Neural population dynamics of human working memory

Working memory (WM) depends on the activity of neural populations to maintain internal representations of information that are no longer available in the environment. The neural population dynamics that support WM in the human cortex remain largely unexplored. Utilizing fMRI and time-resolved multivariate analyses, we found evidence of both stable and dynamic WM representations during a classic memory-guided saccade task. Using dimensionality reduction techniques, we identified robust neural subspaces in early visual, parietal and even frontal cortex that were stable throughout the delay period with memorized locations organized within a two-dimensional representation of visual space. The dynamic subspace, which was largely restricted to early visual cortex, had both an early and late delay component during which the WM representation changed. Leveraging models of the receptive fields of voxels comprising each of the visual field maps, we visualized how the dynamic population code changed over time. Early in the trial, the dynamic code in V1 was dominated by a narrowly tuned activation among voxels with receptive fields centered on the location of the peripheral memory target. Late in the trial, the dynamic code was driven by diffuse activation among voxels with receptive fields along a line between the fovea and the peripheral location of the target. Together, the timing, shape, and spatial distribution of the WM dynamics in early visual cortex suggest that the early component encodes feedforward visual inputs, while the late component encodes abstracted task-relevant mnemonic representations resulting from feedback signals from higher-level brain areas. Significance statement Recent empirical evidence of neural dynamics during working memory (WM) challenges established canonical theories of how WM representations are stored in the brain. Utilizing fMRI and time-resolved multivariate analyses, we identified both stable WM codes in visual and association cortex, as well as changing, dynamic codes in early visual cortex. Each of these population codes were composed of low-dimensional topological representations of visual space. Characteristics of the WM dynamics in early visual cortex suggest that feedback signals from higher-brain areas recode sensory inputs into a format matching the demands of the memory-guided task.