Humans can learn a new ability after practicing a few hours (e.g., driving or playing a game), while to solve the same task artificial neural networks require millions of reinforcement learning trials in virtual environments. And even then, their performances might be not comparable to human's ability. Humans and animals, have developed an understanding of the world that allow them to optimize learning. This relies on the building of an inner model of the world. Model-based reinforcement learning [1, 2, 3, 4, 5, 6] have shown to reduce the amount of data required for learning. However, these approaches do not provide insights on biological intelligence since they require biologically implausible ingredients (storing detailed information of experiences to train models, long off-line learning periods, expensive Monte Carlo three search to correct the policy). Moreover, the storage of long sequences is highly problematic on neuromorphic and FPGA platforms, where memory resources are scarce, and the use of an external memory would imply large latencies. The optimal way to learn and exploit the inner-model of the world is still an open question. Taking inspiration from biology, we explore an intriguing idea that a learned model can be used when the neural network is offline.

The occurrence of sleep passed through the evolutionary sieve and is widespread in animal species. Sleep is known to be beneficial to cognitive and mnemonic tasks, while chronic sleep deprivation is detrimental. Despite the importance of the phenomenon, a theoretical and computational approach demonstrating the underlying mechanisms is still lacking. In this paper, we show interesting effects of deep-sleep-like slow oscillation activity on a simplified thalamo-cortical model which is trained to encode, retrieve and classify images of handwritten digits. If spike-timing-dependent-plasticity (STDP) is active during slow oscillations, a differential homeostatic process is observed. It is characterized by both a specific enhancement of connections among groups of neurons associated to instances of the same class (digit) and a simultaneous down-regulation of stronger synapses created by the training. This is reflected in a hierarchical organization of post-sleep internal representations. Such effects favour higher performance in retrieval and classification tasks and create hierarchies of categories in integrated representations. The model leverages on the coincidence of top-down contextual information with bottom-up sensory flow during the training phase and on the integration of top-down predictions and bottom-up thalamo-cortical pathways during deep-sleep-like slow oscillations. Also, such mechanism hints at possible applications to artificial learning systems.