Sentence Comprehension (I)

  1. The structure of a sentence
  2. Parsing
  3. Theories of parsing and interpretation: The garden-path theory and minimal attachment
  4. Memory and sentence comprehension
  5. Vocabulary or lexical semantics
  6. Brain damage and aphasia

1. The structure of a sentence

Comprehension of language is a complex activity that involves many processes. To start with, we will simplify things by considering just two types of processes, parsing and interpretation. One thing we do as we comprehend is to analyze the structure of the language we hear, a process called parsing. This structure is conveyed by word order from left to right in English and by the hierarchical connections between words, to form something vaguely like a family tree:

Figure 1. Tree diagram

The listener then needs to link up this structure (syntax) to the meaning of the sentence: what is being described or reflected upon by the speaker. This process of determining the meaning of spoken language is called interpretation; it is the second process we will consider, shortly. To give you an idea of the first aspect of language we will cover, parsing, take a look at the following sentence:

(1) It was the dorry that the lutter prated.

Even though this sentence does not make sense, you should still be able to pick out the words representing the participants, dorry and lutter, and the word for the action, prated. These clues come from the appropriate endings attached to the words, and from the small connecting words, like "the" and "that." So, it is these word endings (or inflections, such as -ed on prated), and grammatical words (or functors/function words, such as "it," "was," "the" and "that") called, in psycholinguistics, "the closed class vocabulary," which determine the structure of the sentence. The major lexical items in the above sentence (1), replacing nouns, verb and adjective (called content words), are nonsensical, made-up words, which is why you cannot determine what was actually happening in the sentence.

[Back to top]

2. Parsing

We will consider now how our brains work out the structure of a sentence. In parsing, the brain has to determine how the words are grouped into ever higher and sparser layers, like a pyramid. The groupings adhere to a set of rules, specific for each language, which most speakers seem to have internalized as young children from the language around them. People can use these rules without being able to specify what they are; so we call them tacit knowledge. To see how you, as a comprehender, are aware of rules without knowing what they are exactly, take this sentence:

(2) The horse raced past the barn fell.

Up to the second-last word, barn, you perhaps expected it to continue something like this: "The horse raced past the barn and into the yard." Probably the last word, "fell," came as a surprise, though. Did you go back and try again to work out a different way of saying the sentence? The way it should have read was as though there had been commas after The horse and barn, to mean "the horse that was raced past the barn." This is a classic sentence devised by a famous psycholinguist, Bever, in 1970. It illustrates several characteristics of parsing: Firstly, the reason for your being misled, or garden-pathed, as it is called, is that the word "raced" is grammatically ambiguous. That means: it can be used two different ways in English. One way is to signify that the event (the action entailed in the main verb) was completed in the past; this is the sense which your brain first obtained. The other use is to signify a more succinct version of "which was raced"… This latter use of raced occurs in a reduced relative clause. It is "reduced" because we have the option to leave out the conjunction "that" altogether. A relative clause is a group of words with a main verb which modifies, or says something about, the participant in an event or state.

[Back to top]

3. Theories of Parsing: The garden-path theory and minimal attachment

The fact that you arrived at one meaning at first and then had to revise it, is a phenomenon which has lead to a famous theory of parsing. It is called the garden-path theory and was proposed by Lyn Frazier in her doctoral dissertation in 1978. Put simply, this theory (based on earlier work by Bever, 1970, and another person called Kimball, 1973), claims that the brain forms the simplest structure it can at first (respecting the principle of minimal attachment, such that there are the fewest phrases in the structure). It is thought that, when a syntactically ambiguous word signals that the simple structure could not be correct, the brain backtracks. More consciously now, it makes a second run through the parser. On this second pass, it reclassifies the words for grammatical class (main verb, subject noun, etc.) and follows the grammatical rules of the alternative, more complex, but correct, structure. The garden-path theory is called a two-stage model of parsing: stage 1 where the simplest structure is formed; stage 2 where the revised, correct structure is found.

3.1 Characteristics of modular systems in relation to parsing and interpretation
3.1.1 Modularity

What characterizes the garden-path model is that it operates according to Fodor's (1983) idea of a modular cognitive system (a cognitive system is what we call a mental system which processes information). A modular system has components or modules working independently of others (such as the module for determining the meanings of words), each passes its output on to the next module in line. So, information passes along, a bit like on a conveyer belt, with stations or stages that do something to the information before allowing it to pass on to the next stage.

You can understand what a module is in language processing if you think of the codes each one would use. The garden-path theory considers that the parser is handling information coded for the grammatical class of words. What codes do you think the interpreter would be using to work out the roles participants have in the meaning of a sentence? To find out, look at these two sentences:

(3) The boy kissed the girl.
(4) The boy envied the girl.

In the two sentences, "the boy" has two different roles. In the first, he is an agent of the action of kissing: it is he who kissed the girl. What is his role in the second sentence? Is he the agent again, doing some action, or do you think he could be the experiencer of the emotion of envy? He is not doing anything to the girl in envying her, so he is not an agent; but he can certainly experience envy, and so his role is called experiencer. It is these thematic roles which form the information that the interpreter component makes use of in a modular language system.

In the garden-path theory, the second process of interpretation happens after the first run of the parser when an initial expectation of the structure has been determined. Parsing on this first run does not consider the roles the participants are taking. (Variations on this theory allow earlier use of role information but nevertheless keep the mechanisms handling the different types of information separate).

3.1.2 Information encapsulation
The idea of separate parsing and interpretation, handled by different mechanisms rather than a common one, is an example of information encapsulation, one of the properties of cognitive systems specified by Fodor (1983).

3.2 Constraint-based theories of parsing and interpretation
Another group of theories proposes that comprehension is not so sequentially organized (i.e., first in-depth parsing then interpretation). Instead, the two happen in an interactive way from the start. These constraint-based theories consider that all sources of information are brought to bear in comprehending a sentence, particularly the information that is stored in memory concerning each word in our vocabulary. Some of the types of information stored in the lexical entry of each word in our long-term memory for words (semantic memory) include the meaning of the word, its role in grammar, the structures it can appear in, and the role it plays in sentence meaning. The constraint-based theories assume that the type of information used depends on the probability of finding it in the language and the language-user's experience with it (frequency).

3.3 Connectionism and parsing-interpretation
Constraint-based theories still keep the idea of separate sources of information, unlike yet another theory called connectionism, proposed by McClelland and his collaborators (amongst others). In this third theory, there is no separation in memory of the representations of the different types of information, just different connections between them.

3.4 The concurrent model of parsing-interpretation
Finally, there is now a hybrid theory, which amalgamates two of the ones we have considered. It was proposed by Boland in 1997 and called a concurrent model because it comprises aspects of the garden-path and constraint-based theories, operating simultaneously rather than sequentially. Multiple possible structures, rather than one (as in the garden-path model), are generated by the listener at each incoming word. The constraints as to which one wins out come from lexically stored information, as described in constraint-based theories (and any other relevant information). Because of this parallel processing going on, meaning may precede syntax at different points; but in the end syntax prevents the acceptance of ungrammatical structures, even if they are plausible.

[Back to top]

4. Memory and sentence comprehension

While what has been said so far seems no doubt complex, it is not the end of the story! Comprehension is not just parsing and interpretation. The brain must use memory capacity for the different types of information in determining the structure and interpretation of the language to which we attend. This memory system is now commonly called working memory (a term attributed to Baddeley, one of the most well-known of memory researchers). Working memory is presumed by some researchers to be a system that holds information while it is being processed. Various theories as to the nature of working memory exist amongst researchers who believe in modularity. We will consider four basic types here, because of their relevance to comprehension. Connectionists, on the other hand, see working memory as just a product of the network in operation, not anything with a capacity for storage of information.

4.1 Baddeley's model of working memory
Baddeley’s model of working memory has special storage systems for visual and auditory-sound information. The auditory-sound information memory capacity is called phonological, because it is thought not to be acoustic (i.e., sound based) or articulatory (i.e., related to pronunciation) but some more-abstract code, nonetheless based on the sound patterns of a language. Lots of research, however, indicates that semantic information is also coded in short-term memory, and must be held temporarily in memory while comprehension is going on (e.g., words are recalled better than made-up nonwords, in the short term). To experience this difference for yourself, try the STM recall task comparing word and nonword list recall.

From what you have learned about parsing, it seems logical that the brain must hold partial syntactic analyses while it is comprehending sentences in a discourse. So, another group of theories has more than just the two storage systems that Baddeley and Hitch proposed in 1974.

4.2 Caplan and Water's model of working memory for language
Opinions differ as to how many of these additional storage systems there are: Caplan and Waters (1999) propose one separate system for all the processes to do with language (such as parsing and interpretation) and a third system for handling the output of the language processor in tasks.

4.3 Carpenter, Just and Miyake's model of working memory for language
Yet another idea, from the team of Carpenter, Just and Miyake, is that there is just one general capacity system of memory for both the short-term recall of lists and all the different aspects of language processing. Gibson (1998, 2000) has a general memory capacity for language processing in his model, too, but he has been more specific in describing how its capacity is depleted in terms of parsing and interpretation.

4.4 Martin's model of working memory for language
The fifth and last hypothesis that we will deal with here is that from Martin’s lab (e.g., Martin and Romani, 1994). She believes that there are storage systems in memory for each and every type of information involved in comprehension. Thus, there would be separate ones for phonology, for the semantics of words (lexical semantics), for syntax, for interpretation, and so forth. Her research with patients shows that there are indeed separate systems for phonology and semantics, at least. The evidence comes from the different areas damaged by strokes and the behavioral differences. Evidence for a separate memory system for syntax has yet to be demonstrated conclusively. One team, Miyake, Carpenter and Just (1994), has tried to show that the complexity of syntactic analyses affects working memory. There are objections to their work, however, so we need better evidence of a syntactic storage buffer.

4.5 Working memory as a cognitive system
Consdier how these ideas on memory systems are like the disputes over modular systems in comprehension: some have a general storage facility undifferentiated for the type of information; others have different degrees of specialization in memory storage, depending upon the type of code or information involved. Such versions exemplify the modularity of memory systems and their information encapsulation.

[Back to top]

5. Vocabulary or lexical semantics
Probably the first thing that came to mind, when you started to read this topic of comprehension, was vocabulary. When we talked earlier about the constraint-based theories of sentence comprehension, we met this topic of vocabulary, or lexical knowledge. Vocabulary is portrayed in various ways in psycholinguistics, depending upon its different uses. When you hear someone speak, the sounds of the words you hear have to be recognized as those words in your English vocabulary. This dictionary of heard words from your language is called a lexiconæan input phonological lexicon to be exact. Then you have to attach meaning to each word, and, if the word is part of a sentence, your brain makes use of the other types of stored information connected with each word mentioned earlier: the grammatical category of each word; the assignment of roles to participants in an event; the structures that different verbs can appear in; the grammatical attachments to words, especially verbs. Apart from needing to know what a sentence is about, it is these other types of information which are the focus of this chapter, and not vocabulary (lexical semantics). These other types of information figure prominently in the sentence-comprehension research to do with aphasia, which we come to now.

[Back to top]

6. Brain damage and aphasia
Aphasia is the name given to a whole host of language impairments that may follow brain disease and traumatic brain damage. This group of impairments may arise from: bleeding into the brain ( hemorrhage) or blockage of the blood supply to the brain (thrombosis and embolism); bruising or tearing of brain tissue (contusion); pressure on brain tissue (from a tumor, depressed skull fracture, pooling of tissue fluid or blood); and atrophy or other generalized changes in brain tissue (as in the dementias, although aphasia used to be applied to focal brain damage only). We do not know how many different types of aphasia there are, because they are still being described, but the knowledge we have so far is quite extensive. Here, we will limit ourselves to what is topical in sentence-comprehension impairments. Aphasia can be studied in its own right, in order better to understand what has become dysfunctional for the patient and how best to remediate it. It can also be studied from the point of view of what light it sheds on intact processes. In our approach, we will consider just comprehension impairments from the point of view of what it tells us about how the brain should work if unimpaired. In considering comprehension deficits, we will confine ourselves to those that seem to affect parsing and interpretation.

[Back to top]