Control over declarative and procedural working memory

Executive summary:

Most of our daily tasks require keeping information in mind and updating it frequently as the task unfolds. Working memory (WM) provides these core abilities, enabling robust maintenance of information over time, as well as its rapid updating according to new input. Theories of WM propose that updating is controlled by a gate that buffers between the perceptual input and the maintained information. Control over WM updating is achieved through of a cascade of sub-processes, including continuous monitoring, detection of a relevant change, gate-opening, information substitution, and gate-closing. While previous work had started to characterize the above behavioral processes, their neural correlates were previously unknown and underspecified.

The present project developed new experimental paradigms and empirical markers for distinguishing between the above sub-processes, using both behavioral experiments and fMRI studies. This was done in parallel in two WM domains: declarative and procedural. In the declarative domain, we combined fMRI with the reference-back task, specifically designed to tease apart these subprocesses. Participants compared externally presented face stimuli to a reference face held in WM, while alternating between updating and maintaining this reference, resulting in opening versus closing the gate to WM. Gate opening and substitution processes were associated with strong BG, thalamic, and frontoparietal activation, but intriguingly, the same activity profile was observed for sensory cortex supporting task stimulus processing (i.e., the fusiform face area). In contrast, gate closing was not reliably associated with any of these regions. These findings provided new support for the involvement of the BG in gate opening, as suggested by the gating model, but qualify the model's assumptions by demonstrating that gate closing does not seem to depend on the BG and that gate opening also involves task-relevant sensory cortex. In the declarative domain, a procedural version of the reference-back paradigm allowed for the decomposition of WM updating processes into gate-opening, gate-closing, task switching, and task cue conflict components. Significant behavioral costs were observed for each of these components, with interactions indicating facilitation between gate-opening and task switching, and modulation of cue conflict by gate state. In neural terms, opening the gate to procedural WM was associated with activity in medial prefrontal cortex, posterior parietal cortex, BG, thalamus, and midbrain, but only when the task set needed to be updated. Closing the gate to procedural WM was associated with frontoparietal and BG activity specifically in conditions where conflicting task cues had to be ignored.

Together, the results of this project deepened and broadened our understanding of the mechanisms involved in controlling the contents of WM and helped to inform cognitive and physiologically-based theories in the field.