The Production-Distribution-Compre-hension (PDC) approach to language processing has
the potential to be enormously productive because of its breadth and its elegant simplicity.
As the name suggests, this framework relates phenomena from language production, typology,
and comprehension in a specific direction: speakers' preferences determine the frequency
with which linguistic forms occur; those frequencies determine form distributions;
and frequent forms are predicted to be easier to comprehend, an idea that is based
on classic and well-established principles of learning. Because the production system
essentially leads this parade, the MacDonald article (MacDonald, 2013) focuses more
on production than the other systems of language, which in many ways is a welcome
change from the standard bias in psycholinguistics. Thus, in production, the approach
neatly captures the sometimes competing forces that influence lexical and grammatical
choices, which MacDonald (2013) refers to as Easy First, Reuse, and Reduce Interference.
My goal in this commentary is to extend the PDC approach to two domains that are not
discussed in the article: prosody and disfluency. These domains of language processing
are obviously critically important for dialog and conversation, and they appear also
to follow the basic principles of PDC, and in fact nicely illustrate its strengths.
As mentioned, MacDonald's analysis of language production assumes that speech is influenced
by three preferences: Easy First, Reuse, and Reduce Interference (see article for
details), which can be caricatured as Blurt, Mimic, and Space, respectively. Blurt
is the tendency to initiate production with material that is more available or accessible
in memory, Mimic is the tendency to repeat what you or your interlocutor has said,
and Space is the tendency to separate difficult items using a range of devices including
syntactic alternations, optional words such as that, and potentially also pauses,
fillers (um, er), and syllable elongation.
In the realm of prosody and production, these three forces might lead to the following
tendencies: First, Blurt encourages speakers to begin their utterances with the concept
that is most semantically or phonologically ready (Levelt and Maassen, 1981; Bock,
1987), and to articulate accessible words in a way that requires minimal effort—that
is, to make the words shorter, quieter, and generally less prominent. Blurt would
thus lead to a tendency toward the so-called Given-New strategy, which states that
speakers prefer to begin their utterances with information that has been established
in the context, and to place novel contributions near the end (Haviland and Clark,
1974). Moreover, the Nuclear Stress Rule states that, in English, the default pattern
is for main sentence stress to occur at the end of the utterance (Chomsky and Halle,
1968). The combination of Given-New and the NSR leads to sentences in which given
elements tend to be early and less prosodically prominent, and new elements tend to
be late and accented.
Mimic predicts that prosodic patterns can be primed just like syntactic structures.
This idea has not received a great deal of attention in the literature thus far, but
certainly some degree of prosodic repetition seems to be at work in conversation.
To see how these two forces might influence prosodic forms, consider the question:
Who frightens Bill? The Given-New strategy, now reformulated as a version of Blurt
or Easy First, predicts that Bill is frightened by everybody would be a felicitous
reply, and intuition suggests that it is. But if Mimic (Reuse) is more dominant than
Blurt, the second speaker might be pushed into replying with Everybody frightens Bill—that
is, to reuse the first speaker's syntactic and prosodic forms, the second speaker
might also choose to create an active sentence, which now puts the easier material
late in the sentence—contrary to Given-New. It's particularly interesting that these
two strategies have different consequences for the prosody of the overall sentence.
If Blurt wins, then the new information everybody is prosodically even more prominent
than what is mandated by the NSR (Bill is frightened by EVERYBODY). But if Mimic wins,
then the new information will get an early position in the utterance, but it also
must be prosodically prominent (EVERYBODY frightens Bill). Thus, it can be argued
that Blurt or Easy First pushes speakers to conform to the Given-New strategy, whereas
Mimic or Reuse pushes them to use prosodic devices such as contrastive prominence.
However, some languages, including Italian and other Romance languages, do not allow
prosodic prominence early in utterances, as in EVERYBODY frightens Bill; syntactic
devices for varying constituent prominence must be used instead (Samek-Lodovici, 2006).
These languages would seem to somehow have developed grammatical constraints that
favor efficiency related to lexical-phonological availability over efficiency related
to pattern reuse. Finally, the third force, Reduce Interference or Space, is also
potentially applicable to prosodic patterns in language. Space predicts that phrase-final
lengthening and pausing will be used to separate elements that are potentially interfering,
which seems consistent with findings in the literature (e.g., Smith and Wheeldon,
2004) and is assumed in some models of language production and prosody (Watson and
Disfluencies are a closely related topic of increasing interest in the field of psycholinguistics
(e.g., Arnold et al., 2003; Ferreira et al., 2004). The PDC framework offers a useful
way to think about the relationship between disfluency production, distribution, and
comprehension. Again, in production, three forces are at work: Blurt, Mimic, and Space.
Put very simply, it seems plausible that when Blurt and Mimic dominate, speakers will
tend to generate sequences that will occasionally need to be repaired; and when Space
dominates, people might tend to produce “pauses” of various sorts, including not just
silent intervals but also uhs, ums, and repetitions (the the aardvark). If a speaker
begins an utterance with a concept that is highly available—that is, if she blurts—she
might find herself producing an infelicitous sequence, or possibly even an opening
that appears unlikely to have a grammatical outcome (e.g., if the object of a preposition
is the most active concept and the speaker begins an utterance with it, a repair may
be required, as in the trunk—uh put the beer in the trunk). This tendency has been
observed in our studies of language production focusing on individuals diagnosed with
Attention Deficit Hyperactivity Disorder (ADHD) (Engelhardt et al., 2009): People
with ADHD who were required to generate an utterance with a past participle form such
as ridden were more likely to begin their utterances with an animate entity (the girl
ridden) which then necessitated a repair (the girl—uh the bicycle was ridden by the
girl) than were age- and demographically matched subjects with no history of ADHD.
Given that ADHD is associated with problems relating to inhibitory control, it might
be expected that individuals suffering from it would be more likely to follow Blurt
or Easy First, often triggering repairs. Mimic or Reuse should work similarly—given
that simply mimicking another person's utterance will sometimes impede communicative
success, we can predict that individuals in whom this force dominates over Blurt or
Space will tend to produce sequences that are infelicitous or just plain wrong, leading
to the need for repair. This idea has not been tested yet, to the best of my knowledge,
but the logic seems fairly straightforward.
Reduce Interference, as mentioned, leads speakers to space apart difficult elements,
and therefore might lead speakers to produce uhs, ums, and repeats. Indeed there is
evidence that concepts that are hard to retrieve are sometimes preceded by filler
disfluencies (Arnold et al., 2003). Perhaps more interesting are cases in which speakers
appear to take advantage of lexical and syntactic alternatives to buy themselves extra
planning time during production, in effect treating these linguistic elements as disfluencies.
As MacDonald notes in her article, speakers more often include the optional that in
sentence complement structures when the first phrase of the embedded clause is difficult
to retrieve. One especially compelling demonstration of this tendency comes from Ferreira
and Firato (2002), who used a sentence recall paradigm to induce speakers to produce
sentences with conceptually similar noun phrases, as in The author, the poet, and
the biographer recognized (that) the writer/golfer was boring. They observed that
speakers were more likely to include the optional that and to produce disfluencies
when the subject of the embedded clause was writer rather than golfer. This effect
was observed because interference builds up over the matrix subject, and so the speaker
takes advantage of the optional that to space the difficult-to-retrieve item from
the ones that preceded it. Recently, the Uniform Density Hypothesis has been offered
to capture speakers' tendency to distribute information evenly across an utterance,
which in part explains the inclusion or omission of optional grammatical words such
as complementizers (Jaeger, 2010). The PDC framework assumes that Uniform Density
is a product of Reduce Interference or Space, and so is only one force at work during
production. Indeed, it would be expected that when Blurt or Mimic dominate over Space,
information would not be uniformly distributed over the utterance.
This framework can also be turned around to help explain how linguistic devices encourage
fluency. Blurt allows the speaker to begin an utterance without the need for uhs and
ums, because the speaker can start with what is already available. Utterance priming
associated with Reuse or Mimic allows the speaker to focus on lexical choices rather
than syntactic or prosodic planning, and promotes alignment between interlocutors
(Pickering and Garrod, 2004). And Space would allow the use of prosodic, syntactic,
and lexical devices to separate potentially interfering linguistic elements without
the need for repeats or uhs and ums. Fluent production, in other words, requires that
the three forces MacDonald has identified be optimally balanced. Conversely, disfluencies
will occur when one of the forces is out of balance; depending on which one that is,
different types of disfluency are predicted to occur.
The PDC framework thus appears to be a powerful approach for understanding language
processing. Its distinctive contribution is to unite production and comprehension
in a highly specific and testable way, and I hope that this piece demonstrates the
extent to which it may be applied to phenomena not explicitly discussed in the article.
Of course, many outstanding questions and issues remain. For example, it is not clear
how this approach to linking the production and comprehension systems differs from
that of Pickering and Garrod (2004), who appear to have different but not incompatible
goals: whereas MacDonald would like to explain how distributional patterns arise which
then influence comprehension, Pickering and Garrod's goal is to describe how the two
systems work together in real time to facilitate conversation and dialog. It goes
without saying that such a treatment is well beyond the scope of the MacDonald article
(MacDonald, 2013), but eventually it would be useful to reconcile or at least compare
and contrast these different frameworks, given the similar aims of both. It also would
be helpful to see more examples of linguistic ambiguities that the PDC approach can
explain. But at this point it is enormously helpful to have a clear, coherent framework
from which to ask these questions and to conduct future empirical investigations.