  Despite the remarkable evolution of deep neural networks in natural language
processing (NLP), their interpretability remains a challenge. Previous work
largely focused on what these models learn at the representation level. We
break this analysis down further and study individual dimensions (neurons) in
the vector representation learned by end-to-end neural models in NLP tasks. We
propose two methods: Linguistic Correlation Analysis, based on a supervised
method to extract the most relevant neurons with respect to an extrinsic task,
and Cross-model Correlation Analysis, an unsupervised method to extract salient
neurons w.r.t. the model itself. We evaluate the effectiveness of our
techniques by ablating the identified neurons and reevaluating the network's
performance for two tasks: neural machine translation (NMT) and neural language
modeling (NLM). We further present a comprehensive analysis of neurons with the
aim to address the following questions: i) how localized or distributed are
different linguistic properties in the models? ii) are certain neurons
exclusive to some properties and not others? iii) is the information more or
less distributed in NMT vs. NLM? and iv) how important are the neurons
identified through the linguistic correlation method to the overall task? Our
code is publicly available as part of the NeuroX toolkit (Dalvi et al. 2019).
