Mental compression of spatial sequences in human working memory using numerical and geometrical primitives

Abstract How do humans encode spatial and sequential information in working memory? We tested the hypothesis that participants do not merely store each consecutive location in a distinct memory slot, but instead compress the whole sequence using an abstract language-like code that captures geometrical regularities at multiple nested levels. We exposed participants to sequences of fixed length but variable regularity, while their brain activity was recorded using magneto-encephalography. The entire sequence could be decoded from brain signals. Anticipation signals could also be decoded, and both behavior and anticipatory brain signals were modulated by sequence complexity, defined as the minimal description length provided by the formal language. Furthermore, abstract primitives of rotation and symmetry could be decoded both in isolation and within the sequences. These results suggest that humans encode the transitions between sequence items as rotations and symmetries, and compress longer sequences using nested repetitions of those primitives.