Top-down feedback can explain the existence of working memory traces in early visual cortex

The temporary storage of visual memories is known to engage early visual cortex (EVC), where mnemonic content is presumed to be stored in weak or latent states. Two mechanisms have been suggested to underlie this storage: Top-down feedback from anterior cortical sites, and short-term synaptic plasticity (STSP) traces laid down by previous sensory inputs. Here, we tested if these mechanisms can account for two hallmarks of working memory, namely, the flexible selection of information from working memory, and the robustness of working memory against distraction. Simulating firing rates in a neural network model of working memory, we show that STSP cannot easily account for selection of information from within the working memory store, and is not robust to visual distraction. Rather, any input that passes through the visual system is briefly remembered, irrespective of its relevance. In contrast, top-down feedback can establish durable mnemonic signals in EVC that reflect currently task-relevant working memory contents and are robust to distraction. We argue that feedback is a parsimonious explanation for the existence of mnemonic signals in EVC, given that the storage of working memories is distributed across a hierarchy of cortical sites, which are known to have widespread feedback projections to visual cortex.