Retention of Verbal and Nonverbal Sequence Information in Working Memory. Analysis of Functional and Effective Connections

The studies reported here addressed differences in the structures of cerebral systems mediating the encoding and retention in working memory (WM) of two types of information: sequences of verbal (letter names) or non-verbal (open broken line segments) presented either statically or dynamically. Cerebral systems were characterized in terms of the strength of functional and effective connections between eight approximately bilaterally symmetrical cortical loci, including the dorsolateral prefrontal cortex (dlPFC) and areas of the temporal (STG), parietal (IPS), and occipital (v2) cortex. Studies in a group of subjects using high-density EEG recordings, employing an eight-channel vector autoregressive model in the space of cortical EEG sources to show that: 1) the cerebral organization of WM during retention of a letter sequence differs from that during retention of a broken line segment sequence; 2) the cerebral organization of WM depends on the mode of presentation of sequences: the strength of the functional connection is different for dynamic and static presentation of sequences; 3) differences in the structures of functional and effective connections are not clearly frequency-selective and are seen in all the encephalographic frequency bands studied, from θ (4–8 Hz) to high-frequency γ (50–60 Hz); 4) the most significant differences between the task of retaining a sequence of letters and the task of retaining a sequence of broken line segments on static visual presentation of sequences were seen in the α- and β-frequency bands in terms of the strength of the functional connection between the left-hemisphere dlPFC and the right-hemisphere STG measured in terms of coherence, as well as in the θ range between the right-hemisphere dlPFC and the left visual cortex v2; 5) the most significant difference between static and dynamic presentation modes in the task of retaining broken line segments was observed in the γ frequency band (50–60 Hz) between the dlPFC in the right hemisphere and the left visual cortex v2.