Diffuse predictions stabilize and reshape the neural code during working memory encoding

Predictions can alter working memory (WM) representations. However, its effects may have been mischaracterized due to the use of precise predictions in previous experiments, where exact properties of upcoming memory items are cued in advance. Here we investigated a more ecologically valid scenario, in which we assessed the impact of diffuse predictions, where advance cues provided only partial knowledge about the targets. To investigate the resultant nature of the target representations in WM, we performed a series of multivariate analyses of EEG data. Forty participants judged whether a probe grating was rotated clockwise or counterclockwise relative to a memorized orientation, which was either predictable or unpredictable. Each memory item was preceded by a central color cue (red, green, or blue). In half of the trials, two of these (predictive) colors cued two non-overlapping 90{degrees} segments of orientations that the grating was sampled from. Thus, participants knew the range of possible orientations of these items, but not their exact orientation. In the other half of the trials, a third (non-predictive) color was presented, signaling that the item could have any possible orientation. Behavioral results revealed higher accuracy for predictable items, with systematic biases toward the center of the cued segment. EEG results revealed equally successful decoding of orientation for both predictable and unpredictable items during memory encoding. However, cross-condition decoding was significantly weaker than within-condition decoding, suggesting that the encoding format changed between conditions. Representational similarity analysis showed higher similarity between predictable items, with a representational bias towards the cued segment. Covariance matrices showed lower variance for predictable items while the representational space of predictable items was shrunk. These effects were absent during the maintenance phase. Together, our findings suggest that diffuse predictions alter the geometric layout of the neural representations and stabilize the neural code during WM encoding.