1 Introduction
Humans share with other species the ability to associate sounds and meanings
in a particular way (K aminki, Call and Fisher 2004). However, the unbounded
nature of the association is uniquely designated to humans. While other species
convey a limited, fixed array of meanings by using a rigid order of body motions,
humans convey an unbounded set of different meanings by using language.
This unique property of unboundedness is generated by the internalized mecha-
nisms (internalized language, I-language) in the brain. The basic property of
I-language is to provide “an unbounded array of hierarchically structured expres-
sions that receive interpretations at two interfaces, sensorimotor for externaliza-
tion and conceptual-intentional for mental processes”(C homsky 2013: 647).
Linear order is a part of externalization required by properties of the sensorimotor
system, during the process of which hierarchical structure is mapped onto linear
sequences of words. Linear or sequential information is not specific to humans.
In this sense, hierarchical structures are a defining feature of human language,
forming a division between human and nonhuman communication systems
(A nderson 2008; M oro 2011).
F rank et al. (2012), however, challenge the taken-for-granted assumption
that hierarchical structure is basic to human language (C homsky 1957, H auser
et al. 2002). Drawing from evidence related to psycholinguistics, computa-
tional linguistics and cognitive neuroscience, they claim that sequential struc-
ture is more fundamental to language use (i.e., production and comprehension)
than hierarchical structure. C homsky (1957) has already shown in a convincing
way that sequential information based on probabilistic generalizations about
surface distribution cannot characterize the essential properties of human
language. In fact, many important generalizations about linguistic knowledge
(e.g., knowledge regarding subject-verb agreement, reflexive pronouns, nega-
tive polarity items; knowledge regarding syntactic constituency (i.e., syntactic
unit) in terms of pronominal anaphora, phrasal movement, ellipsis) have been
made in terms of hierarchical structure. Frank et al. (2012), however, deny
the involvement of hierarchical structure in language use or sentence
processing. They claim that even if the offline (with no time limit) mental