REAPING THE BENEFITS OF INTERACTIVE SYNTAX 
AND SEMANTICS* 
Kavi Mahesh 
Georgia Institute of Technology 
College of Computing 
Atlanta, GA 30332-0280 USA 
Internet: mahesh@cc.gatech.edu 
Abstract 
Semantic feedback is an important source of informa- 
tion that a parser could use to deal with local ambigu- 
ities in syntax. However, it is difficult to devise a sys- 
tematic communication mechanism for interactive syn- 
tax and semantics. In this article, I propose a variant of 
left-corner parsing to define the points at which syntax 
and semantics should interact, an account of grammat- 
ical relations and thematic roles to define the content of 
the communication, and a conflict resolution strategy 
based on independent preferences from syntax and se- 
mantics. The resulting interactive model has been im- 
plemented in a program called COMPERE and shown 
to account for a wide variety of psycholinguistic data 
on structural and lexical ambiguities. 
INTRODUCTION 
The focus of investigation in language processing 
research has moved away from the issue of seman- 
tic feedback to syntactic processing primarily due 
to the difficulty of getting the communication be- 
tween syntax and semantics to work in a clean and 
systematic way. However, it is unquestionable that 
semantics does in fact provide useful information 
which when fed back to syntax could help elimi- 
nate many an alternative syntactic structure. In 
this article, I address three issues in the commu- 
nication mechanism between syntax and semantics 
and provide a complete and promising solution to 
the problem of interactive syntactic and semantic 
processing. 
Since natural languages are replete with ambi- 
guities at all levels, it appears intuitively that a 
processor with incremental interaction between the 
levels of syntax and semantics which makes the best 
and immediate use of both syntactic and semantic 
information to eliminate many alternatives would 
win over either a syntax-first or a semantics-first 
mechanism. In order to devise such an interactive 
mechanism, one has to address three important is- 
sues in the communication: (a) When to communi- 
cate: at what points should syntax and semantics 
interact, (b) What to communicate: what and how 
*The author would like to thank his advisor Dr. 
Kurt Eiselt and his colleague Justin Peterson for their 
support and valuable comments on this work. 
much information should they exchange, and (c) 
How to agree: how to resolve any conflicting pref- 
erences between syntax and semantics. 
In this article, I propose (a) a particular variant 
of left-corner parsing that I call Head-Signaled Left 
Corner Parsing (HSLC) to define the points where 
syntax and semantics should interact, (b) an ac- 
count of grammatical relations based on thematic 
roles as a medium for communication, and (c) a 
simple strategy based on syntactic and semantic 
preferences for resolving conflicts in the communi- 
cation. These solutions were motivated from an 
analysis of a large body of psycholinguistic data 
and account for a greater variety of experimen- 
tal observations on how humans deal with struc- 
tural and lexical ambiguities than previous models 
(Eiselt et al, 1993). While it also appears that the 
proposed interaction with semantics could make 
improvements to the efficiency of the parser in deal- 
ing with real texts, such a conclusion can only be 
drawn after an empirical evaluation. 
WHEN TO COMMUNICATE 
Syntax and semantics should interact only at those 
times when one can provide some information to 
the other to help reduce the number of choices be- 
ing considered. Only when the parser has analyzed 
a unit that carries some part of the meaning of the 
sentence (such as a content word) can semantics 
provide useful feedback perhaps using selectional 
preferences for fillers of thematic roles. We need 
to design a parsing strategy that communicates 
with semantics precisely at such points. While pure 
bottom-up parsing turns out to be too circumspect 
for this purpose, pure top-down parsing is too eager 
since it makes its commitments too early for seman- 
tics to have a say. A combination strategy called 
Left Corner (LC) parsing is a good middle ground 
making expectations for required constituents from 
the leftmost unit of a phrase but waiting to see the 
left corner before committing to a bigger syntactic 
unit (E.g., Abney and Johnson, 1991). In LC pars- 
ing, the leftmost child (the left corner) of a phrase 
is analyzed bottom-up, the phrase is projected up- 
ward from the leftmost child, and other children of 
the phrase are projected top-down from the phrase. 
310 
While LC parsing defines when to project top- 
down, it does not tell us when to make attachments. 
That is, it does not tell when to attempt to at- 
tach the phrase projected from its left corner to 
higher-level syntactic units. Should it be done im- 
mediately after the phrase has been formed from its 
left corner, or after the phrase is complete with all 
its children (both required and optional adjuncts), 
or at some intermediate point? Since ambigui- 
ties arise in making attachments and since seman- 
tics could help resolve such ambiguities, the points 
at which semantics can help, determine when the 
parser should attempt to make such attachments. 
LC parsing defines a range of parsing strategies 
in the spectrum of parsing algorithms along the 
"eagerness" dimension (Abney and Johnson, 1991). 
The two ends of this dimension are pure bottom- 
up (most circumspect) and pure top-down (most 
eager) parsers. Different LC parsers result from 
the choice of arc enumeration strategies employed 
in enumerating the nodes in a parse tree. In Arc 
Eager LC (AELC) Parsing, a node in the parse tree 
is linked to its parent without waiting to see all its 
children. Arc Standard LC (ASLC) Parsing, on the 
other hand, waits for all the children before making 
attachments. While this distinction vanishes for 
pure bottom-up or top-down parsing, it makes a 
big difference for LC Parsing. 
In this work, I propose an intermediate point 
in the LC Parsing spectrum between ASLC and 
AELC strategies and argue that the proposed 
point, that I call Head-Signaled LC Parsing 
(HSLC), turns out to be the optimal strategy for in- 
teraction with semantics. In this strategy, a node 
is linked to its parent as soon as all the required 
children of the node are analyzed, without waiting 
for other optional children to the right. The re- 
quired units are predefined syntactically for each 
phrase; they are not necessarily the same as the 
'head' of the phrase. (E.g., N is the required unit 
for NP, V for VP, and NP for PP.) HSLC makes 
the parser wait for required units before interacting 
with semantics but does not wait for optional ad- 
juncts (such as PP adjuncts to NPs or VPs). The 
parsing spectrum now appears thus: 
(Bottom-Up --~ Head-Driven -~ ASLC -~ HSLC 
-~ AELC --~ Top-Down) 
Algorithm HSLC: 
Given a grammar and an empty set as the initial 
forest of parse trees, 
For each word, 
Add a new node T~ to the current forest of 
trees {Ti} for each category for the 
word in the lexicon 
mark T~ as a complete subtree 
Repeat until there are no more complete trees 
that can be attached to other trees, 
Propose attachments for a complete 
subtree Tj 
to a T~ that is expecting Tj, or 
to a T~ as an optional constituent, or 
to a new Tk to be created if Tj can be 
the left corner (leftmost child) of Tk 
Select an attachment (see below) and attach 
If a new Tk was created, add it to the forest, 
and make expectations for required units 
of Tk 
If a T~ in the forest has seen all its required 
units, 
Mark the T~ as a complete subtree. 
Consider a PP attachment ambiguity and the 
tree traversal labelings produced by different LC 
parsers shown in Figure 1. It can be seen from Fig- 
ure la that AELC attempts to attach the PP to 
the VP or NP even before the noun in the PP has 
been seen. At this time, semantics cannot provide 
useful feedback since it has no information on the 
role filler for a thematic role to evaluate it against 
known selectional preferences for that role filler. 
Thus AELC is too eager for interactive semantics. 
ASLC, on the other hand, does not attempt to at- 
tach the VP to the S until the very end (Fig lb). 
Thus even the thematic role of the subject NP re- 
mains unresolved until the very end. ASLC is too 
circumspect for interactive semantics. HSLC on 
the other hand, attempts to make attachments at 
the right time for interaction with semantics (Fig 
lc). 
6 / 
(a) AELC 22D~ T 26"~ (b) ASLC 
6 
1%R ~ 2~, 
22DE" T 24 N 
(©) HSLC 
Figure 1: LC Parsers at an Attachment Ambiguity 
WHAT TO COMMUNICATE 
The content of the communication between syntax 
and semantics is a set of grammatical relations and 
thematic roles. Syntax talks about the grammati- 
cal relations between the parts of a sentence such 
311 
as Subject, Direct-object, Indirect-object, preposi- 
tional modifier, and so on. Semantics talks about 
the thematic relations between parts of the sen- 
tence such as event, agent, theme, experiencer, 
beneficiary, co-agent, and so on. These two closed 
classes of relations are translated to one another 
by introducing what I call "intermediate roles" 
to take into account other kinds of linguistic in- 
formation such as active/passive voice, VP- vs. 
NP-modification, and so on. Examples of inter- 
mediate roles are: active-subject, passive-subject, 
VP-With-modifier, subject-With-modifier, and so 
on. While space limitations do not permit a 
more detailed description here, the motivation for 
intermediate roles as declarative representations 
for syntax-semantics communication has been de- 
scribed in (Mahesh and Eiselt, to appear). 
The grammatical relations proposed by syntax 
are translated to the corresponding thematic rela- 
tions using the intermediate roles. Semantics eval- 
uates the proposed role bindings using any selec- 
tional preferences for role fillers associated with the 
meanings of the words involved. It communicates 
back to syntax a set of either an Yes, a No, or 
a Don't-Care for each proposed syntactic attach- 
ment. A Yes answer is the result of satisfying one 
more selectional preferences for the role binding; a 
No for failing to meet a selectional constraint; and 
a Don't-Care when there are no known preferences 
for the particular role assignment. 
HOW TO AGREE 
Since syntax and semantics have independent pref- 
erences for multiple ways of composing the different 
parts of a sentence, an arbitrating process (that I 
call the Unified Process) manages the communica- 
tion and resolves any conflicts. This unified process 
helps select the alternative that is best given the 
preferences of both syntax and semantics. In ad- 
dition, since the decisions so made are never guar- 
anteed to be correct, the unified process is not de- 
terministic and has the capability of retaining uns- 
elected alternatives and recovering from any errors 
detected at later times. The details of such an er- 
ror recovery mechanism are not presented here but 
can be found in (Eiselt et al, 1993) for example. 
Syntax has several levels of preferences for the 
attachments it proposes based on the following cri- 
teria: Attachment (of a required unit) to an expect- 
ing unit has the highest preference. Attachment as 
an optional constituent to an existing (completed) 
unit has the next highest preference. Attachment 
to a node to be newly created (to start a new 
phrase) has the least amount of preference. These 
preferences are used to rank syntactic alternatives. 
The algorithm for the unified process: 
Given: A set of feasible attachments {AI} where each 
Ai is a fist of the two syntactic nodes being attached, 
the level of syntactic preference, and one of (Yes, No, 
Don't-Care) as the semantic feedback, 
If the most preferred syntactic alternative has 
an Yes or Don't-Care, select it 
else if no other syntactic alternative has a Yes, 
then select the most preferred syntactic 
alternative that has a Don't-Care 
else delay the decision and pursue multiple 
interpretations in parallel until further 
information changes the balance. 
DISCUSSION 
The model of interactive syntactic and semantic 
processing proposed accounts for a wide range psy- 
cholinguistic phenomena related to the handling 
of lexical and structural ambiguities by human 
parsers. Its theory of communication and the arbi- 
tration mechanism can explain data that modular 
theories of syntax and semantics can explain as well 
as data that interactive theories can (Eiselt et al, 
1993). For instance, it can explain why sentence 
(1) below is a garden-path but sentence (2) is not. 
(1) The officers taught at the academy were very 
demanding. 
(2) The courses taught at the academy were very 
demanding. 
HSLC is different from both head-driven pars- 
ing and head-corner parsing. It can be shown that 
the sequence of attachments proposed by HSLC is 
more optimal for interactive semantics than those 
produced by either of the above strategies. HSLC 
is a hybrid of left-corner and head-driven parsing 
strategies and exploits the advantages of both. 
In conclusion, I have sketched briefly a solution 
to the three problems of synchronization, content, 
and conflict resolution in interactive syntax and se- 
mantics. This solution has been shown to have dis- 
tinct advantages in explaining psychological data 
on human language processing. The model is also 
a promising strategy for improving the efficiency of 
syntactic analysis. However, the latter claim is yet 
to be evaluated empirically. 

REFERENCES 
Steven P. Abney and Mark Johnson. 1991. 
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Kurt P. Eiselt, Kavi Mahesh, and Jennifer K. Hol- 
brook. 1993. Having Your Cake and Eating It Too: 
Autonomy and Interaction in a Model of Sentence 
Processing. Proc. Eleventh National Conference 
on Artificial Intelligence (AAAI-93), pp 380-385. 
Kavi Mahesh and Kurt P. Eiselt. To appear. Uni- 
form Representations for Syntax-Semantics Arbi- 
tration. To appear in Proc. Sixteenth Annual Con- 
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