Language Without A Central Pushdown Stack 
Carson T. Schtitze and Peter A. Reich 
Department of Linguistics 
University of Toronto 
Toronto, Canada M5S 1A1 
E-mail: carsontr@ dgp.toronto.edu 
Abstract 
We will attempt to show how human performance 
limitations on various types of syntactic embedding 
constructions in Germanic languages can be modelled 
in a relational network linguistic framework. After 
arguing against centralized data stores such as 
pushdown stacks and queues, we will demonstrate how 
interconnections among levels of linguistic structure 
can account for many of the psycholinguistic facts. 
1. Introduction 
The long-range goal of our research project is to 
develop and implement in computer simulation a uni- 
fied, psycholinguistically realistic model of language 
behaviour--specifically, language production, language 
comprehension, and language development. Our model 
is constructed in the framework of relational network 
linguistics (also known as cognitive-stratificational 
grammar (Copeland & Davis 1980)), while also 
incorporating features of spreading activation (Collins 
& Loftus 1975), competition (Bates & MacWhinney 
1987), and other models. In this paradigm, linguistic 
information is represented in the form of an 
asynchronous, massively parallel network (in the sense 
of Schnelle et al. 1988) whose nature can be seen as 
intermediate between that of mainstream connectionist 
networks and traditional generative grammars, 
incorporating aspects of both formalisms. The basics of 
relational network grammars will be set forth in §3. 
Our specific goal here is to describe our attempts to 
simulate the psychological facts about the production of 
syntactic embedding in a non-ad hoc way within the 
above framework. We will show that adequately ac- 
counting for human performance requires an ex- 
amination of at least two other representational levels 
(or strata) in addition to the syntax, namely the event 
level and a level we call the lexotacticso In the process 
we also hope to demonstrate the unified nature of our 
theory across linguistic levels. 
Particular consideration will be given to the 
cognitive data structures and processing mechanisms 
required. Our claim in this regard is that temporary 
memory in the form of a centralized data store, whether 
it be a pushdown stack, a queue, a collection of stacks, 
or whatever, is an inappropriate component for 
modelling human linguistic processing. We will argue 
instead for a large collection of very simple, localized 
processing units which possess small amounts of 
storage by virtue of the possible states which they may 
be in; in short, a network of finite-state devices. These 
processors will be general, in the sense that they are 
useful for cognitive processes outside the domain of 
language. 
64 
2. The Phenomena 
The types of sentences we want to account for 
include centrally embedded and crossed serial structures 
(see below), in contrast to right- and left-branching 
structures. This is one area where computational 
linguists find it useful to look at human limitations, 
because if there were strict limits on the former forms of 
embedding, they could allow us to build simpler lan- 
guage processors. Many linguists continue to claim that 
because of potentially indefinite embedding, something 
more powerful than a finite-state device is needed. But 
in some sense all computational linguistics is done on 
machines which have a limited number of states, and 
some of us believe that the human brain also has this 
property. 
The matter first arose when Chomsky (1957) 
argued that natural language cannot be produced by a 
finite-state device, because of sentences with central 
embedding to an arbitrary depth. Chomsky's argument 
was that such sentences are all grammatical because 
they are "formed by processes of sentence constn~ction 
so simple that even the most rudimentary English 
grammar would contain them" (1957: 23). What 
Chomsky meant by rudimentary processes was 
recursion, and within the generative framework there 
was no way to account for people's performance 
limitations in terms of recursive phrase structure rules. 
The issue of whether embedding to arbitrary depth 
is part of natural language has been debated in the 
psychological and linguistic literature ever since (e.g., 
Miller 1962; Labov 1973; de Roeck et al. 1982). While 
it is still a point of contention, recent carefully 
controlled experiments suggest that the human syntactic 
mechanism, without semantic or pragmatic cues, and 
without the aid of pencil and paper to construct 
sentences, does have a sharp limit of one or two levels 
of embedding (Bruner & Cromer 1967; Reich & Dell 
1977). In this paper we make no claim as to how many 
levels people are able to process, only that there is some 
small, finite bound. See Reich & Schiitze 1990 for 
further discussion. 
The following are some examples of the types of 
sentences we are particularly concerned with. Although 
some of these are judged to be marginal or unacceptable 
by some informants, none of the subsequent discussion 
depends on precisely where the limits of acceptability 
are drawn. 
• Centrally embedded relative clause constructions in 
English. A clause B is centrally embedded within a 
clause A if and only if part of A occurs before B and the 
remainder of A occurs after B. 
1. The man that the dog chased ate the malt. 
2. The man that the dog that bit the cat chased ate 
d~e malt. 
. Verb-final complement constructions in German, 
which involve nested dependencies. Nested 
dependencies occur when the verbs at the end of the 
sentence are produced in reverse order from their 
associated arguments at the beginning. Thus, they must 
be paired up by working from the edges in toward the 
middle. 
3. Die Manner haben Hans die Pferde fiittern 
lehren. 
\[the men have Hans the horses to feed taught\] 
"The men taught Hans to feed the horses." 
4. Johanna hat die Manner tlans die Pferdefiittern 
lehren helfen. 
\[Joanna has the men Hans the horses to feed to 
teach helped\] 
"Joanna helped the men to teach tlans to feed 
the horses." 
• Verb-final complement constructions in Dutch, 
which involve crossed serial dependencies. Crossed 
serial dependencies occur when the verbs at the end of 
the sentence are produced in the same sequence as their 
associated arguments at the beginning. Sentences 5 and 
6 are synonymous with 3 and 4 respectively. 
5. De mannen hebben Hans de paarden leren 
voeren. 
\[the men have Hans the horses taught to feed\] 
6. Jeanine heeft de mannen Hans de paarden 
helpen leren voeren. 
\[Joanna has the men Hans the horses helped to 
teach feed\] (Bach et al. 1986) 
- Additional variations which are claimed to occur in 
Swiss-German (Shieber 1985): scntence 7 is in crossed 
serial order, 8 is in nested order, and 9 shows a variation 
of crossed serial order with the upper subject and dative 
object transposed; all three are synonymous. Analogous 
variations on 10 are also claimed lo be possible. 
7. Jan s',iit, das mer em ttans es huus hgilfed aastri- 
iehe. 
\[Jan says that we Hans the house helped to paint\] 
8. Jan sait, das mer em tlans es huus aastriiehe 
hi~lfed. 
\[Jan says that we Hans the house to paint helped\] 
9. Jan ~sait, das em llans meres huus hfilfed aastri- 
iche. 
\[Jan says that Hans we the house helped to paint\] 
"Jan says that we helped llans to paint the 
house." 
10. Jan ~t, das met d'ehind em ltans es huus 16rid 
hdlfe aastriiche. 
\[Jan says that we the children Hans the house let 
help paint\] 
"Jan says that we let the children help Hans 
paint the house." 
3. The Basics of Relationai Networks 
A relational network consists of a collection of 
nodes of various types, and connections between them~ 
known as wires. Language processing consists of ac- 
tivity flowing through the network or, in the case of ac- 
quisition, growing new wires and nodes in the network. 
Signals move in both directions along the wires from 
one node to the next. Each node is an independently 
operating processing unit. All nodes of the same type 
have the ~me finite-state definition. Their behaviour 
consists of sending signals out along the wires to which 
they are connected, and possibly changing state. Signals 
are one of a small number of possible types, e.g. 
production (also called positive),feedback,failure (or 
negative) and anticipation. 
There are currently approximately 25 types of 
nodes required in our system, expressing various ex- 
plicit and implicit features found in context-free phrase 
structure grammars and other formalisms. Each type of 
node is represented graphicaUy by a distinct shape. One 
basic building block is the concatenation node, 
equivalent to the phrase structure rule a ~' b c (see 
Figure 1). When a positive signal comes into the node 
via its top wire, which we label the a-wire, a positive 
signal will be sent down the b-wire to produce the first 
element, and the node will change state to 'remember' 
this fact. If the production of that element succeeds (as 
indicated by a positive feedback signal returning on the 
b-wire), a positive signal will be sent down the c-wire to 
produce the second element in the concatenated 
sequence, and the node will change skate again. Upon 
the c-wire's successful completion, positive feedback is 
sent up the a-wire and the node returns to the initial 
state, known as stale zero. 
FIGURE 1: Concatenation Node 
Other major types of nodes include: disjunction, 
which allows a choice of one alternative among many 
paths of production (e.g., a verb may be realized as 
"sing", "think", "walk", etc.); precedence disjunction, 
which also allows a choice of alternatives, but tries 
them one at a time, slopping as soon as one succeeds; 
conjunction (used in the Boolean sense), which simply 
fires all its downward w~res at once when a production 
signal comes in the top; and inverse conjunction, which 
requires that two or more separate conditions must be 
signalling for a path to be followed (e.g., the pronoun 
"we" can only be produced if plural, first person, and 
nominative case are all being signalled). 
In its gross structure, the network we propose 
connects a general memory for events to a semotactic 
level, a lexotactic level, a syntactic level, a 
phonological level, an articulatory level, and an 
auditory level. The same type of structure is found in all 
the levels, and except for the last two, all strata are used 
in both production and understanding. The syntax 
defines the sequence in which morPhemes arc'. built into 
words, phrases, and clauses. The phonotactics defines 
the sequence in which the sounds are combined into 
2 
65 
clusters, syllables, and phonological feet. The 
semotactics defines which concepts can be associated 
with which types of participants. (For example, the 
concept "fill" allows an agent and an affected partici- 
pant, among others, and the affected must be a type of 
container.) The lexotactics constrains the choice of 
vocabulary and its syntactic position on the basis of 
which elements of an event are to be expressed. The 
lexicon of the language, itself in the form of a network, 
is connected to all major areas of language structure, 
and is in part what binds the levels to one another. Each 
word, morpheme, or idiom connects to meaning, syntax, 
and phonological representation. In addition, there are 
some wires which pass control information between 
strata. 
Thus, the major differences between relational 
networks and connectionist ones h la Rummelhart and 
McClelland (1986) are that the former use several types 
of nodes with different behaviour, and the actions of 
each node depend on an internal state in addition to the 
incoming signals. Furthermore, output signals can be a 
more complex function of the input signals than simple 
weighted sums. 
4. Right Branching: Iteration with Clean-Up 
We now focus our attention on the production of 
embedded clauses in relational networks. Relational 
network syntax makes a strong distinction between 
right-branching clauses and centrally embedded ones. 
(Left-branching is handled similarly to right-branching.) 
In a right-branching structure, once an embedded 
clause is complete, the superordinate clause will also be 
complete (as in "This is the cat \[that killed the rat \[that 
ate the malt \[that was stored in the house \[that Jack 
built\]\]\]\]"). In such sentences there is no need to 
preserve any information about the superordinate clause 
once an embedded one has begun; in fact, from a 
psychological point of view, it is undesirable--we do 
not wish to posit more demands on working memory 
than are actually required to do the job. Hence, in our 
model, the superordinate clause is explicitly cleaned up 
before a right-branching embedded clause is begun. By 
cleaned up, we mean that the nodes involved in its 
production are returned to state zero by sending a 
positive feedback signal up through the syntactic 
network; that signal eventually reaches the top of the 
clause structure, at which point the embedded clause 
may begin. 
For this clean-up to happen when it does, namely 
before the start of an embedded clause, the syntax must 
'know' whether the current syntactic constituent is the 
final element of its superordinate clause. There is no 
independent way for the syntax to make this 
determination, since a direct object, say, might be 
followed by an indirect object, or by any number of 
prepositional phrases. Therefore, we must add 
something elsewhere in the network to allow this 
condition to be recognized. What we add is a control 
wire from the lexotactics to the clause-heading node in 
the syntax, which will signal when the final participant 
(defined broadly) is underway. (The lexotactics already 
has access to all participants of a clause right from its 
start, and is notified when each participant begins to be 
realized, for independent reasons.) We note that in some 
66 
sense the syntax is no longer completely autonomous. 
Whether this is actually a drawback is partly an empir~ 
ical psycholinguistic question; studies of Wernicke's 
aphasics could be relevant. 
5. From Iteration to Recursion: Central 
Embedding 
We have now described how, in cases of right- 
branching, each clause starts with an essentially pristine 
syntactic network, and therefore little more needs to be 
said about how indefinite iteration is possible in a finite- 
state device. The more difficult cases are centrally 
embedded and crossed serial constructions. In such 
structures, it is clear that a portion of some clauses is 
delayed, i.e. prevented from being realized, until some 
time after its usual (simplex clause) position. In most 
computational approaches, a centralized (though 
possibly implicit) data structure, be it a stack or a 
queue, is used to store these elements until it comes 
time to real~e them. We see a number of problems with 
this approach. 
First of all, there is a certain intuitive appeal to the 
suggestion that in people, currently active information 
is distributed in shallow storage across the cognitive 
network, rather than localized in a single, deep 
data store. (For instance, parking your car multiple 
times leads you to forget previous parking spots, but not 
how much money is in your wallet.) Secondly, it is not 
clear what a central store should look like in order for it 
to account for both 'queue' and 'stack' types of 
languages, i.e. crossed serial and strictly nested-order 
ones, especially since both orders may occur in a single 
language. Models which have been proposed to handle 
both cases have typically involved powerful 
formalisms, such as a sequence of stacks in Joshi's case 
(1985, 1990). The resulting processor is more powerful 
than a pushdown automaton (it can recognize some 
strictly context-sensitive languages), and it is our belief 
that structures in the brain are simply not this powerful. 
These two arguments are independent of what has often 
seemed to be the central quarrel relational network 
theorists have with other computational models, namely 
that they allow for nesting to unlimited depth. It might 
be simple enough, if somewhat arbitrary, to impose a 
finite limit on the size of stacks or queues in other 
models, and thus limit their generative power to that 
which we believe humans are capable of. However, this 
would not address our other objections. 
Our proposal is as follows. When the cognitive 
representation of an event becomes active, all its 
composing elements, i.e. the action and all the 
participants in it, are fired simultaneously, However, the 
realization of those elements is held 'in check:' until the 
syntax allows them to come out, one at a time. The 
realization of any given participant may involve 
producing one or more clauses which describe it (e.g., 
relative clauses, sentential complements), and these 
expansions may be produced before all the elements of 
their superordinate clause (in particular, the verb) have 
come out. However, since all aspects of an event fire 
simultaneously, the superordinate verb will have 
already been signalled. This is necessary because the 
choice of verb may affect the realization of its 
associated participants; in particular, it may determine 
in which syntactic position they must be realized. For 
example, the sentences "George borrowed the book 
ti'om Sue" and "Sue loaned the book to George" both 
describe the same event, but the choice of verb has 
forced George into subject position in the former case, 
and indirect object in the latter. 
The already-signalled superordinate verb will have 
generated an anticipation signal up towards the verb- 
completion wire of the syntax. (German and Dutch 
syntax allow only one verb immediately after the 
subject. Any remaining verbal elements are placed at 
the end of a clause, in what we will call the verb- 
completion position.) It is the handling of this signal, 
and any subsequent verb anticipations which come up, 
which determines the eventual order of production. (We 
are assuming for simplicity that verbs come out only in 
verb or verb-completion positions, although possibly as 
part of the 'wrong' clause.) The limit on how many 
nested clauses are possible turns out to be totally 
unrelated to this structure, deriving instead from the 
finite-state definitions of the nodes themselves (see § 7). 
How does the network keep track of which order 
verbs should come out in? The dashed box of Figure 2 
shows the relevant structure. This structure contains n 
placeholder nodes, where n = 1 + the number of 
possible embeddings in the language. (In this 
discussion, we will assume n = 3.) The placeholders are 
labelled pl, p2 and p3 in the figure. Each is connected 
to every verb of the language, and acts as a 'slot' for 
remembering one verb. Whenever a verbocompletion is 
required by the syntax, the network attempts to realize 
the first verb slot, then the second if the first was empty, 
and ,so on. These realizations will succeed if verbs have 
already been signalled from the events which they 
describe, the E's in the diagram. A verb signalling in 
this way tries to 'turn on' one of the positions in the 
sequence of possible verb-completions, and succeeds at 
doing so if and only if no other verb has already filled 
that position. In the case of crossed serial orders such as 
Dutch, verbs try to occupy the first slot first, exactly as 
to syntax 
erb-completion 
(D 
c 03 c 
0 (D 
c- 
o_ 
fl) 
d2: 
tactics to mo rphology 
FIGURE 2: Fragment of the network for Dutch 
shown in Figure 2. For nested-order languages such as 
German and English, the wiring from r's to p's is 
exactly reversed, so that a verb first tales to fill the last 
available slot, then works its way forward to em'lier 
slots until it finds one unoccupied (see Figure 3, which 
would replace the dashed box of Figure 2). 
,/ 
r3 i r2 Irl 
FIGURE 3: GetTnan network fragment 
6. A Detailed Example 
As an example, let us consider the production of 
the Dutch sentence 6, assuming sentences of such 
complexity to be possible under some circumstances. 
The sentence involves three events, represented by the 
conjunction nodes El, E2 and E3 in Figure 2. The 
events are: <doanna helping the men>> ~1), <<the men 
teaching Hans>> (E2), and <4~ans feeding the horses>> 
(E3). These events will fire in the order just stated, since 
this is how they are hierarchically arranged in the event 
structure (E2 and E3 each modify or constitute a par- 
ticipant of the next higher event). As production begins, 
"Jeanine", "helpen" and E2 fire simultaneously. 
"Jeanine" is realized immediately, but "helpen" cannot 
be immediately realized, for the tollowing reason. The 
first verb position of the clause has been filled by the 
auxiliary verb "heeft", which is the realization of a 
semantic element which marks this scenario as having 
taken place in the past. Since "helpen" could not be 
realized in this position, it must come out in the verb- 
completion position. Therefore, it causes an anticipation 
to fire up from inverse conjunction node il towards that 
position. That anticipation will be directed up to the 
first verb position, namely placeholder node p 1, by the 
routing node rl. (A routing node is an inverted 
precedence disjunction which attempts to send a signal 
up its leftmost wire to the placeholder at the other end. 
If that fails, it tries to send the signal up its remaining 
wires in order from left to right.) Since "helpen" is the 
first verb to signal in this sentence, pl will accept the 
anticipation and remember which of its wires the signal 
came in on. 
While all this is taking place, the syntax has begun 
realizing the subordinate event E2, <<the men teach 
ttans>>. "Mannen" can be immediately realized. "Leren" 
cannot, but it will again send an anticipation signal, this 
time via i2 up into the verb structure. This will be 
routed by r2 to the first position (pl) once again, but 
this time the anticipation will be rejected, because there 
is already a verb pending for this position. A failure 
signal is sent down by pl to signal this fact, and r2, see- 
ing that cancellation, now tries sending the anticipation 
up to the second position, p2. This time the anticipation 
will be accepted, since nothing has previously come up 
4 
67 
to this point, and the verb's wire will be remembered. 
Similarly the third verb, "voeren", will be routed to the 
tlfird slot when E3 fires, and remembered by p3. 
Now, as E3 is realized by the syntax, the syntax 
will license a verb-completion following the object 
"paarden", since there are no more participants in the 
lowest clause. As the verb-completion signal comes 
down, it passes through the precedence disjunction node 
dl, which tries each of its output wires in turn from left 
to right until one succeeds. Its first output leads to pl, 
which will succeed (since an anticipation has previously 
come up to it), and finally permit the first verb, namely 
"helpen", to be phonetically realized. (Node pl 
remembered the wire which led to the morphological 
representation of that verb.) Positive feedback from this 
production will trigger pl to return to state zero, and 
pass the feedback on up. Since the first alternative wire 
of the precedence disjunction dl succeeded, none of the 
others will be touched. The end of the verb-completion 
is signalled by dl, to which the syntax responds by 
'unwinding' the complement clause loop (not shown in 
Figure 2) once. 
We are now at the point of having finished all the 
participants in the middle clattse (the ~the men teaching 
Hans>~ clause), so all that remains at this level is the 
verb. Again the syntax signals the verb-completion 
wire, again the precedence disjunction dl tries the first 
path, but this time it will fail, because no verb is waiting 
at pl. In state zero, this placeholder node sends a 
negative signal up to the precedence disjunction, which 
must therefore try its next wire. This one will succeed, 
producing the verb which was remembered by p2, 
namely "leren". Similarly, as the syntax unwinds once 
more and signals for a verb-completion once more, the 
precedence disjunction will eventually find the verb 
held by p3 at the third position, namely "voeren", and 
the sentence will be complete. 
Incidentally, a close analogue to this method can be 
used to account for the order of appearance of noun 
phrases across embedded clauses as well. In the case of 
English, we can use a structure like Figure 3 to hold 
onto the direct object of a superordinate clause until 
after the direct object of an embedded relative clause 
has come out. For exanrple, in "The man who liked the 
dog hated the cat", "the cat" is the first direct object to 
be made available by the event structure, since it is a 
participant in the superordinate event, but the first direct 
object to come out is "the dog", so object NPs are 
realized in last-in, first-out order. Thus the handling of 
participants provides additional motivation for the types 
of nodes and structure tlmt we have posited to handle 
verbs. 
7. Some Theoretical Issues 
In a syntax in which nodes are finite state devices, 
the job of remembering the status of a clause falls on 
each and every node in the network, as follows. 
Suppose a node requires a set of s states to handle 
processing within one clause. Then in the worst case, 
for each of those states it will need a copy of the entire 
set to use for processing an embedded clause. Each set 
corresponds to remembering a different place where the 
superordinate clause was left off. The total number of 
states in the node will be s n, where n once again is the 
number of possible pending clauses. This approach is 
analogous to a programming language that does not 
allow subroutines. In such a language, a copy of a 
recurring block of code must appear at each place where 
it could be needed. This tendency to exponential growth 
could account for why languages seem to impose such 
severe restrictions on the amount of central embedding 
or crossed serial dependency. 
The interesting and crucial thing about the way the 
process described in §6 was carried out is that the 
mechanism for remembering verbs was totally 
independent of the mechanism which ordered them for 
output. That is, while a particular set of nodes each 
remembered which one verb was associated with its 
slot, the connections between nodes determined the 
order of output relative to the order of signalling. This 
means that all the various orderings which occur cross- 
linguistically can be accounted for by the same 
inventory of nodes. No additional data structure is re- 
quired; all that we must do to 'convert' from Dutch to 
German word order is to rewire the connections 
between the upward routing nodes and the placeholder 
nodes, so that slots are tried in exactly the opposite 
order. To handle the fact that a single language (like 
Swiss-German) may use different orders depending on 
syntactic context (or even stylistic factors), all we need 
to do is have the verb-completion wire branch into all 
the options, each of which will have its own precedence 
disjunction and set of placeholder nodes. The upward 
anticipation from a particular verb will be sent 
simultaneously to all the different orderings, and the 
syntax will choose the appropriate one and cancel the 
others. 
The close symmetry between German and Dutch in 
our model would seem to be a psycholinguistic 
shortcoming, given Bach et al.'s (1986) result that 
Dutch is easier to process than German. However, we 
believe that, to the extent that their results are 
meaningful, they are not attributable to a queue versus 
stack difference, but rather to something along the lines 
of Joshi's (1990) proposed Principle of Partial 
Interpretation, whereby the syntax can't forget about a 
clause unless an argument slot to place it in has already 
been processed. 
One could argue that, viewed somewhat abstractly, 
our collection of nodes and wires in fact implements a 
finite-sized convertible queue/stack. Our basic response 
to this is to point out once again that that is essentially 
an artifact, having been built up out of independent, 
lower-level components. As for the particular size 
('depth') of the data structure being stipulated, this 
really is not troubling. Note that such a structure could 
be any size--nothing in the node definitions would limit 
it to size three or four, since expanding it only requires 
adding more nodes. However, more than some small 
fixed number of verbs can never be realized nestedly, 
because the syntax simply will not be able to call for 
them. As described above, the definitions of the nodes 
which the syntax makes use of simply break down after 
a couple of nestings. It is therefore reasonable to postu- 
late that the acquisition process would have no reason to 
build the verb structure any larger than the syntax had 
ever called for. And with regard to the node definitions 
68 
themselves being arbitrary in their maximum nesting 
limitations, this is certainly true in the sense that we 
define them to Ix'. precisely powerful enough to do what 
humans can do with syntax. (It is possible to imagine 
that humans could have evolved with the capacity for, 
say, one fewer or one more nesting; we would not 
expect that number to follow from mlything else.) The 
point once again is that this limitation is distributed 
throughout the network, rather than being a function of 
the total amount of storage available. 
8. Areas for Further Research 
"Fhrough detailed computational modelling we have 
made significant progress in analyzing our theory, 
finding flaws and oversights, and making it more 
rigorous. We believe that, with the complexity of lin- 
guistic ruodels as it is today, no theory can lay strong 
claims to adequacy, completeness, correcmess, etc. 
unless it has been tested in a computer simulation. 
Having reworked the theory several times over a period 
of only a few months, we cannot stress this point 
vigorously enough. 
There are Several important questions which our 
research has not yet addressed. One m~or issue involv- 
ing high-level control between strata is that of precisely 
where and how the decision is made that a subordinate 
clause is to be I~oduced. in a highly interconnected 
semantic network of events, there will almost always be 
'extra' information available which could be used to 
expand the desc~ption of any participant in the tbrrn of 
a relative claus. We believe that ninny factors go into 
the decision as to whether or not to carry out this 
expansion. The~ would include pragmatic issues such 
as the purpose of communication, urgency of the 
conversation, amount of relevant knowledge believed to 
be possessed by the audience, etc. Even assuming we 
can wire in the relevant decision criteria, it still remains 
to show how the lexotactic and syntactic strata are 
notified ttmt an additional clause is being produced. One 
possibility is that the tiring of a new action (and/or the 
associated verb) is the trigger. 
Additionally, if we look back at the stated goals of 
the theory in our introduction, it is evident that only one 
of the three main areas of language behaviour has been 
explored, namely production. The whole question of 
how this system works for comprehension has barely 
been addressed for relational network models in 
general. 'I\]~e specific issue of embo~ding is sure to add 
more wrinkles. Furthermore, accounting for the 
acquisition of both iteration and recursion is a serious 
hurdle for any connectionist model of language to 
overcome. In our case, it will involve the network 
growing new structure, in addition to modifying 
connection weights. So far we have concentrated on 
convincing ourselves that a viable language processor 
can be created in network form, whereas connectionists 
more often are concerned with exploring how much 
information can be acquired when starting from a tabula 
rasa. While we have no clear ideas on how acquisition 
should proceed in our framework, we believe we have 
at least come up with a possible structure as an end-goal 
for future acquisition models to strive towards. 
Acknowledgements 
We would like to thank Elizabeth Cowper, Jan 
Wiebe and Graeme Hirst for their comments on a draft 
of this paper. This research was supported by a grant to 
the second author from the Social Sciences and 
Humanities Research Council of Canada. 

References 

Bach, Emmon, Colin Brown & William Marslen-Wilson 
(1986) Crossed and nested dependencies in German and 
Dutch: A psycholinguistic study. Language and 
Cognitive Processes 1:4, 249-262. 

Bates, Elizabeth & Brian MacWhirmey (1987) Competition, 
variation, and language learning. In B. MaeWhinney, 
Mechanisms of language acquisition, Hillsdale, N.J.: 
Lawrence Erlbaum, 157-193. 

Bruner, Jerome S. & R. Cromer (1967) An unpunished study 
of eye movements reported in Harvard Center for 
Cognitive Studies Seventh Annual Report, p. 7. 

Chomsky, Noanl (1957) Syntact& Structures. The Hague: 
Mouton. 

Collins, Allan M. & Elizabeth Lofms (1975) A spreading 
activation model of semantic processing. Psychological 
Review 82, 407-428. 

Copeland, James E. & Philip W. Davis, Eds. (1980) Papers in 
Cognitive-Stratificational Linguistics. Rice University 
Studies, Vol. 66. Houston, TX: Rice University. 

Joshi, Aravind K. (1985) Tree adjoining grammars: How 
much context-sensitivity is required to provide 
reasonable structural descriptions? In D. Dowry, L. 
Karttunen & A. Zwicky, eds., Natural Language 
Parsing: Pn2cchologieal, computational and theoretical 
perspectives, New York: Cambridge University Press, 
206-250. 

Joshi, Aravind K. (1990) Processing crossed and nested 
dependencies: An automaton perspective on the 
psycholinguistic results. Language and Cognitive 
Processes, to appear. 

Labov, William (1973) The place of linguistics research in 
American society. In Eric Hamp, ed., Themes in 
linguistics: The 1970s, The Hague: Mouton. 

Miller, George A. (1962) Some psychological studies of 
grammar. American Psychologist 17, 748-762. 

Reich, P.A. & G.S. Dell (1977) Finiteness and embedding. In 
R.J. DiPietro & E.L. Blansett, Jr., eds., The thirdLACUS 
forum, Columbia, S.C.: Hornbeam Press, 438-447. 

Reich, Peter A. & Carson T. Schiltze (1990) Syntactic 
Embedding: What Can People Really Do? Working 
paper in the Computer Applications Group, Department 
of Linguistics, University of Toronto. 

de Roeck, Anne, Roderick Johnson, Margaret King, Michael 
Rosner, Geoffrey Sampson & Nino Varile (1982) A 
Myth About Centre-Embexlding. Lingua 58, 327-340. 

Rumelhart, David E. & McClelland, James L. (1986) Parallel 
distributed processing: Explorations in the 
microstructures of cognition. Cambridge, MA: MIT 
Press. 

Schnelle, Helmut (moderator), with (alphabetically) G. 
Cottrell, P. Dey, J. Diederich, P. A. Reich, L. Shastri & 
A. Yonezawa (panelists) (1988) Panel: Parallel 
Processing in Computational Linguistics. In Drnes 
Vargha, ed., Proceedings of Coling Budapest, 
Association for Computational Linguistics, 595-598. 

Shieber, Stuart M. (1985) Evidence against the context- 
freeness of natural language. Linguistics and Philosophy 
8, 333-343. 
