Sentence Comprehension (II)
7. Assessing comprehension
8. Theories of asyntactic comprehension
9. Current thinking
7. Assessing comprehension
Imagine what a sentence would have to be like that forced the
comprehender to rely solely on the grammar and not be comprehendible
from gestures, expression in the voice, general knowledge, and
all the other cues that we pick up on in conversation. Here is
a sentence that does just that:
(1) It was the boy that the girl kissed.
This sentence only has word order and grammatical words to indicate
who was kissing whom. If you had to say who did the kissing, which
one of the participants would you pick? This is a slightly stilted
way of speaking and we do not use a sentence in this form very
often, but I hope you can see that it is the girl who is the one
kissing. It is this sort of sentence that the patients with comprehension
deficits find difficult to understand. They may end up guessing,
or going for the first one mentioned, or picking the more likely
one based on their knowledge of the world. When they cannot work
it out from the grammar, their scores are near 50% for chance
(guessing on a two-choice judgment) or maybe even below chance
(if using some strategy, such as picking the first participant
each time). You yourself probably would find this next sentence,
(2), easier than the one above, (1):
(2) It was the girl that kissed the boy.
It means the same thing as (1) but is easier to understand, isn't
it? The patients we are going to talk about also find (2) easier
than (1) and make many fewer errors on it. The trouble is, they
could be working it out on the basis of left-to-right allocation
of roles: the first person does the action to the second person.
This would be a consciously applied strategy, though it does reflect
a predominant feature of English. (3) is another difficult structure
for patients with sentence-comprehension difficulties, while (4)
is not, except for the most severely affected.
(3) The boy was kissed by the girl.
(4) The girl kissed the boy.
Sentence (3), like (2), is easy because the one on the left does
it to the one on the right, which is the more frequent arrangement
in English. In other words, many English sentences have a subject,
verb and object, in that order, and the actor-action-victim in
(4) are in the same order; so it is easy for the patient to interpret
this structure. Sentence (3) is called a passive structure and
has the reverse arrangement: the first one mentioned is the one
acted upon. So English expectations will lead the listener astray,
in the case of the passive, unless the comprehender can interpret
from the grammar.
To get an idea of the sort of task used to assess
sentence comprehension, click here.
7.1 Structures used in assessing comprehension in aphasia
Sentence (6) is called the object cleft construction. Various
ideas exist as to why it is harder than other structures, for
all of us, to comprehend. The listener has to understand, from
the order of the words, that the subject of the sentence is the
girl, the second one mentioned, not the first participant, the
boy. Then this structure has to be linked up to the roles of kisser
and kissee to understand who kissed whom. Just by a simple change
in word order, to that in (7), the subject of the sentence becomes
the first participant, who is the kisser. Sentence (7) is easier
to comprehend, perhaps because of something to do with this aspect
that the subject and role of Agent (kisser, here) are filled by
the same noun phrase. We do not yet know why there is an exaggerated
effect of complexity for patients.
7.2 Tasks used in assessing asyntactic comprehension
Lots of reasons have been put forward as to why some patients
find this structure of object cleft, and often the passive too
(The boy was kissed by the girl), as well as others, hard to handle
in a comprehension task. It is likely that there are as many reasons
as patients! Not many patients are investigated in enough detail,
on enough different tasks, to know why they have trouble with
such sentences. A big obstacle to this research is that we have
few if any tasks which directly reflect function of the parser
interpreter. The ones that are used for college students (such
as one called cross-modal priming) do not always work with patients.
Often tasks are limited to decisions at the end of the sentence,
whereas the processes involved in understanding have started as
soon as a word is heard. The one you tried earlier, actor identification,
is the kind where comprehension is tapped after the sentence.
It is thought that this method allows the listener to introduce
his or her own ideas on how to do the task. These ideas are likely
to involve short-cuts that are easier than applying the grammar.
As a consequence, performance may reflect the strategy the person
employs, which may or may not be effective, and may not be what
the person is normally doing in comprehending conversation in
everyday life. It is better to use tasks that conceal the real
measure of comprehension and better still to use ones that measure
comprehension as it is taking place.
7.3 The profile of asyntactic comprehension
In light of all these problems, we will now consider the theories
for why certain types of sentences are problematical for some
aphasic patients. Many theories no longer have adherents, so we
will cover just the ones that remain viable contenders. The disorder
we have been talking about, asyntactic comprehension, can best
be described with a profile of the performance that often occurs
on a particular task, called sentence-picture matching. Although
this task is problematical, for the reasons outlined above, it
is still valuable as a screening instrument for this disorder.
A person is considered to have so-called asyntactic comprehension
if, on the sentence-picture matching task, s/he can handle sentences
like (5), but is poor or at-chance on sentences like (6):
(5) It was the girl that the boy chased.
(6) It was the apple that the boy ate.
Try this task for yourself by clicking here.
Note that there will only be sentences like (5).
Can you work out why (6) should be possible for these patients
to get right? Think of some way in which you could guess the correct
answer without using the grammar; that is, without using word
order and the little (grammatical) words. By just noting the participants,
boy and apple, and the action, ate that is, using the major lexical
items you can guess that it must be the boy eating the apple,
not the apple eating the boy. So it is possible to ignore the
syntax and just use the meanings of the individual words (lexical
items) and knowledge of what goes on in the world to guess the
picture which matches this particular sentence (of which one ate
[Back to top]
8. Theories of asyntactic comprehension
Before dealing with individual theories, we need to clarify the
two types that they fall into in the main. These two types concern:
(1) theories to do with processing; and (2) theories to do with
the storage of knowledge.
Another matter to mention is that, when the renewed interest
in sentence- comprehension difficulty in aphasia started in the
1970s, there was one type of aphasic patient that had everyone
excited. The disorder these patients exhibited is called agrammatism.
Because the vast majority of patients with agrammatism tend to
have damage in the front part of the brain on the left side (frontal
lobe), the disorder is linked to Broca's aphasia, originally thought
to be from damage located there. Broca was a nineteenth century
physician-anthropologist who proposed that an area at the base
of the left frontal lobe in most people was the control center
for speech. Although Broca's first patient had no speech beyond
a recurrent utterance, and would more properly be said to have
a global aphasia, with poor comprehension for language as well
as no connected speech, the name, Broca's aphasia, has come to
be linked to the halting, slow, limited output of agrammatism,
which resembles a telegram in having a majority of major lexical
More recently, it is thought that Broca's aphasia arises from
more extensive damage, particularly around the Sylvian fissure,
called the peri-Sylvian area.
The reason psycholinguists and aphasiologists were interested
in these patients was that their comprehension difficulties might
shed light on whether there was a module for syntax, separate
from that for the meanings of words (lexical items). If it turned
out that these patients, who understood the meanings of individual
words well, had trouble comprehending the grammatical words and
inflections that they could not produce in speech, then this parallelism
in comprehension and production would be strong support for there
being a separate module for handling syntax. So the earliest theories
for asyntactic comprehension focused on agrammatic patients and
tried to link their production deficits to their apparent difficulty
with comprehending the same items that they could not produce.
Zurif is probably the main person responsible for the psycholinguistic
interest in aphasia that started in the 1970s. It was he and his
graduate students (e.g., Zurif, Caramazza & Myerson, 1972)
who proposed the idea of parallelism between comprehension and
production in use of the closed class vocabulary. It was also
Zurif, along with his former student, Caramazza, who made the
discovery about these patients' difficulty with sentences where
the participants are both capable of the action. Before dealing
with those theories on why word order is difficult, we need first
to discuss the contribution of the closed class vocabulary hypotheses.
8.1 Closed class hypotheses
At first, in the 1970s and 1980s, the whole class of closed class
items was implicated as a problem for agrammatic patients. An
influential hypothesis at the time, was put forward by Bradley,
a doctoral student under Zurif. They proposed that the grammatical
words were stored in two places, including an additional lexicon
that could be accessed rapidly. Speed of accessing is related
to how frequently words are used in the language. Bradley, Garrett
and Zurif (1980) argued that the normal rapid accessing procedures
were lost and that only the slower route to the general lexicon
holding all lexical items was available. This idea is no longer
in favor, partly because it was not replicable (though some other
supporting evidence Bradley produced has stood the test of time),
but also because subsequent researchers showed that many patients
are able to distinguish grammatical from ungrammatical sentences
as efficiently as control subjects, solely on the basis of these
words (e.g., Baum,1989; Shankweiler, Crain, Gorrell & Tuller,
Investigations into the closed-class vocabulary, from a comprehension
point of view, have been prolific over the last quarter of a century.
Certain studies in the early days, however, stood out as important.
One such study is that by Linebarger, Schwartz and Saffran in
1983. Because it marks a turning point in the field, we will deal
with it in detail, and give you a chance to try the task yourself.
Linebarger, Schwartz and Saffran (1983) showed that patients
who failed on sentence-picture matching could nevertheless determine
whether the following sentences were grammatical or ungrammatical:
|I want you to go to the store now.
||I want you will go to the store now.
|The boy put the book in the kitchen.
||The boy put the book that was in the kitchen.
|Has the girl eaten her breakfast yet?
||Has the girl has eaten her breakfast yet?
|Which animal did the girl want her mother to
||Frank thought was going to get the job.
|Which animal did the girl want her mother to
||The hungry girl wanted her mother to feed.
As you can see, the ones on the right have had particular grammatical
words substituted, omitted or added in order to make the sentence
ungrammatical (there were other items in which word order was
tampered with, but we will just address this set here). Thus,
it seems that these patients, who could not match sentences to
pictures with participants reversed, nevertheless must have possessed
quite sophisticated grammatical knowledge in order to be able
to judge whether the above items were grammatical. Firstly, grammaticality
judgments tap awareness of grammar, but do not require interpretation.
On the other hand, sentence-picture matching does require that
the listener interpret sentence meaning, in order to be able to
match the sentence to the correct picture. So these patients seemed
to have quite a lot of knowledge, more than had been realized
up until that time. Other laboratories have found the same thing.
Link to the Grammaticality-Judgments task to
see how you find making these decisions rapidly, using another
set of items.
| Figure 1:
patients on good (gray bar), and poor (red and/or blue bars),
subsets of Grammaticality Judgments (Linebarger et al.,
8.2 Parallelism Hypothesis
Another way in which the Linebarger, Schwartz and Saffran
(1983) article was important was that not all their four subjects
were Brocas aphasic, yet all four had poor sentence-picture
matching and good grammaticality judgments. Caramazza and Zurif
(1976) also had included patients who were not Brocas aphasic
(they had another syndrome, called conduction aphasia, where the
principal sign is difficulty with repetition), yet who produced
similar performance profiles. Over the years, more and more evidence
has accumulated on the range of expressive disorders that may
be accompanied by performance lumped under the asyntactic banner.
Thus, the parallelism proposal, expounded by Berndt and Caramazza
(1980), among others, has disappeared from view in the writings
of most researchers in the nineties and later. Apart from the
Linebarger et al. study, what disposed of the parallelism hypothesis
even more effectively was the finding of patients who had agrammatic
output but no comprehension deficit, at least as far as assessment
went (e.g., Nespoulous, Dordain, Perron, Ska, Bub, Caplan, Mehler
& Lecours, 1988; Miceli, Mazzucchi, Menn & Goodglass,
8.3 Dissociations within the closed class
There is now abundant evidence that ayntactic comprehenders
understand some closed class types better than others. Various
people have tried to find some commonality in the types they can
cope with, which would differentiate them from the types they
handle poorly. Friederici, for example, long championed the idea
that semantically rich functors were well handled by these patients
whereas syntactically important ones were not. A semantic one
would be the preposition by when it denotes location (e.g., The
box was standing by the door), whereas a syntactic use would be
by used with the passive, The girl was kissed by the boy. This
difference does not always follow through, however, perhaps because
different tasks tap awareness at different levels, or confound
performance with task demands.
The same idea of semantic-syntactic dichotomy has been pursued
in regard to the endings on words (inflections; e.g., raced).
What confuses the picture is the fact that some patients do not
discriminate ends of words in general too well (e.g., Tyler, 1992).
8.4 The link from loss theories to processing theories
Interestingly, asyntactic comprehenders can access the information
contained in these grammatical markers when there is redundancy,
such as several sources of information which converge. Yet when
this same information is all that is available, they may not be
able to use it. Bates and Smiths teams have each done
useful work to show this effect. A lot of their work is in languages
more reliant than English on inflections. They also have shown
that word order is more preserved in English agrammatic patients
than the closed class, while the relative importance is reversed
in those languages where inflections or functors carry the major
grammatical information. Thus, there is general agreement that
asyntactic comprehenders still have in long-term memory the grammatical
information they need; but what is problematical for them is to
access it or to use it as it arrives, in the normal flow of speech.
The remaining theories to be covered will all be variations on
this idea of a processing deficit to explain asyntactic comprehension.
8.5 Dissociations and double dissociations
Ever since the landmark study of Caramazza and Zurif in 1976,
there has been awareness of the difficulty asyntactic comprehenders
have with understanding sentences in which the action verb can
be carried out by either participant, especially in structures
where the predominant word order of English is not followed (called
non-SVO, standing for non-Subject-Verb-Object word order). As
you saw above, sentences in which either participant can do the
action have now become the standard way of screening for asyntactic
comprehension. Few studies do enough assessments to know the extent
to which people with this word-order difficulty also have a vulnerability
in understanding the closed class. It is known in at least a few
patients that they do only have one or the other to a marked extent.
This aspect of some patients having difficulty with one
sort of information and not with another sort is called a dissociation.
If two patients have the reverse pattern to each other, then we
call it a double dissociation. These terms do not apply to just
any difference, but certain criteria need to apply: for example,
the difference needs to be quite marked (e.g., chance versus near
perfect; or better than chance and worse than chance); efforts
should also have been expended to rule out confounds and leave
just a difference on a single variable that is important to information
8.6 Verb deficits and sentence comprehension
One of the most studied dissociations in aphasia has relevance
to asyntactic comprehension. This dissociation is between the
production of verbs and that of nouns. It was originally intriguing
because the dissociation seemed to be related to the site of lesion/type
of aphasia. In other words, Brocas aphasic patients seemed
more often to have a problem with verbs, whereas Wernickes
aphasic patients appeared vulnerable on nouns. We now know that
many different kinds of patients have a verb deficit, including
patients whose major difficulty is with nouns. Thus, it does not
necessarily follow that a verb-production deficit leads to a sentence-comprehension
impairment. In addition, there can be a deficit which seems to
affect meaning in some people but the word forms (phonology) in
others. The deficit itself differs in what tasks it affects (picture
naming versus story telling) and different types of verbs may
be more vulnerable than others. It is a problem to control confounds
such as the frequency and imageability of verbs relative to nouns,
but verb deficits still remain where people have made an effort
to control these aspects.
Why a verb deficit is interesting in connection with asyntactic
comprehension is because patients difficulties working out
who did the action to whom in a sentence may be due to a problem
accessing such information about particular verbs. A few group
studies, however (Breedin & Martin, 1996; Berndt et al., 1997),
have shown that not all asyntactic patients have trouble accessing
verb information per se. There are sufficient case studies available
now to realize that accessing information in the verbs lexical
entry may affect the ability to use that information to comprehend
sentences in some patients, but not with everyone. It seems that
there may be a whole host of reasons for patients failure
to handle interpretation of sentences, only one of which is the
accessibility or utility of information about the roles of participants
an action verb takes.
8.7 The mapping hypothesis
Closely related to this idea of a verb deficits underlying
asyntactic comprehension is the explanation Linebarger and her
colleagues (1983) gave for good grammaticality judgments in patients
with poor sentence-picture matching. Recall that this task dissociation
was thought to be due to the interpretive demands of sentence-picture
matching which were not required for detecting an ungrammaticality.
What Linebarger, Schwartz and Saffran (1983) proposed was that
these patients may not be able to work out, after grammatically
processing a sentence, which participant in the verbs action
goes with the subject and which with the object (called mapping),
especially when the agent is not the first one mentioned. They
proposed, further, that either this mapping process itself is
faulty (though they have given no details), or else the person
has insufficient working memory resources to do both parsing and
interpretation: parsing takes place (hence good grammaticality
judgments) but interpretation exceeds resources (hence poor sentence-picture
8.8 Capacity-limitation hypotheses
8.8.1 The trade-off hypothesis
This second alternative is called the Trade-off Hypothesis
and introduces us to another group of theories which have some
sort of capacity limitation in working memory. The trouble with
this idea is that some of the resources required to do sentence-picture
matching seem to be non-linguistic. Non-linguistic working memory
demands were discussed earlier, when we introduced the sentence-picture
matching task. So, patients could fail on this task, but not from
a limitation in linguistic working memory resources, but rather
from the excessive demands of the task itself, outside language
(in the cognitive processes required to make their choice between
the matching and non-matching picture) as demonstrated by Cupples
and Inglis (1993).
Since then, Saffran, Schwartz and Linebarger (1998) have favored
a model of comprehension akin to the constraint-based theories,
in which syntactic and semantic (i.e., roles of participants in
the action) information is processed interactively. In their current
view of the asyntactic disorder, weak exploitation of syntactic
information is overridden by lexical information, so that the
person tends to put together the participants in the action as
they exist in the real world. The task they and others have tended
to use, instead of sentence-picture matching or actor identification,
is so-called plausibility judgments. Here, the action between
two participants is plausible in one order but implausible when
the participants are reversed, as in (7) and (8):
(7) The man was carried by the horse.
(8) The horse was carried by the man.
To give you an idea of this task, try for yourself
Plausibility Judgments task.
The response required is Yes (for sensible) for (7) and No (for
silly) for (8). Unless the comprehender can use the word order
and the grammatical words like was and by, s/he will find this
decision difficult to make, especially when the structure has
a non-SVO word order.
To view a patient's performance on plausibility
judgments, click here.
8.8.2 Working memory and limited capacity
Another working memory hypothesis was proposed by Just, Carpenter
and Miyake, appearing in various articles in the early 1990s.
In their view, there is one working memory capacity for all linguistic
tasks, including holding lists of words briefly. Furthermore,
this all-embracing capacity is different in different individuals,
and is reduced in brain damage. In asyntactic patients, the capacity
limitation is supposed to have more impact on complex structures
than on simple ones. The big weakness in this hypothesis is that
it is only one possible account, which is hard to falsify, yet
which explains the data no better than others (see Martin, 1995,
for a critique).
8.8.3 Working memory for language
Yet another working memory hypothesis comes from Caplan and
Waters (1999). They expand on Miyake et al.s (1994) model
in the number of working memory systems, by having one for language
and another for what they call post-linguistic processes; in other
words, what we earlier described as the nonlinguistic cognitive
demands of comprehension tasks. This post-interpretive capacity
is the same memory underpinning span tasks, in their view. So,
in that respect, it differs from Cupples and Inglis (1993)
conception, which is not shared by span tasks (their two cases
had normal or borderline-normal spans). The problem for Caplan
and Waters model is that STM-deficit patients should not
be able to handle the plausibility judgments involving misordered
items, where there is a silly meaning in the syntax but a sensible
meaning from the logical combination of participants), if span
and post-linguistic functions rely on the same memory system;
yet there are STM patients who do.
8.8.4 Working memory and multiple capacities
If you are not totally confused by the number of different
working memory hypotheses around, there is one final one we need
to consider: that from Martins lab (Martin & Romani,
1994; Martin & He, in press). This is the most elaborate yet
in that it has a memory buffer for each particular code involved
in language and list recall. Thus, there would be one to handle
phonological information (to do with speech sounds), another to
handle semantic information (word meaning and semantic roles),
presumably one to handle syntax, and so on. When brain damage
occurs, it would be conceivable to find patients where these different
buffers are differentially damaged, leading to dissociations.
Accordingly, Martin and Romani (1994) and Martin and He (in press)
describe two patients who provide data conforming to the double
dissociation pattern: ML who has a semantic STM deficit and EA
who has the phonological STM pattern. How these different memory
deficits have an impact on comprehension is governed by the type
of information involved. If the sentence has a heavy semantic
load, such as strings of adjectives which must be held in memory
before the noun that they modify for example, the shiny, new,
blue SAAB, then the semantic STM patient will have difficulty,
but not when the same adjectives are re-arranged to follow the
noun: The SAAB he drove was shiny, blue and new. Phonological
STM is thought by most people in the field now, not to be required
for parsing or interpretation in real time. That is because the
information being processed, as we saw earlier, is no longer phonological.
Hence, a phonological STM patient typically copes with sentence-comprehension
Where phonological STM may become important is if the patient
also has some temporal processing disorder (Martin, 1993). What
Martin (1993) suggests is that such a patient may need to hold
onto the phonological input longer than normal if his/her language
processes are operating more slowly since the stroke. This hypothesis
has yet to be put to an empirical test, but it deserves attention.
8.9 Slow access versus rapid decay
The most likely candidate
for a temporal processing disorder is the asyntactic comprehender.
A number of different research labs have been pursuing that idea,
ever since Bradley, Garrett and Zurifs (1980) important
study. The exact nature of the problem is still not clear, and
it seems possible that different problems may afflict different
people. Two ways in which people have thought temporal processing
may be aberrant is: (1) slowed access to stored information, and
(2) rapid decay of activation after stored information has been
accessed (Kolk, 1995; Haarmann & Kolk, 1991). While there
is evidence for both of these patterns, there is quite a lot of
inconsistency between laboratories, so that the story has yet
to unfold as to what the range of deficits is, and whether the
variety of deficits explains corresponding difficulties with sentence
comprehension. It is noteworthy that not just Brocas aphasic
patients show the patterns (see Tyler, Ostrin, Cooke & Moss,
1995). While Kolk does not believe that these two patterns will
likely exist in the one patient, the first seems to be a problem
with accessing, something that is well recognized in other, more
easily tested areas, such as lexical semantics or the acquired
dyslexias, while the second has to do with decay in memory. While
the same piece of information may either not be accessible or
may quickly lose its activation after access, it would be necessary
to explain how damage produces one deficit and not the other.
[Back to top]
9. Current thinking
One of the problems
in researching this topic is the difficulty in tapping real-time
language processing. A few labs (especially Swinneys) have
advocated cross-modal tasks. In these paradigms, the research
participant has to make a decision regarding a written word (such
as whether it is a real word in the language lexical decision)
appearing on the screen at some specific point during the presentation
of an auditory sentence. The idea is to look for a fast reaction
to the written word when it is related to some other word in the
auditory sentence. More than one laboratory has found that aphasic
patients are not able to respond reliably on this task (e.g.,
Tylers and Martins). While alternative tasks exist
(e.g., word monitoring but it is only appropriate for parsing),
other options are to use modern sophisticated equipment, such
as ERP (event-related potentials a form of EEG), fMRI (functional
magnetic resonance imaging a non-invasive scan of the brain),
or PET (positron-emission tomography an invasive scan of the brain
using a radioactive substance). In addition to being more or less
invasive, these techniques vary in whether they better assess
localization of function (fMRI, PET) or temporal processing (ERP).
This last technique of ERP is the best equipped to tease apart
the temporal aspects of language processing, but it has not yet
been used extensively with aphasic patients.
Irrespective of the difficulty of tapping real-time language
processing, the research done so far seems to indicate that the
entity called asyntactic comprehension is far from "asyntactic"
and is not likely to be due to a single etiology. In view of the
variability in performance profiles and varying degrees of difficulty
with different structures, there is a need for comprehensive single-case
studies using a range of tasks. Syndrome labels like this one
are no more than a useful shorthand to convey some idea of a patient's
presenting profile; they convey little useful information when
it comes to understanding what processes have been affected in
any particular individual.
Another characteristic of recent research into sentence comprehension
is that patients with etiologies other than stroke have become
the focus of attention. In particular, Alzheimers sufferers
have been of interest because of their relative preservation of
syntactic processing compared to lexical semantics. It seems that,
in testing them on the range of sentences from aphasic asyntactic
comprehension research, these patients have trouble with the non-linguistic
working memory demands of tasks and not with linguistic parsing
and interpretation per se.
The research on patients with sentence comprehension difficulties
is only now beginning to bear fruit in terms of explaining aspects
of normal processing of language. The early hope that agrammatic
patients' perceived difficulty with comprehension would indicate
the existence of a syntactic processor common to both comprehension
and production has not survived. The reason for failure may just
be the lack of sensitive enough assessment tools or the rarity
of the right patients to demonstrate a damaged central parser.
On the other hand, Martin's demonstration of an effect of semantic
information on comprehension is a breakthrough in showing a link
between memory and language.
[Back to top]