Spoken language comprehension in a fluent … · COGNITIVE NEUROPSYCHOLOGY, 1988, 5(3) 375-400 Spoken Language Comprehension in a Fluent Aphasic Patient Lorraine K. Tyler M. R. C

COGNITIVE NEUROPSYCHOLOGY, 1988, 5(3) 375-400

Spoken Language Comprehension in a Fluent Aphasic Patient

Lorraine K. Tyler M . R . C. Language and Speech Group, University of Cambridge, U. K.

This paper describes an aphasic patient, RH, who, on standard tests of language comprehension, appears to have a severe comprehension deficit. We compared performance on these standard tests with that obtained on tasks which measure the processes involved in constructing a representation of the speech input as it is heard (so-called “on-line” tasks). Performance on these on-line tests was essentially normal, whereas performance on standard tests of comprehension was no better than chance. These data are interpreted in terms of a distinction between the mental processes involved in constructing a representation and those involved in consciously reflecting upon it.

INTRODUCTION

Many attempts have been made to investigate comprehension deficits in aphasia. Some have involved examining groups of patients, categorised according to various diagnostic tests, for their ability to comprehend a variety of sentence structures, while others have focused on describing the comprehension deficit of a single patient in more detail. Almost all of this research has shared the implicit assumption that comprehension deficits- whatever their nature-can be measured by more-or-less any experimental task. Attempts are rarely made to relate the experimental task to a particular aspect of the comprehension process.This is partly because to do so requires both a theory of the task and a well-developed theory of the processes involved in language comprehension, neither of which is readily forthcom- ing.

Requests for reprints should be sent to Dr. L. K. Tyler, M.R.C. Language and Speech Group, Dept. of Experimental Psychology, University of Cambridge, Downing Street, Cam- bridge CB2 3EB. I thank the members of the SpeechTherapy Department at Queen Mary’s Hospital, Sidcup, for their co-operation in testing RH. I also thank Howard Cobb for carrying out much of the testing and data analyses, and along with Peter Hagoort, Debby Burke, Karalyn Patterson, and William Marslen-Wilson, for useful discussions about the data. This research was supported by an M.R.C. programme grant to LKTand WMW.

0 1988 Lawrence Erlbaum Associates Limited

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In particular, current accounts of language comprehension deficits have little to say about the distinction between constructing a representation of the speech input and other, post-perceptual, analysis processes. Researchers have, on the whole, been primarily interested in whether a patient (or group of patients) has difficulty with particular types of linguistic information, but they rarely attempt to locate the source of the difficulty in any particular aspect of the comprehension process.

To do so requires considering in some detail the processes and representations which appear to be involved in comprehending spoken language. To understand a spoken utterance, a listener has to translate the speech input into a meaningful representation. The way in which this translation process is currently conceived of is in terms of a set of mental processes which operate upon internally represented symbols to generate mental representations (cf. Fodor, 1983). It is generally assumed that some representation of the speech input is mapped onto internal representations of lexical form.The semantic and syntactic properties of these lexical representations then become available to those processes which construct higher- level representations of the speech input-syntactic, semantic, and discourse representations. It is only when these higher-level representations have been constructed that the listener can be said to have “comprehended” the utterance.

These mental processes which are involved in consrrucring representations of a spoken utterance have been variously termed “nonconscious” (Fodor, 1983) or “preconscious” (Hampshire, 1983) processes. Their hallmark is that they are not available to conscious awareness and, by and large, they are not under voluntary control (cf. Marslen-Wilson &Tyler, 1981). It has been claimed (Marslen-Wilson &Tyler, 1980; 1981 ;Tyler & Marslen-Wilson, 1982) that these nonconscious, automatic processes involved in spoken language comprehension are best reflected in “on-line” tasks. These are tasks in which the listener’s response is closely tied in time to particular stretches of the speech input, thus reducing the time for introspection. The close temporal relationship between input and response is important because the input to the language processing system-the speech signal-is distributed over time. To determine what kinds of analysis the listener performs on this input as it accumulates over time, and at what points in time different kinds of information can be extracted from the speech input, we have to use tasks in which the listener’s response can be related to specific stretches of speech. We can then try to determine what kind of representation the listener has developed, given the input available at the point the response is made. Using tasks which elicit fast responses is one way of tapping these so-called “on-line” processes. Fast-response tasks are useful because the closer in time a response is to some relevant portion of the speech input, the more closely we can specify the properties of the

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internal mapping processes involved. Tasks in which responses are not closely tied in time to the speech signal run the risk of producing data which reflect conscious, introspective analyses of the speech input.

The research reported here was an attempt to examine these on-line comprehension processes in an aphasic patient. We selected a patient (RH) who, according to conventional methods of assessment, suffered from a profound comprehension deficit. On tasks such as theTokenTest (De Renzi & Wgnolo, 1962), sentence-picture matching, and grammaticality judgements, his ability to comprehend spoken language was assessed as being no better than chance. The question we asked was whether his comprehension deficit could be located in one of the processes involved in the comfruc- tion of a representation of the speech input or in some later post-perceptual process. To try to answer this question we focused, in particular, on RH’s lexical processing system, and carried out a series of studies designed to probe his ability to map the sensory input onto representations of lexical form, to access lexical, syntactic, and semantic representations, and to use these lexical representations to develop higher-level representations of the utterance.

Although our primary motivation was to examine RH’s on-line processing capabilities, our experiniental tests provided the opportunity to evaluate the various claims which have been made to account for the deficit known as Wernicke’s aphasia.This is because RH conforms to the traditional specification of a Wernicke patient-fluent, paragrammatic speech accompanied by a severe comprehension problem. (cf. Caramazza & Berndt, 1978; Goodglass & Kaplan, 1972).’

Assessment of the comprehension capabilities of Wernicke patients has been limited by the fact that most data come from off-line tasks. The tasks are off-line in the sense that either the whole stimulus item is presented (e.g. whole word, entire sentence) before the subject makes a response, or subjects are not encouraged to make a rapid response (e.g. sentence- picture matching, grammaticality judgements). In either case, these tasks are most suited either for probing the nature of the final representation which the listener has constructed, or for assessing the patient’s ability to perform a metalinguistic judgement on this representation.* They are not appropriate for determining the intermediate processes involved in the construction of that representation.

The exact nature of the comprehension deficit in Wernicke’s aphasia is unknown. Although there is widespread agreement that the disorder

‘To evaluate these claims does not imply that we subscribe to the traditional classification

*We do not intend to imply that off-line tasks form a homogeneous group, but merely that schema for aphasic patients.

they probe a different set of processes from on-line tasks.

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involves lexical-semantic information (Caramazza & Berndt, 1978; Zurif, Caramazza, Myerson, & Galvin, 1974), there is some controversy as to whether it also involves syntactic information. Some recent studies suggest that Wernicke patients do indeed have problems with various aspects of syntactic processing (Goodglass, Gleason, & Hyde, 1970; Goodglass & Menn, 1986; Heeshen, 1980; Parisi & Pizzamiglio, 1970), while others suggest that they do not (Caramazza, Berndt, Basili, & Koller, 1981; Friederici, 1983; Rosenberg, Zurif, Brownell, Garrett, & Bradley, 1985; Zurif & Cararnazza, 1976).

Apart from disagreements over the types of information involved in the deficit, there is also dispute as to its cuuse. Assuming that the problem can indeed be located primarily in the lexicon, there are at least three distinct ways that the lexicon could be affected by brain damage. First, lexical structure itself could be disrupted (Goodglass & Baker, 1976; Whitehouse, Caramazza, & Zurif, 1978; Zurif et al., 1974), in the sense that some lexical entries become incompletely specified. Secondly, the lexicon could remain structurally intact but the processes by which information is accessed might be disrupted. Thirdly, both structural organisation and access processes might be normal, but patients might be unable to use the information which they access appropriately-either to construct a structural representation of an utterance, or to make metalinguistic judgements by operating on the information in some explicit way (cf. Blumstein, Milberg, & Shrier, 1982; Milberg & Blumstein, 1981).

What kinds of performance deficit might plausibly result from these different effects of brain damage? A strong interpretation of the structural disruption hypothesis would claim that if particular types of lexical information are lost, then patients should never be able to access that information- whatever the task used. This is unlikely-at least as an explanation of any semantic deficit-in the light of Milberg and Blumstein (1981) and Blums- tein et al.’s (1982) data. Their Wernicke patients were sensitive to various kinds of lexical-semantic relations, suggesting that instead of a disruption in the underlying semantic organisation of the lexicon, they had a problem in either accessing or using information from the lexicon. Unfortunately, there is no comparable data on the representation and use of syntactic information (for the purpose of comprehension) in Wernicke patients.

Access problems could be caused by a variety of factors. The process of mapping the sensory input onto lexical entries could be disrupted, or there could be insufficient activation of lexical entries such that lexical information does not become available to the listener. Given the Milberg and Blumstein and Blumstein et al. data, some Wernicke patients seem able to contact the lexicon and, also, to access at least some semantic information contained in lexical entries. These patients were faster at making lexical decisions to words which were semantically related to a previously-


presented prime, suggesting that specific types of lexical-semantic information had been activated and accessed.

However, the authors claim that their patients did suffer from some deficit in either access to or use of lexical-semantic information, because their overall latencies were much longer than for either normal controls or Broca patients. Wernicke patients’ ability to use lexical information was tested by asking them to judge the semantic relatedness of the semantically related word-pairs used in the priming study. They found no correlation between performance on this task and performance on the priming tasks, suggesting that patients can access lexical information without being able to use it correctly in a metalinguistic task. The question which Milberg and Blumstein (1981) raise is whether this stems from “an impairment in the access of semantic information for its ultimate linguistic usage or interpretation”.

Their experiment does not answer this question since their materials were single words rather than sentences. At best their results show that patients can access particular types of lexical-semantic information, but they say nothing about whether they can use this information in the process of interpreting an utterance.The other limitation of the Milberg and Blums- tein study, in terms of constraining models of language comprehension deficits, concerns the nature of the semantic information they manipulated in their semantic priming experiments. It is quite plausible that the semantic associations between words which are activated in the semantic priming task are not relevant for the construction of a meaningful interpretation of an utterance (cf. Foss, 1982).

By examining the lexical procesiing system of an aphasic patient who could be classified as a “typical” Wernicke, we were able to evaluate these various claims for the type of information involved inwernicke’s comprehension deficit and its cause. The experiments were designed to determine whether RH’s language comprehension deficit could be attributed to an abnormality in any of the phases of lexical processing-initial contact of the lexicon, accessing syntactic and semantic information, using this information to construct both “local” and “global” representations of an utterance, or explicitly manipulating these representations. To do this, we carried out four experiments.

In the first, we examined the process of making initial contact with the lexicon. Before testing RH’s ability to access and use lexical information we needed to establish that the process of mapping the sensory input onto lexical representations is unimpaired. Without testing these processes, we cannot determine whether problems in lexical processing are due to an inability to contact the lexicon or to access information from it. In this first study, we examined these lexical mapping processes by estimating the amount of sensory input RH needed to contact the correct lexical entry.

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In the second study, we examined RH’s ability to access and use lexical information. We examined his ability to use both lexical-syntactic and semantic information to construct ‘‘local’’ relationships-those involving structural relations between adjacent elements-within an utterance. The question we asked was whether RH can access syntactic and semantic information marked on verbs and use this information appropriately in the on-line construction oi verb-argument structures.

The third experiment was carried out to test the generality of any effects found in the second study. If RH’s ability to use lexical information to construct ‘‘local’’ relationships is intact, can he also construct a “global” representation spanning an entire utterance? To establish this latter point, we examined his ability to use different types of information to construct a representation which spans an entire utterance. To do this, we had the patient monitor for a target word occurring at various word-positions across three different types of prose contexts-normal sentences, anomalous sentences (where the syntactic structure was intact but the material was mean- ingless), and scrambled strings of words. Varying the word-position of the target and the presence or absence of syntactic and semantic structure allowed us to track the availability of different types of processing information across an entire utterance. We used these particular contrasts because they have been shown to be sensitive to the global properties of on-line processing in normal listeners (cf. Marslen-Wilson & Tyler, 1975; 1980).

In the final experiment, we tested RH’s comprehension in an off-line task, using the materials from the second study. By comparing his on-line and off-line performance on the same stimuli we can determine whether he has a deficit in either the process of constructing a representation, or in subsequent metalinguistic analyses. The data from the off-line task also provide a more appropriate comparison with his comprehension data from the standard diagnostic tests.

Description of Patient

RH was born in 1920 and had a stroke in 1977. His speech is fluent and contains relatively few content words, many stereotypical phrases and occa- sional verbal paraphasias. An example of his speech follow^:^ E: RH: E: That must be frustrating. RH:

Do you do much reading? No I don’t no its all gone I don’t.

I’d to I...I...like to me that’s very good ... I’d have something here a long time and have this but now.. .I suppose all I can do the.. . box.

? h i s is a carefully selected sample of RH’s speech to illustrate that he can sometimes be understood. However, most of his speech is nothing like as transparent as this sample.


E: RH:

Television? What do you like to watch? Well ... no anything I want really because I can’t the others at all ... I can’t this going ... er ... there’s nothing for me to go or sometimes the., .erm.. .phew.. .wait a minute.. .er.

On the basis of the Boston exam (Goodglass & Kaplan, 1972), adminis- tered in 1979,4 RH was classified as a Wernicke with poor auditory comprehension (z score = -0.5). His comprehension was also poor when measured on the Token Test (De Renzi & Vignolo, 1962). His score of 7/36 indicated that he has a severe aphasia. He also scored poorly on the T.R.O.G. test (Bishop, 1982), only getting 7 blocks out of 20 correct. In informal interaction, RH appears to have a severe comprehension deficit. This impression is, no doubt, partly due to his profound difficulty in producing coherent language.

His digit span (as measured on the B.D.A.E.) was only three. However, he was able to match four digits with 100% accuracy on a digit-matching task, and five digits with 66% accuracy, suggesting that he does not have a severe S.T.M. deficit for digits.’ He completed the trail-making test in 2 0 5 ~ e c , ~ indicating that he has severe brain damage, but his general intel- lectual abilities are above average as measured by the Ravens Progressive Matrices.

EXPERIMENT ONE THE UPTAKE OF SENSORY INFORMATION

This first study was devised to examine lexical mapping processes. It has been claimed (Marslen-Wilson & Tyler, 1980; Marslen-Wilson & Welsh, 1978) that these processes are maximally efficient in their use of the acoustic input. The speech signal activates a large set of word-candidates (the word- initial cohort) which gradually diminishes in size (as the sensory input accumulates over time) until only a single candidate remains. I t is at this point of separation from all other candidates-which is the first possible point at which the word can be identified-that a listener recognises the word.

To determine whether listeners do indeed recognise a word at this separation point we need to use a task which allows us to measure that point. One such task is the gating task (Grosjean, 1980; Salasoo & Pisoni, 1985; Tyler, 1984;Tyler &Wessels, 1983). In this task, subjects are presented with

vesting of this patient began 18 months afterwards, in 1981. ’In this test he had to say whether two strings of digits were the same or different. On each

trial the length of the strings increased by one digit. The sequences ranged from 1-6 digits in length. The two sequences were spoken with a 2sec pause separating them.

%e range for the age-matched controls was 29-69sec.

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successively larger fragments of a word and after each fragment they either say or write down the word they think they are hearing. We can thus determine the point in the speech stream at which a word is recognised. Although the standard version of the task does not require subjects to make a rapid response (they merely have to write down their word-choice and are not placed under time constraints), research has shown (Tyler & Wessels, 1985) that the task is still on-line in the sense that listeners’ responses can be directly related to specific stretches of the speech input.

The stimuli we used in the present study were a set of words which varied in their separation points. These points correlate highly with the point at which normal listeners recognise spoken words (Marslen-Wilson, 1984;Tyler & Wessels, 1983). For this study we chose words which became unique either early or late in the word. If RH’s processing of the speech input is normally efficient in the sense that he recognises a word at the point at which it separates from its competitors, then he should require less sensory input to identify words with early uniqueness points compared to those with late uniqueness points. We can then be confident that he is mapping the sensory input onto lexical representations in much the same way as normal listeners do.

Method

Control Subjects

The control group consisted of 8 subjects (mean age of 67 years), who were all native speakers of English.

Materials

We selected 24 polysyllabic words (a mixture of nouns, verbs, and adjectives).Twenty of these were test items and the remaining four were practice items. Half of the test words had early separation points and half had late separation points.These were determined, first, on the basis of a dictionary search and, second, by examining recordings of the words on an oscillos- cope. We used the same mixture of adjectives, nouns, and verbs in the two groups. The mean frequency of the words in the early group was 31 and in the late group it was 38 (Francis & Kuqera, 1982).

These words, pseudo-randomly ordered, were recorded by a female native speaker of English. They were digitised at a sampling rate of 20kHz and segmented into fragments from word-onset, with each fragment increasing in duration by 50msec.The first fragment consisted of the first lOOmsec of the word, the second consisted of the first 150msec and so on, throughout the whole word.The total number of fragments for each word ranged from 9 to 16.


Procedure

The eight control subjects were tested in pairs in a quiet room. They were instructed to listen to the material carefully and to write down after each fragment the word they thought they were hearing.

RH was tested individually in the hospital where he goes for speech therapy. Testing was spread over two sessions. He was given the same instructions as the control subjects with one minor change-he was asked to say the word instead of writing it down.

Results and Discussion

Control Group Data

Subjects’ responses were examined to locate the point at which they correctly identified the word and did not subsequently change their minds. These “isolation points” were a measure of the amount of sensory input needed to identify the target words. The isolation points for each word were entered into an ANOVAwith items as the random effect.This revealed that the mean isolation point for the early group of words (285msec) was significantly earlier than the mean isolation point (453msec) for the late group (F(1,19) = 42.36, P<O.OOl).

Patient Data

RH correctly identified 19 out of the 20 test items. His isolation points for each word were located (using the same criteria as above) and entered into an ANOVA. This revealed a large difference between early and late groups (F(1,18) = 17.471, P<O.OOl).The mean isolation point (IP) for the early group of words was 335msec. and for the late group it was 500msec. These isolation points were, respectively, 50 and 47msec later than the isolation points for the control group (F (1,18) = 12.37, RO.01) . These slightly later isolation points can undoubtedly be explained in terms of RH’s high-frequency hearing loss. Apart from showing a hearing loss at 3-6kHz on an audiogram, he also has considerable difficulty distinguishing phonemes on the basis of high-frequency information. In a phoneme discrimination task, he made 33% errors on voiceless fricatives and 23% errors on voiceless plosives. His performance on voiced plosives and fricatives was much better (only 8% errors on each), presumably because of t h e extra low-frequency information these speech sounds contain. This interpretation is supported by the results from one of the control subjects (AK) who had a comparable degree of hearing loss. Her mean IPS were also later than those of the controls (380msec for the set with early uniqueness points and 513rnsec for the set with late uniqueness points).

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The most important aspect of the data is that both R H and the controls showed the same difference between early and late isolation points (F( 1,18) = 0.066, P = 0.7998).

We also examined the words which FUI produced at each fragment. Almost all of his responses began with the correct phoneme (91%) and 75% of the words he produced were also produced by the control group. These results suggest that he does not have any problem in accessing the appropriate cohort on the basis of the sensory input and that the structure of his cohorts are similar to those of normal listeners.

The fact that RH needed more sensory input to recognise words with late separation points compared to those with early separation points suggests that the processes involved in making contact with the lexicon are not disrupted.This, plus the fact that the words he produces are very similar to those produced by control listeners and begin with the correct initial phoneme, indicates that the sensory input, accumulating over time, is being mapped onto the lexicon and is serving to activate lexical representations. It appears, then, that RH’s processing of the speech input (with respect to accessing the lexicon) is not impaired.

EXPERIMENT TWO

Now that we have established that the process of contacting the lexicon via the sensory input is essentially unimpaired in RH, we can determine whether he can access and use lexical information. In this study we tested his ability to exploit verb-argument relations in the on-line interpretation of an utterance.

We focused on the processing of verbs because part of their representation includes syntactic and semantic information which constrains possible verb- argument relations. The syntactic constraints on possible verb arguments involve, for example, restrictions on whether the verb can take an indirect object. The sequence “He slept the house” would violate what Chomsky (1965) has termed syntactic subcategorisation constraints between the verb and its argument. When subcategorisation constraints are violated a syntactic structure cannot be constructed and the semantic relations between verb and noun are not semantically interpretable.’

The major type of semantic constraints on verb-argument relations have been termed semantic selection restricrionr (Chomsky, 1965). Semantic selection restrictions on the verb eat, for example, are assumed to include that it must take as its argument something which is edible. So, for example,

’Initially Chomsky assumed that both subcategorisation constraints and selection restrictions were part of the syntactic component of the grammar, but now linguists generally agree that selection restrictions are part of the semantic component (e.g., Jackendoff, 1972).

VERB-ARGUMENT RELATIONS


the sequence “He ate the house” would violate the semantic restrictions on the verb and its argument. This type of semantic information is usually contrasted with pragmatic comfraints on verb-argument relations. “He licked his door” is an example of an utterance which is pragmatically implausible. Traditionally, pragmatic information is assumed to be distinct from selection restriction information. Whereas selection restriction information is assumed to be coded lexically, pragmatic information is not. Rather it is assumed to involve a process of interpreting information which is lexically represented with respect to real-world knowledge. In practice, however, it is often difficult to motivate the basis for the distinction (cf. Marslen-Wilson & Tyler, 1987).

Linguistic analyses (e.g. Bresnan, 1978) claim that both subcategorisation and semantic selection restrictions are part of the specification of a verb in the mental lexicon. Psychological data suggest that listeners access this information-and pragmatic information-when they recognise a word, and use it to constrain the representation under construction. For example, Marslen-Wilson, Brown, and Tyler (1988) and Tyler (1985) have found that violating either the semantic, syntactic, or pragmatic contraints between verbs and their arguments disrupts normal listeners’ processing of the speech input, and causes word monitoring latencies to increase. If RH also accesses this type of syntactic and semantic information when he recognises a verb, and uses it to constrain possible verb-argument configurations, then his processing performance should also be disrupted when constraints on verb- argument relations are violated.

Method

Subjects

We used the same age-matched control subjects as in the first experiment.

Materials

We selected 32 common nouns as target words and constructed sentence- pairs for each of them. Each target occurred as the object noun in the second sentence of each pair. The first sentence provided a minimal, and not highly constraining, context for the interpretation of the second sentence. The second sentence always had the same construction-subject NP + verb + object NP (the target word). The sentence continued with at least one other clause after the target. These 32 sentence-pairs provided the normal base-line against which the violation conditions could be evaluated.

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There were three types of violation of verb-argument relations. An example of these, together with the normal base-line (l), follow.The target word is in bold type.

1. The crowd was waiting eagerly. The young man grabbed the guitar and . . .

2. The crowd was waiting eagerly.The young man drank the guitar and . . . 3. The crowd was waiting eagerly.The young man slept the guitar and . . . 4. The crowd was waiting eagerly.The young man buried the guitar and . . . A selection restriction violation was created by replacing the verb with one in which the relationship between verb and noun was semantically anomalous (condition 2). The argument slot for “drink” is restricted to liquid substances, and the object noun violates that constraint. Similarly, to violate syntactic constraints on verb-argument relations, we replaced the verb with an intransitive verb (condition 3). Thus, the target could not be the object noun of the verb. A pragmatic violation was created by replacing the verb preceding the target with another verb which made the sentence pragmatically (but not linguistically) anomalous, given the semantics of the preceding verb and the prior context (condition 4). The mean frequency (Kuqera & Francis, 1967) of the verbs used in the 4 conditions was 27 per million (no violation), 19 (pragmatic violation), 24 (selection restriction violation), and 26 (subcategorisation violation) per million.

So that the subject would hear each target word only once per session, 4 versions of the materials were constructed. Each version contained one quarter of the targets in each of the 4 conditions. Items in the 4 conditions were pseudo-randomly distributed across a version with 44 filler items interspersed between the test items. The filler items were designed to obscure the regularities in the test items. Thus, there were a total of 76 items in each version, preceded by 9 practice items.

The 4 versions were recorded and then timing pulses were placed on the non-speech channel of the tape at the onset of each target word. These pulses triggered a timing device which was stopped by the subjects pressing a key in response to the target.

Procedure

RH was tested individually in a quiet room in the hospital where he has speech therapy. At each session he was tested on one version of the materials. Testing sessions were separated by intervals of one month. At each session, he was first shown the target word printed on a card, and was asked to name it. He was instructed to listen to the materials (presented over headphones) and to press the response button as soon as he heard


that particular word. Throughout each trial the printed word remained in front of him so as to minimise the possibility that he might forget the word he was listening for.

The control subjects were also tested individually in a quiet room on four separate occasions, each separated by a month’s interval.


Extreme values (defined as those which exceeded the mean +/- 2 S.D.s) were replaced by the mean +/- 2 S.D.s, and missing values’ were replaced by the appropriate condition mean. Fifteen percent of RH’s latencies were replaced in this way. His mean latencies for each of the four conditions are given in Table 1. The data from the normal control group are included for comparison. His latencies are, overall, slower than the control group. Col- lapsing across conditions, RH’s mean latency was 413msec. This contrasts with the control group’s mean latency of 353msec. RH’s longer latencies are most probably due to the general effects of brain damage, which fre- quently slows down psychomotor performance-irrespective of the location of the damage (Benton & Joynt, 1958; Dee &Van Allen, 1973).9

Latencies in the undisrupted condition provide the baseline for evaluation of latencies in the three disruption conditions. If latencies in a disrupted condition are significantly longer than in the undisrupted condition, we take this as evidence that the presence of the disruption has affected the

TABLE 1 Mean R.Ts in Msec

Condition

Selection Progmotic Restriction Subcaregorisotion

Normal Violation Violation Violation

RH 367 436 442 406 Controls 303 355 368 389

‘RH missed the target a total of four times. A total of two targets were missed by the control group.

%e have a 65-year-old subject who has no detectable residual effects of a stroke she suffered in April 1986. She behaves as normal on a variety of on-line tasks, but her latencies (496msec in the present study) are longer than normal and longer than RH’s. This supports our claim that RH’s longer latencies are the result of brain damage.

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listener’s processing of the speech input. For this to be the case, the listener must be attempting to use the information which is disrupted. If latencies in a disrupted condition are not longer than in the undisrupted condition, we assume that the listener does not use that particular type of information when processing the speech input.

Normal listeners produced significantly slower responses when sentences contained any type of violation. ANOVAs showed a significant effect of anomaly [Min F (3,47) = 14.56, P<O.OOl]. A set of a posteriori compari- sons (Newman-Keuls) showed that all three types of anomaly significantly increased reaction times (R.T.s) compared to the undisrupted condition. Moreover, the presence of different kinds of anomaly had different effects on latency. Pragmatic violations had the smallest disruptive effect over normal (41msec). Selection restriction violations had a slightly larger effect, increasing latencies by an average of 64msec. The difference between the two types of semantic anomaly (13msec) was not significant, lending support to the claim that these two types of semantic information are not, in fact, qualitatively different. Molations of subcategorisation restrictions had the largest effect, increasing latencies by 86msec. Responses to subcategorisation violations were significantly slower than to violations of selection restrictions.

For RH, just as for normals, the presence of either type of semantic violation significantly increases latencies over the undisrupted condition. Pragmatic violations increased R.T.s by 69msec (t = 2.818, P<O.Ol), and violations of selection restrictions increased latencies by 75msec (t = 2.939, kO.01) . This difference between the two semantic anomaly conditions was not significant (t = 0.268, P = 0.7904).

The significant increase in latencies for both types of semantic violations suggests that R H has no problems either in accessing semantic information, or in using it to constrain the semantic relationships between words in an utterance. He thus shows no evidence of a semantic deficit which affects the on-line interpretation of the speech input.

Where RH differs from normal is in his response to subcategorisation violations. He is clearly affected by the presence of a subcategorisation violation because his latencies increased over the undisrupted case by 39msec (t = 1.994, R0.05). However, in contrast to normal listeners, R.T.s to subcategorisation violations were significantly faster than latencies to selection restriction violations (t = - 1.750, P<0.05). What this suggests is that R H is less affected than normals by the presence of a word which violates the subcategorisation constraints on the prior verb.

The question we now have to ask is whether this ability to use semantic information is restricted to conditions in which the relevant semantic relationships involve adjacent items-as was the case here. Verb-argument structures are one example of what we can call ‘‘local’’ relationships between


lexical items. It may be that RH is only able to construct this type of local semantic structure. He may be unable to develop a representation spanning an entire utterance. We can evaluate this possibility by looking at RH’s performance in an experiment which tests his ability to construct a “global” representation of an utterance.

EXPERIMENT THREE GLOBAL STRUCTURAL REPRESENTATIONS

RH, then, appears to be able to access semantic information from the lexicon and use it appropriately to construct local structural relationships- in this case, verb-argument relations. However, he does have some difficulty in the on-line use of syntactic information. What we now need to know is whether these local effects generalise to more global representations that span an entire utterance.

Does he use lexical semantic information to constrain the representation he develops of an entire utterance, as opposed to a local representation involving only adjacent elements? How extensive are his difficulties in using lexical syntactic information? It might be very restricted and apply only to the types of local syntactic constraints manipulated in the previous study. On the other hand, it might extend to other types of syntactic information that are involved in developing a global representation of an utterance.

Experiment three allows us to answer these questions by manipulating the availability of syntactic and semantic information across an utterance. We had R H monitor for target words heard in three types of prose contexts: 1. Normal prose (where both syntactic and semantic structural information

2. Anomalous prose (which is grammatical but semantically anomalous); 3. Scrambled strings (where there is neither syntactic nor semantic struc-

By positioning target words at different serial positions throughout each type of context, we were able to track the availability of different kinds of structural information across the sequences. If R H can develop a structural representation spanning an entire utterance then, like normal listeners (Marslen-Wilson & Tyler, 1980), his monitoring latencies should decrease across normal and anomalous prose sentences.

Method

are available);

tural information).

Subjects

We used the same 8 control subjects (mean age = 67 yrs) as we used in the first experiment.

CN W 3 - H

390 TYLER

Materials

We constructed 54 pairs of normal prose sentences, each of which contained a target word in the second sentence of each pair. The first sentence provided a minimal context for the interpretation of the second sentence. The targets were all names of common objects. Forty-one of the targets were monosyllabic, ten were bisyllabic and three were trisyllabic. The serial position of the target varied, with targets appearing either early, middle, or late in the sentence. The early category covered word-positions 2-4, the middle category covered positions 6-43, and the late group covered positions 10-14.

Anomalous prose versions of the normal prose sentences were made by replacing each content word (except the target) with another word of the same form-class and frequency, so that the sentences, although semantically anomalous, were grammatical. Scrambled strings were made by pseudo- randomly mixing the words in the normal prose sentences so that the resulting strings had no semantic or syntactic suucture.The position of the target word was unchanged throughout these manipulations. Examples of the three types of context, with target emphasised in bold type, are:

1. NORMAL PROSE: Everyone was outraged when they heard. Appa- rently, in the middle of the night some thieves broke into the church and stole a golden crucifix.

2. ANOMALOUS PROSE: Everyone was exposed when they ate. Appa- rently, at the distance of the wind some ants pushed around the church and forced a new item.

3. SCRAMBLED STRINGS: They everyone when outraged heard was. Of middle apparently the some the into the broke night in thieves church and crucifix stole a golden.

Three versions of the materials were made so that every target could appear in each of the three types of prose without being repeated in a single session. Each version contained one occurrence of each of the 54 targets, with 18 targets in each of the 3 types of material. To distribute any effects due to learning, each version was divided into two parts, each containing half of the targets. Targets were blocked according to type of material in order to reduce the possibility of confusing the patient by constantly chang- ing from one type of material to another. Within each part there were nine normal prose sentences, followed by nine anomalous prose items, followed by nine sets of scrambled strings. This order of presentation meant that any decrease in latency due to practise would not disproportionately benefit the normal prose materials. Four practice sentences preceded each block of nine test sentences.


Each version was recorded in the order described, and timing pulses, which triggered the timing device, were placed at the onset of each target word.

Procedure

The procedure was the same as for Experiment two.


Extreme and missing values" were replaced (9% in total) before the data were analysed. RH's overall mean latency was 530msec, which was longer than the mean of the control group (40lmsec). Again, this is undoubtedly due to a combination of age differences between patient and controls and the effect of brain damage."

Apart from this difference in overall response latency, the pattern of responses across prose types was essentially the same for RH and controls (seeTable 2). Control subjects produce faster latencies (342msec) in normal prose than in anomalous prose (399msec), and these in turn are faster than latencies (462msec) in scrambled strings (Min F [2,80] = 21.61, P<0.001).12 RH's mean R.T. to targets in normal prose (473msec) was also significantly faster ( t = 2.133, P = 0.04) than his responses in anomalous prose (523msec). Latencies to words occurring in scrambled strings (595msec) were significantly slower than R.T.s in both normal prose ( t = 4.311, P<O.OOl) and anomalous prose ( t = 2.728, P<O.Ol). For all

TABLE 2 Mean R.Ts in msec

~~ ~~

Target Position

Prose 7jpe Early Middle Late

RH Normal 524 519 377 Anomalous 571 538 459 Scrambled 550 600 634

Control Normal 380 341 305

Scram bled 469 458 460 subjects Anomalous 425 394 379

"'RH missed a total of three targets, while six targets were missed by the controls. "Our recovered stroke subject produced a mean latency of 546msec. '*Latencies in the three types of prose were all significantly different from each other at

the 0.01 level on the Newman-Keuls test.

392 TYLER

three prose types, RH’s latencies were about 130msec longer than those of the control group. These overall differences between the three types of prose material provide a general measure of RH’s ability to use syntactic and semantic information to develop a representation of an utterance.

A more direct measure of his ability to develop syntactic and semantic representations across an entire sentence is the word-position effects for each of the three types of prose. For control subjects, R.T.s decrease across both normal prose (Fl (2,14) = 19.27, PCO.01; R (2,51) = 5.94, P<O.Ol) and anomalous prose sentences (Fl (2,14) = 10.6, Pc0.01; I2 (2,51) = 3.07, P<0.05), but not across scrambled strings (Fl(2,14) = 0.851, P = 0.45; R (2,51) = 0.033, P = 0.97). The absence of a word-position effect in scrambled prose is important because it validates the claim that faster latencies across normal and anomalous prose sentences reflect the development of syntactic and semantic structural representations.

RH’s pattern of responses across word-positions are almost the same as normals. Mean latencies for targets in early, middle, and late positions in normal prose were 523, 519, and 377msec. The difference between early and late positions was significant [t (34) = 3.42, P<O.Ol], as was the difference between middle and late positions [r(34) = 2.88, P<O.Ol].The difference between early and middle positions was not significant [r (34) = 0.07, P >0.05]. Similarly, in anomalous prose there were significant differences between R.T.s in early (571msec) and late positions (459msec) [t (34) = 2.66, PcO.011, and between R.T.s in middle (538msec) and late positions [r (34) = 2.46, P<O.O5]. The only way in which RH’s reponses differed from normal is that, instead of showing no serial position effects in scrambled strings, his latencies to late-occurring targets (634msec) were slower than to early-occurring targets (550msec) [t (34) = - 1.78, P<O.O5].

In general, RH’s pattern of results here is very similar to that of the normal controls. He shows increasingly faster latencies across both normal and anomalous prose utterances, indicating that he is able to develop both syntactic and interpretative representations spanning an entire utterance. His ability to construct global representations, then, seems to be intact.

EXPERIMENT FOUR

In most respects RH’s on-line comprehension was not substantially different from normal, and yet his off-line performance (as measured by the B.D.A.E.,TokenTest, andT.R.O.G.Test) was very poor. Since it is difficult to interpret results when different tasks are carried out on different materials, we decided to compare on-line and off-line responses to the same set of stimuli by testing RH on an off-line version of the second experiment.

OFF-LINE COMPREHENSION


Method

Materials

We used the same tapes as those described in Experiment two.

Procedure

Fifteen months after RH had been tested on all four tapes using the monitoring task, he was tested on an off-line version of the experiment. Each of the four testing sessions was separated by a month’s interval. He was instructed to listen to each sentence-pair and to indicate whether there was anything wrong with it by saying either “good” or “bad”.

Each of the control subjects tested in the on-line experiment was tested in the same way as RH.


The judgements of normal listeners were rarely incorrect. Their level of accuracy ranged between 90-96% correct. RH’s performance was very different from this. He responded correctly 75% of the time when the sentences were undisrupted, but this may primarily reflect a tendency on the part of patients to respond positively when asked to make any type of decision. His performance on all types of anomalies was very poor (50%, 53%, and 59% accuracy), and no better than chance (Binomial test >0.05). There was no difference in accuracy for any of the three anomaly conditions (chi square >0.05). It is unlikely that this poor performance can be accounted for in terms of a memory deficit since RH does not have a severe memory problem.

What is striking here is the difference between the on- and off-line responses to the same set of stimuli. The date from the off-line experiment and the Boston exam are consistent with the description of RH as a “typical” Wernicke with a severe comprehension deficit. The on-line data present a different picture.They indicate that RH is able to develop an interpretation of the speech input as he hears it.

GENERAL DISCUSSION

In almost all respects, RH’s ability to construct representations of the speech input as it is heard appears to be unimpaired. Where he does deviate slightly from normal in his on-line processing is in his sensitivity to violations of syntactic subcategorisation information. For normal listeners, subcategorisation violations have the most disruptive effect on latencies, caus-

ing them to increase over and above the increase for any other type of violation. When the syntactic argument slot required by a prior verb cannot be filled appropriately, listeners' attempts to interpret an utterance are more severely disrupted than when the semantic specifications of the verb cannot be satisfied. This suggests that the syntactic relations between a verb and its argument have priority, in the sense that it is only when the appropriate syntactic relations between a verb and its argument have been constructed that the semantic implications of this structure are made available to the rest of the processing system.13

What seems to be different about RH is this priority of syntactic relations. He clearly is sensitive to subcategorisation information, because when it is disrupted his R.T.s increase over the undisrupted case. But he is less disrupted by this type of syntactic violation than by selection restriction violations-which is exactly opposite to the normal pattern. This suggests that, for RH, subcategorisation information plays a less important role in constraining the relations between verbs and their arguments.

But R H does not have a generalised syntactic deficit. He is able to develop a structural representation of an utterance, as evidenced by his increasingly faster latencies throughout anomalous prose sentences in the final experiment. He can also construct structural groupings-such as noun phrases and prepositional phrase-n the basis of local syntactic information. This was shown by his performance in an experiment in which we manipulated the availability of cues to local syntactic constituents, such as prepositional phrases and noun phrases (Tyler &Warren, 1987). For example, one of the cues which informs the listener that the unit which is currently under construction is a prepositional phrase, is the presence of a preposition. Similarly, the definite article cues the existence of a noun phrase. The question we asked was whether R H was sensitive to these local structural cues. In a word-monitoring task, the monitoring latencies of normal listeners increased when these local cues were disrupted. We took this as evidence that listeners use these types of information when constructing syntactically coherent groupings. RH showed a similar sensitivity to these local syntactic cues.

These data suggest, then, that although RH has no difficulty in using either word order or syntactic structural information appropriately, he is unable to maximally exploit syntactic constraints on verbs.I4 In contrast, he had no difficulty in appropriately using both the semantic and pragmatic properties of verbs in the process of developing a structural representation of an entire utterance. There was no evidence in the data from either the

'?his does not mean that syntactic processing necessarily precedes semantic analysis, but merely that syntactic structure has consequences for semantic interpretation.

'Since we have not exhaustively examined the entire variety of syntactic information, we cannot be sure that his deficit is exclusively restricted to this particular type of lexical information.


second or third experiments of a semantic deficit in the on-line processing of the speech input-ither in the processes involved in identifying words or in accessing and using semantic representations attached to lexical items.

In contrast to his relatively normal comprehension as assessed by the on-line tasks, RH’s comprehension as measured by off-line tasks was extremely poor. This distinction is most marked in the contrast between his on- and off-line responses to the verb-argument stimuli. The fact that R.T.s increase in the presence of an anomaly means that he must be trying to establish an appropriate relationship between the verb and its argument and finding that this is not possible. Thus, the processes involved in constructing a representation of the speech input are sensitive to the presence of an anomaly. But as soon as the relationship between the verb and its argument has to be explicitly evaluated for the purpose of making a decision about the relationship, his performance is barely better than chance.

If, in keeping with most other research in this area, we had only tested comprehension by using off-line tasks, we would have concluded that R H had the severe comprehension deficit typical of Wernicke patients. A comparison of the on- and off-line data implies that the picture is more complex than this. What it suggests is that RH has problems with some particular types of comprehension processes but not others. The comprehension processes which appear to be intact are those that are involved in constructing representations of the speech input.

However, what we think of as being “language comprehension” involves more than just the processes involved in constructing linguistic representations. These representations, once constructed, form the output of the language processing system. They have to be made available to conscious awareness so that, for example, listeners can act in response to an utterance they hear. However, the extent to which a listener needs to be aware of the output of the system in order to fully comprehend an utterance remains unclear.

Listeners also need to be aware of the output of the language processing system in order to carry out tasks which require any kind of metalinguistic decision. These are tasks which require the listener to pay attention to particular aspects of the linguistic output. For example, listeners need to be awarc of different aspects of the representation they have constructed depending on whether they are performing a sentence-picture matching task or a grammaticality judgement task. In either case, the listener has to make a decision about some aspect of the representation and it is this decision process which, we would argue, is an optional part of the comprehension process and can therefore be distinguished from the obligatory mental processes without which no representation could be constructed.

”Dretske (1981) draws a similar distinction between those representations that can become available for further cognitive processing and those that cannot.

396 TYLER

In RH’s case, it appears that the obligatory mental processes remain unimpaired, while at least some of the processes which require conscious awareness no longer function normally. So, we would argue that RH can develop a normal representation of an utterance but he has difficulty gaining access to it for the purposes of making voluntary decisions about it.”What we are currently trying to establish is whether his ability to operate upon the output of the processing system is as impaired in ‘‘normal’’ listening situations as it is in experimental situations. This turns out to be a very difficult task for a variety of reasons. However, what is clear is that R H does have some difficulty in performing the correct action in response to an utterance he hears. Therefore, it is likely that his comprehension deficit is not restricted only to experimental tasks requiring a metalinguistic judgement.

For this to be the explanation of RH’s comprehension problems, we have to explain how he manages to carry out the gating and word-monitoring tasks successfully. One possibility is that responses in these tasks derive from the products of nonconscious, automatic mental processes.This neces- sitates making the assumption that, although the action of pressing a response key in the word-monitoring task is a conscious activity, the event requiring the response is not. We would have to claim that RH knows that a relevant event has occurred (in this case, a successful match between written and spoken word)16 but not what word has occurred. For responses in the gating task to be based on the outcome of these nonconscious processes we have to conclude that the process of initiating a spoken response does not have to be a conscious activity. Marslen-Wilson (1985) has made a similar claim to account for the behaviour of close shadowers.”

If RH’s comprehension problem can indeed be located in his inability to operate voluntarily on structural representations, then we can be more explicit about the basis of the distinction between on- and off-line tasks. The important difference between them is not just speed of response, but also whether they require the listener to have access to representations for the purposes of making some explicit decision about them. To test this hypothesis, we devised a task which was both on-line (in the sense of requiring an immediate response) and required conscious access to structural representations. We constructed an on-line version of the verb-

I%e same matching explanation would apply to the phoneme discrimination task. ”One alternative explanation of RH’s ability to carry out the word-monitoring and gating

tasks is that his problem is confined to relational aspects of representations. That is, while he can reflect on single words, he cannot reflect on the structural aspects of representations. This explanation would also account for why he can accurately match a spoken word to a picture although he cannot match a spoken sentence to a picture.

‘mis also ensured that RH’s difficulties with the off-line judgement task (Experiment four) were not due to problems in remembering the sentences.


argument grammaticality judgement task in which RH had to press a response button as soon as he heard something wrong in an utterance.'*

By using the verb-argument materials we were able to compare his performance directly on the same materials with three different tasks:

1. the word-monitoring task which requires a fast response to a particular word but does not depend on any awareness of the relationship between the target and the prior verb;

2. the off-line grammaticality judgement task which only requires awareness of the structural relationship;

3. the on-line anomaly detection task which requires both an immediate response and an awareness of the structural relationship between the verb and its argument.

We predicted that if RH cannot operate upon the representations he con- structs, then his performance should be just as poor on the on-line anomaly detection task as on the off-line tasks, because in both cases he has to examine the representation in order to respond appropriately. The results supported this prediction.The pattern of responses on the on-line detection task was no different from that obtained in the off-line anomaly judgement task. In both tasks he responded correctly to most of the undisrupted sentences but performed at chance level with all three types of anomaly.

CONCLUSIONS

The contrast between automatic and nonobligatory processes and on- and off-line data revealed in these studies raises important questions about the nature of language comprehension. We do not want to conclude that comprehension consists only of those processes and representations which are automatic and nonconscious. This would mean either that our theory of comprehension could not account for the effects of conscious awareness on representations, or it would mean claiming that all comprehension is nonconscious-which is clearly false. Nor do we want to claim that comprehension can be solely defined in terms of representations which we can be conscious of. This would mean that on-line tasks do not reflect comprehension processeeand there is a great deal of experimental data on unimpaired listeners that would refute this. Moreover, we cannot indiscrimi- nately assume that the two types of tasks reveal different aspects of the same phenomenon without specifying what those different aspects are and how they are related to each other. But if it is the case that listeners do not need to reflect on what they hear in order to comprehend the speech input, then what role does conscious reflection of the representation(s) play in normal language comprehension? What additional aspects of comprehension are revealed in off-line tasks?

398 TYLER

To answer these questions we need to develop a more detailed analysis of these different types of tasks. We need to clarify the properties of the numerous different kinds of off-line tasks and determine whether on-line tasks reflect only automatic and obligatory processes and representations or whether the difference between on- and off-line tasks is only one of the degrees to which they tap the “core” processes involved in comprehension.

Finally, the results reported here answer some of the questions left open in Blumstein and Milberg’s research by showing that a patient who conforms to the traditional profile of a Wernicke’s aphasic does not necessarily have difficulty in using lexical semantic information appropriately to construct a representation of an utterance as it is being heard. To the extent that Wernicke’s aphasia corresponds to a clinically definable sub-group of aphasics, the data suggest that the comprehension deficit is not due to a semantic processing deficit, but rather to an a inability to reflect consciously on structural representations for the purpose of making some type of decision about them.

*

Revised manuscript received 3 November 1987

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Documents

Spoken language comprehension in a fluent … · COGNITIVE NEUROPSYCHOLOGY, 1988, 5(3) 375-400 Spoken Language Comprehension in a Fluent Aphasic Patient Lorraine K. Tyler M. R. C