Intermediate-Term Memory as a Bridge between Working and Long-Term Memory

Working memory is the ability to maintain information in an active and readily available state for short periods of time. It is a key component of many cognitive processes, including inference, decision-making, mental calculations, and awareness. One of the dominant models of working memory postulates that memoranda are stored through persistent neuronal activity (Goldman-Rakic, 1995). This model is supported by numerous single-neuron recordings from different brain areas in animals using a variety of paradigms (Constantinidis et al., 2001; Warden and Miller, 2010). Recently, we have corroborated this hypothesis in humans at the single-neuron level (Kamiński et al., 2017). However, results from other studies have led to the proposition of alternative models of working memory. Recently, Lundqvist et al. (2016) recorded local field potentials and single neurons in the prefrontal cortex of macaque monkeys performing a working memory task and found that information in working memory could be maintained through neuronal activity linked to discrete bursts of gamma oscillations. Additionally, Stokes et al. (2013) in another macaque study did not observe stimulus-specific persistent activity in their recordings, and instead suggested that working memory was encoded through complex neuronal dynamics. This led to the proposition that synaptic changes, which are not visible in single-neuron recordings, may represent content held in working memory (the "activity-silent working memory" hypothesis; Stokes, 2015).