previous studies on the neuronal basis of working memory have identified brain regions that temporarily store simple visual features like motion direction or color. In contrast, it remains unknown which brain areas temporarily store information about how those features are bound into an object.
In each trial of a functional magnetic resonance imaging (fMRI) study, participants (N=20) memorized two objects composed of two visual features each: color and motion direction. We manipulated the feature bindings within these objects across trials. To collect a sufficient number of binding repetitions per participant, each feature varied only across three exemplar categories (color: blue, yellow and pink; motion direction: leftwards, rightwards and downwards). Verbal strategies were minimized by slightly jittering color value and motion direction within each category. Together, this allowed us to isolate memorized binding information by comparing delay-related activity patterns between trials in which participants memorized pairs of objects with exactly the same features but the opposite binding. According to previous research and based on a preliminary analysis (from 10 out of 20 participants), we decoded memory information about motion direction and color from activity in the visual cortex. Crucially, we were also able to decode the trial-specific binding information from the memory-related visual cortex activity.
This novel, but preliminary, finding indicates that early visual areas can retain specific information both about individual features and about the way these features are bound together.