Thanks to the availability of large scale digital datasets and massive amounts of computational power, deep learning algorithms can learn representations of data by exploiting multiple levels of abstraction. These machine learning methods have greatly improved the state-of-the-art in many challenging cognitive tasks, such as visual object recognition, speech processing, natural language understanding and automatic translation. In particular, one class of deep learning models, known as deep belief networks, can discover intricate statistical structure in large data sets in a completely unsupervised fashion, by learning a generative model of the data using Hebbian-like learning mechanisms. Although these self-organizing systems can be conveniently formalized within the framework of statistical mechanics, their internal functioning remains opaque, because their emergent dynamics cannot be solved analytically. In this article we propose to study deep belief networks using techniques commonly employed in the study of complex networks, in order to gain some insights into the structural and functional properties of the computational graph resulting from the learning process.
IntroductionRecent strides in artificial intelligence research have opened tremendous opportunities for technological development. In particular, the last decade has been marked by the so-called "deep learning revolution", which is having strong impact both for scientific investigation and for engineering applications [30].Deep learning allows building artificial neural networks composed of many processing layers, which can learn high-level representations of the data by exploiting multiple levels of abstraction [18]. To differ from conventional machine-learning techniques, this allows to automatically discover intricate statistical structure in large datasets without the need for domain expert knowledge: the relevant features needed to 1 arXiv:1809.10941v1 [cond-mat.dis-nn] 28 Sep 2018 describe the data distribution are directly learned by the machine from the raw input (e.g., pixels values in a digital image). An intriguing aspect of deep learning systems is that they are inspired by neuronal networks in biological brains: information processing occurs in a parallel and distributed fashion [46], thereby allowing cognitive abilities to emerge from the orchestrated operation of many simple, non-linear processing units [35,58].Deep learning has dramatically improved the state-of-the-art in challenging cognitive tasks, such as image classification [16,27], speech recognition [40], natural language understanding [11] and even highlevel reasoning [39,51]. It is currently employed by all major IT companies (Google, Facebook, Microsoft, Apple, just to mention a few) to automatically extract knowledge from large digital datasets, and it is achieving impressive performance also in many other domains such as drug discovery [34], genomics [60], high-energy physics [5] and telecommunications [57,63].However, despite the continuous progress and the widespread d...