  There is an ongoing debate about whether the Internet is like a public sphere
or an echo chamber. Among many forms of social media, Twitter is one of the
most crucial online places for political debate. Most of the previous studies
focus on the formal structure of the Twitter political field, such as its
homophilic tendency, or otherwise limit the analysis to a few topics. In order
to explore whether Twitter functions as an echo chamber in general, however, we
have to investigate not only the structure but also the contents of Twitter's
political field. Accordingly, we conducted both large-scale social network
analysis and natural language processing. We firstly applied a community
detection method to the reciprocal following network in Twitter and found five
politically distinct communities in the field. We further examined dominant
topics discussed therein by employing a topic model in analyzing the content of
the tweets, and we found that a topic related to xenophobia is circulated
solely in right-wing communities. To our knowledge, this is the first study to
address echo chambers in Japanese Twitter political field and to examine the
formal structure and the contents of tweets with the combination of large-scale
social network analysis and natural language processing.
