Abstract
Prominent theories of working memory (WM) have proposed that distinct working memory systems may support the storage of different types of information. For example, distinct dorsal and ventral stream brain regions are activated during the storage of spatial and object information in visual WM. Although feature-specific activity is likely critical to WM storage, we hypothesize that a content-independent indexing process may also play a role. Specifically, spatiotemporal pointers may be required for the sustained indexing and tracking of items in space and time, even while features change, within an unfolding event. Past evidence for such a content-independent pointer operation includes the finding that signals tracking the number of individuated representations in WM (load) generalize across colors, orientations and conjunctions of those features. However, overlapping orientation and color codes in early visual cortices may mimic a generalizable signal. Here, we provide a stronger demonstration of content-independence by using pairs of features that are as cortically disparate as possible. Study 1 (n=16) used color and motion coherence stimuli, and showed that load decoding models generalized across these disparate features. In addition, we used representational similarity analysis (RSA) to document “pure” load signals that tracked the number of items stored regardless of attended feature, while simultaneously documenting and controlling for feature-specific neural activity. Extending these observations, in Study 2 (n=24; n=16) we applied similar analytic approaches to demonstrate a common load signature between auditory and visual sensory modalities, while controlling for modality-specific neural activity and the spatial extent of covert attention. Our findings suggest that content-independent pointers may play a fundamental role in the storage of information in working memory, and may contribute to its overall limited capacity.