Although the neural basis of working memory (WM) capacity is often studied by exploiting interindividual differences, capacity may also differ across memory materials within a given individual. Here, we exploit the content dependence of WM capacity as a novel approach to investigate the oscillatory correlates of WM capacity, focusing on posterior 9–12 Hz alpha activity during retention. We recorded scalp electroencephalography (EEG) while male and female human participants performed WM tasks with varying memory loads (two vs. four items) and materials (English letters vs. regular shapes vs. abstract shapes). First, behavioral data confirmed that memory capacity was fundamentally content dependent; capacity for abstract shapes plateaued at around two, whereas the participants could remember more letters and regular shapes. Critically, content-specific capacity was paralleled in the degree of attenuation of EEG-alpha activity that plateaued in a similar content-specific manner. Although we observed greater alpha attenuation for higher loads for all materials, we found larger load effects for letters and regular shapes than for abstract shapes, which is consistent with our behavioral data showing a lower capacity plateau for abstract shapes. Moreover, when only considering two-item trials, alpha attenuation was greater for abstract shapes where two items were close to the capacity plateau than for other materials. Multivariate decoding of alpha activity patterns reinforced these findings. Finally, for each material, load effects on capacity (K) and alpha attenuation were correlated across individuals. Our results demonstrate that alpha oscillations track memory capacity in a content-specific manner and track not just the number of items but also their complexity.

SIGNIFICANCE STATEMENT WM is limited in its capacity. We show that capacity is not fixed for an individual but is rather memory-content dependent. Moreover, we used this as a novel approach to investigate the neural basis of WM capacity with EEG. We found that both behavioral capacity estimates and neural oscillations in the alpha band varied with memory loads and materials. The critical finding is a capacity plateau of approximately two items only for the more complex materials, accompanied by a similar plateau in the EEG alpha attenuation. The load effects on capacity and alpha attenuation were furthermore correlated across individuals for each of the materials. Our results demonstrate that alpha oscillations track the content-specific nature of WM capacity.