Fiebig, F.; Chrysanthidis, N.; Lansner, A.; Herman, P. A.

Working memory (WM) is essential for almost every cognitive task and behavior. The neural and synaptic mechanisms supporting the rapid encoding and maintenance of memories in diverse tasks are the subject of an ongoing debate. The traditional view of WM as stationary persistent firing of selective neuronal populations has given room to newer ideas regarding mechanisms that support a more dynamic maintenance of multiple items, which may also tolerate more activity disruption. Various computational WM models based on different biologically plausible synaptic and neural plasticity mechanisms have been proposed. We show that these proposed short-term plasticity mechanisms may not necessarily be competing explanations, but instead yield interesting functional interactions on a wide set of WM tasks and enhance the biological plausibility of spiking neural network models, in particular of the underlying synaptic plasticity. While monolithic models (WM function explained by one particular mechanism) are theoretically appealing and have increased our understanding of specific mechanisms, they are narrow explanations. WM models need to become more capable, robust and flexible to account for new experimental evidence of bursty and activity-silent multi-item maintenance in more challenging WM tasks, and generally solve more than one particular task. More detailed models also allow for electrophysiological constraints from recordings. In this study we evaluate the interactions between three commonly used classes of plasticity, namely intrinsic excitability, synaptic facilitation/augmentation and Hebbian plasticity. Combinations of these are systematically tested in a spiking neural network model on a broad suite of tasks or functional motifs deemed principally important for WM operation, such as one-shot encoding, free and cued recall, delay maintenance and updating. In our evaluation we focus on the operational task performance and biological plausibility. Our results indicate that a composite model, combining several commonly proposed plasticity mechanisms for WM function, is superior to more reductionist variants. Importantly, we attribute the observable differences to the principle nature of specific types of plasticity.