Precision of neural codes involved in storing phonological information in working memory

Working memory (WM) precision is defined as the quality with which representations are stored in WM, and has to be distinguished from WM capacity, which is the quantity of information that can be maintained in WM. This study is the first to assess the neural precision of WM traces for auditory-verbal information, using a functional magnetic resonance imaging (fMRI) approach. In this experiment, we asked 27 young adults to actively maintain 4-syllable nonwords during a 7-second interval. The nonwords were highly similar or dissimilar at the phonological level. Using multivariate voxel pattern analysis (MVPA), we explored the neural patterns associated with each nonword. We hypothesized that if auditory-verbal WM precision is limited, as indicated by the well-established phonological similarity effect in the WM literature, then dissimilar but not similar nonwords should be associated with distinctive neural patterns during WM, especially during the maintenance stage. Using Bayesian one sample t-tests on whole-brain classification accuracies, we observed that neural decoding of similar nonwords was at chance level, while neural decoding of dissimilar nonwords was clearly above chance during the maintenance stage. Searchlight analyses showed that the informative neural patterns were located in the dorsal language pathway known to support phonological processing. These results provide evidence for the neural basis of the phonological similarity effect in WM and the limited precision of phonological coding in WM.