Imaging phonological similarity effects on verbal working memory

Studies of verbal working memory (VWM) report that performance declines as the phonemic similarity of stimuli increases. To determine how phonological similarity affects brain function during VWM, "standard" and "similarity" versions of the 2-Back task were presented to 34 healthy participants during functional magnetic resonance imaging (FMRI). Letter consonants presented during similarity blocks rhymed, while consonants did not rhyme during standard blocks. Empirical ROIs were identified from significant 2-Back-related activity observed during either condition. A priori ROIs were selected from functional neuroimaging literature on phonological processing. Although VWM-related activity was not modulated by similarity in any of four regions recruited (dorsolateral prefrontal, posterior parietal, anterior insular, and supplementary motor cortices), four of five regions of deactivation exhibited significantly greater deactivation during the similarity compared to the standard condition (posterior cingulate, paracentral lobule, posterior insula, and parahippocampal gyrus). In a priori phonological processing-related ROIs, similarity did not affect observed increases in activity (supplementary motor area, Broca's area, and cerebellum), while two of the three regions exhibiting decreased activity (near Wernicke's area and Heschel's Gyrus) also exhibited more deactivation during similarity. Accuracy was lower during the similarity 2-Back, positively related to activity within recruited VWM-related ROIs, and inversely related to activity in regions of VWM-related deactivation. Based on known functions of these ROIs, we conclude that language, audition, and self-reflection processes may disengage during phonological interference, while activity levels are maintained in regions recruited during VWM processing. Similarity effects likely include suspension of attention to unrelated and distracting processes to improve concentration.