This work examines the presence of modularity in pre-trained Transformers, a
feature commonly found in human brains and thought to be vital for general
intelligence. In analogy to human brains, we consider two main characteristics
of modularity: (1) functional specialization of neurons: we evaluate whether
each neuron is mainly specialized in a certain function, and find that the
answer is yes. (2) function-based neuron grouping: we explore finding a
structure that groups neurons into modules by function, and each module works
for its corresponding function. Given the enormous amount of possible
structures, we focus on Mixture-of-Experts as a promising candidate, which
partitions neurons into experts and usually activates different experts for
different inputs. Experimental results show that there are functional experts,
where clustered are the neurons specialized in a certain function. Moreover,
perturbing the activations of functional experts significantly affects the
corresponding function. Finally, we study how modularity emerges during
pre-training, and find that the modular structure is stabilized at the early
stage, which is faster than neuron stabilization. It suggests that Transformers
first construct the modular structure and then learn fine-grained neuron
functions. Our code and data are available at
https://github.com/THUNLP/modularity-analysis.Comment: Findings of ACL 202