A Linear-time Model of Language Production: some psychological implications 
(extended abstract) 
David D. McDonald 
MIT Artificial Intelligence Laboratory 
Cambridge, Massachusetts 
Traditional psycholinguistic studies of language 
production, using evidence from naturally occurring 
errors in speech \[1\]\[2\] and from real-time studies of 
hesitations and reaction time \[3\]\[4\] have resulted in 
models of the levels at which different linguistic units 
are represented and the constraints on their scope. 
This kind of evidence by itself, however, can tell us 
nothing about the character of the process that 
manipulates these units, as there are many a priori 
alternative computational devices that are equally 
capable of implementing the observed behavior. It will 
be the thesis of this paper that if principled, non- 
trivial models of the language production process are 
to be constructed, they must be informed by 
computationally motivated constraints. In particular. 
the design underlying the linguistic component I have 
developed ("MUMBLE .... previously reported in \[5\]\[6\]) 
is being investigated as a candidate set of such 
constraints. 
Any computational theory of production that is to 
be interesting as a psycholinguistic model must meet 
certain minimal criteria: 
(1) Producing utterances incrementally, in their 
normal left-to-right order, and with a well- 
defined "point-of-no-return" since words 
once said can not be invisibly taken back~ 
(2) Making the transition from the non- 
linguistic "message"-level representation to 
the utterance via a linguistically structured 
buffer of only" limited size: people are not 
capable of linguistic precognition and can 
I. This report describes research done at the Artificial 
Intelligence Laboratory of the Massachusetts Institute of 
Technology. Support for the laboratory's artificial 
intelligence research is provided in part by the Advanced 
Research Projects Agency of the Department of Defence 
under Office of Naval Research contract 
N00014-75-C-0643. 
55 
readily "talk themselves into a corner ''z 
(3) Grammatical robustness: people make very 
few grammatical errors as compared with 
lexical selection or planning errors ("false 
starts") \[7\]. 
Theories which incorporate these properties as an 
inevitable consequence of independently motivated 
structural properties will be more highly valued than 
those which only stipulate them. 
The design incorporated in MUMBLE has all of 
these properties~ they follow from two key 
intertwined stipulations--hypotheses--motivated by 
intrinsic differences in the kinds of decisions made 
during language production and by the need for an 
efficient representation of the information on which 
the decisions depend (see \[8\] for elaboration). 
(i) 
(~) 
The execution time of the process is linear in 
the number of elemenzs in ~he input 
message, i.e. the realization decision for each 
element is made only once and may not be 
revised. 
The representation for pending realization 
decisions and planned linguistic actions (the 
results of earlier decisions) is a surface-level 
syntactic phrase structure augmented by 
explicit labelings for its constituent 
positions (hereafter referred to as the tree). 3 
This working-structure is used 
simultaniously for control (determining 
what action to take next), for specifying 
constraints (what choices of actions are 
Z. In addition, one inescapable conclusion of the research 
on speech-errors is that the linguistic representation(s) 
used during the production process must be capable of 
representing positions independently of the units (lexical or 
phonetic) that occupy them. This is a serious problem for 
ATN-b~sed theories of production since they have no 
representation for linguistic structures that is independent 
front their representation of the state of the process. 
3. The leaves of this tree initially contain to-be-realized 
message elements. These are replaced by syntactic/lexical 
structures as the tree is refined in a top-down, 
left-to-right traversaL Words are produced as they are 
reached at (new) leaves, and grammatical actions are taken 
as directed by the annotation on the traversed regions. 
ruled out because of earlier decisions), for 
the representation of linguistic context, and 
for the implementation of actions motivated 
only by grammatical convention (e.g. 
agreement, word-ordar within the clause, 
morphological specializations; see \[6\]). 
The requirement of linear time rules out any 
decision-making techniques that would require 
arbitrary scanning of either message or tree. Its 
corollary, "Indelibility", 4 requires that message be 
realized incrementally according to the relative 
importance of the speaker's intentions. The paper will 
discuss how as a consequence of these properties 
decision-making is forced to take place within a kind 
of blinders: restrictions on the information available 
for decialon-making and on the possibtUtias for 
monitoring and for invisible self-repair, all describable 
in terms of the usual linguistic vocabulary. A further 
consequence is the adoption of a "lexicalist" position on 
transformations (see \[9\]), i.e. once a syntactic 
construction has been instantiated in the tree, the 
relative position of its constituents cannot be modified; 
therefore any "transformations" that apply must do so 
at the moment the construction is instantiatad and on 
the basis of only the information available at that time. 
This is because the tree is not buffer of objects, but a 
program of scheduled events. 
Noticed regularities in speech-errors have 
counter-parts in MUMBLE's design 5 which, to the 
extent that it is Independently motivated, may provide 
an explanation for them. One example is the 
4. I.e. decisions are not subJeCt to backup-="they are 
~rritten in indelible ink". This is also a property of 
Marcus's "deterministic" parser. It is intriguing to 
speculate that indelibility may be a key characteristic of 
psychologically plausible performance theories of natural 
language. 
5. MUMBLE produces text. not speech. Consequently it 
has no Knowledge of syllable structure or intonation and 
can make no specific contribution= to the explanation of 
errors at that level. 
phenomena of combined-form errors: word-exchange 
errors where functional morphemes such as plural or 
tense are "stranded" at their ori~inal positions, e.g. 
"My locals are more variable than that." 
Intended- "...variables are more local" 
"Why don't we Eo to the 24hr. Star Marked and 
you can see my friend check in E cashes." 
Intended: "...cashing checks." 
One of the things to be explained about these errors is 
why the two classes of morphemes are distinguished-- 
why does the "exchanging mechanism" effect the one 
and not the other? The form of the answer to this 
question is generally agreed upon: two independent 
representations are being manipulated and the 
mechanism applies to only one of them. MUMBLE 
already employs two representations of roughly the 
correct distribution, namely the phrase structure tree 
(defining positions and grammatical properties) and 
the message (whose elements occupy the positions and 
prompt the selection of words). By incorporating 
specific evidence from speech-errors into MUMBLE's 
framework (such as whether the quantifier all 
participates in exchanges), it is possible to perform 
synthetic experiments to explore the impact of such a 
hypothesis on other aspects of the design. The 
interaction with psycholinguistios thus becomes a 
two-way street. 
The full paper 6 will develop the notion of a 
linear-time production process: how it is accomplished 
and the specific limitations that it imposes, and will 
explore its implications as a potential explanation for 
certain classes of speech-errors, certain hesitation and 
self-correction data. and certain linguistic constra_nts. 
6. Regretably, the completion of this paper has been 
delayed in order for the author to give priority to his 
dissertatlon. 
56 
References 
\[I\] Garrett. M.F. (1979) "Levels of Processing in 
Sentence Production", in Butterworth ed. 
Language Production Volume I, Academic Press. 
\[2\] Shattuck Hufnagel, S. (1975) Speech Errors and 
Sentence Production Ph.D. Dissertation, 
Department of Psycholog~v, MIT. 
\['3\] Ford. M. & Holmes V.M. (1978) "Planning units and 
syntax in sentence production", Cognition 6, 35- 
63. 
\['4\] Ford M. (1979) "Sentence Planning Units: 
Implications for the speaker's representation of 
meaningful relations underlying sentences", 
Occasional Paper 2, Center for Cognitive Science, 
MIT. 
\['5\] McDonald, D,D. (1978) "Making subsequent 
references., syntactic and rhetorical constraints", 
TINLAP-g. University of Illinois. 
\[6\] (1978) "Language generation: 
Automatic Control of Grammatical Detail", COLING- 
78. Bergen. Norway. 
\['7\] Fay, D. (1977) "Transformational Errors". 
International Congress of Linguistics. Vienna, 
Austria. 
\[8\] McDonald D.D. (in preparation) Natural Language 
Production as a Process of Decision-making 
Under ConsU'alnt Ph.D. Dissertation, Department 
of Electrical Engineering and Computer Science, 
MIT. 
\[9\] Bresnan, J. (1978) "Toward a realistic theory of 
grammar", in Bresnan. Miller, & Halle ads. 
Linguistic Theory and Psychological Reality Mrr 
Press. 
57 

