Physiological Characteristics of Capacity Constraints in Working Memory as Revealed by Functional MRI

TLDR

Working memory has a limited capacity, yet its physiological basis remains unexplained. The study aimed to identify characteristics of working memory capacity using fMRI in healthy subjects. The authors used a parametric n‑back task with increasing load to probe capacity limits via fMRI. DLPFC exhibited a clear inverted‑U response to load, indicating capacity constraints, while other regions showed heterogeneous patterns and some additional loci also displayed capacity‑constrained activity, confirming regionally specific capacity limits in the working‑memory network.

Abstract

A fundamental characteristic of working memory is that its capacity to handle information is limited. While there have been many brain mapping studies of working memory, the physiological basis of its capacity limitation has not been explained. We identified characteristics of working memory capacity using functional magnetic resonance imaging (fMRI) in healthy subjects. Working memory capacity was studied using a parametric 'n-back' working memory task involving increasing cognitive load and ultimately decreasing task performance. Loci within dorsolateral prefrontal cortex (DLPFC) evinced exclusively an 'inverted-U' shaped neurophysiological response from lowest to highest load, consistent with a capacity-constrained response. Regions outside of DLPFC, in contrast, were more heterogeneous in response and often showed early plateau or continuously increasing responses, which did not reflect capacity constraints. However, sporadic loci, including in the premotor cortex, thalamus and superior parietal lobule, also demonstrated putative capacity-constrained responses, perhaps arising as an upstream effect of DLPFC limitations or as part of a broader network-wide capacity limitation. These results demonstrate that regionally specific nodes within the working memory network are capacity-constrained in the physiological domain, providing a missing link in current explorations of the capacity characteristics of working memory.