Front. Comput. Sci.
3
relation with the meaning of "after then" or Causal with that
of "because". It normally confuses the relation detection sys-
tems which favors the usage of connectives in relation de-
tection, e.g., [13] and [14] connective-to-relation mapping in
relation inference.
[4] gives a first step at disambiguating the relation senses
of a small subset of connectives (e.g., the connectives "since",
"while" and "as") at the paragraph level. By means of syntac-
tic features derived from PDTB and a MaxEnt model, they
distinguish the relations of Temporal, causal Contingency,
contrastive Comparison for the connectives.
But, [5] shows connective disambiguation can only pro-
vide no more than 1% improvement for relation detection (on
nearly all sections of PDTB). Correspondingly, [14] proves
that the disambiguation doesn’t work when connectives ei-
ther occur as mood particles or are purely pragmatic failures.
In this paper, we specialize in implicit relation disam-
biguation. As will being seen, ambiguous implicit relations
are much harder to identify and eliminate than explicit ones,
more easily cheating relation detection systems because of
their imperceptibility.
3 Definition of IDRD
IDRD aims to identify and eliminate ambiguous implicit re-
lations, reducing confusion of multiple relations implied in
pairwise arguments. Ambiguous implicit relation has the
characteristics as:
• Implicit: relation between arguments isn’t directly sig-
naled by an overt connective or any other explicit mark.
• Ambiguous: a pair of arguments implies more than one
type of potential relation but only one is truly correct.
• Deceptive: some of the implied relations are pseudo-
correct, which easily cheats the relation detection sys-
tems.
Given a pair of arguments as input, IDRD is required to gen-
erate results in two forms. One is to output all the relations
implied in the arguments along with corresponding triggers,
where a trigger is made of sentence constituents that can be
linked to reflect a specific relation. The other is to give the
correct relation. See (4), which shows the standard output of
IDRD for the instance of (3).
(4)INPUT:
Tom is young (Arg1), even younger than old Billy (Arg2).
OUTPUT:
Relation 1→ Expansion.Instantiation
Trigger 1→ "young"(Arg1) + "younger than Billy"(Arg2)
Relation 2→ Comparison.Contrast
Trigger 2→ "young"(Arg1) + "old"(Arg2)
(Correct Relation→Relation1)
(Pseudo-Correct Relation→Relation2)
where, there are two relations probably considered as reason-
able, Contrast and Instantiation. In this case, however, the
truly correct relation is Instantiation. It is because that Arg2
actually presents an example (younger than Billy) to support
the opinion in Arg1 (Tom is young). The other relation,
Contrast, is incorrect because the main point is not to prove
that Tom is younger than Bill. It is easily-misjudged to be
correct, therefore named as a pseudo-correct relation.
A general discourse relation detection system is only re-
quired to output the type tag of the relation held between ar-
guments. Though the system may output more than one type
of relation no matter whether all of them are correct, such as
Instantiation and Contrast in (4). By contrast, an IDRD sys-
tem will provide richer information, including relation tags
and triggers. Besides, if there are more than one type of re-
lation detected, the system is required to make a final detec-
tion on the correctness, marking out the sole genuine relation
type. For example, it adds a ”Correct Relation" to Relation1
in (4). Correspondingly the Relation2 is pseudo-correct.
As will be seen, we use a Relation Search System (RSS) to
obtain implied relations and triggers. On the basis, we detect
truly correct relations and eliminate pseudo-correct ones with
a focus-sensitive relation disambiguation model. But in the
writing of this paper, we begin with a discussion of the dis-
ambiguation model (Section 4), aimed at getting to the point
quickly, secondly we show RSS (Section 5), thirdly we intro-
duce eight relation detection systems in Section 6, including
two the-state-of-the-art systems and six modified ones by our
disambiguation model, and finally we show the experiments
in Section 7.
4 Focus-sensitive Relation Disambiguation
4.1 Disambiguation Model
We suppose that truly correct relations between arguments
are triggered by focal sentence constituents while pseudo-
correct ones are triggered by complementary or ancillary sen-
tence constituents. Accordingly, if a pair of arguments im-
plies multiple relations, the correct one can be determined by
the following model:
r
∗
= argmaxP(r| f ) (1)