Deficits in phonology and past tense morphology

Deficits in phonology and past tense morphology Helen Birda Matthew A. Lambon Ralphb Mark S. Seidenbergc James L. McClellandd Karalyn Pattersona

a. MRC Cognition and Brain Sciences Unit, Cambridge, UK b. Department of Psychology, University of Manchester, UK c. Department of Psychology, University of Wisconsin, Madison, USA d. Department of Psychology and the Center for the Neural Basis of Cognition, CarnegieMellon University, Pittsburgh, USA

Running title: Phonological and morphological deficits

Address for Correspondence: Karalyn Patterson MRC Cognition and Brain Sciences Unit 15 Chaucer Road Cambridge CB2 2EF, UK Tel: +44 (0)1223 355294 Fax: +44 (0)1223 359062 [email protected]

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Abstract Neuropsychological dissociations between regular and irregular past tense verb processing have been explained in two ways: (a) separate mechanisms comprising a rulegoverned process for regular and a lexical-associative process for irregular verbs; (b) a single system drawing on phonological and semantic knowledge. The latter account invokes phonological impairment as the basis of poorer performance for regular than irregular past tense forms, due to greater phonological complexity of the regular past. In ten nonfluent aphasic patients, the apparent disadvantage for the production of regular past tense forms disappeared when phonological complexity was controlled. In a same-different judgement task on spoken words, all patients were impaired at judging regular present and past tense verbs like press and pressed to be different, but equally poor at the phonologically matched non-morphological discrimination between cress and crest. These results provide strong evidence for a central phonological deficit that is not limited to speech output nor to morphological processing; under such a deficit distinctions lacking phonological salience, as typified by regular past tense English verbs, became especially vulnerable.

Key words: aphasia, nonfluent, verbs, connectionist models

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Introduction How we transform the phonological form of a present tense verb into that of its past tense has attracted fierce debate. The controversy raises a significant issue at the heart of language processing: does the language system comprise words stored in memory and a separate set of rules operating upon and transforming stored lexical entries, or does a single system operate upon all verbs? Alternative accounts of past tense processing have been pursued in many domains: speed and accuracy of processing and the differential effects of word frequency in normal adults and children (Marchman, 1997; Prasada, Pinker, & Snyder, 1990; Seidenberg, 1992; Ullman, 1999), ERP studies (Newman, Izvorski, Davis, Neville, & Ullman, 1999; Penke, Weyerts, Gross, Zander, Munte, & Clahsen, 1997), and studies of developmental disorders (Clahsen & Almazan, 1998; Gopnik & Crago, 1991; Thomas et al., 2001; Ullman & Gopnik, 1999). Theories have been further refined as attention has shifted to impairments following brain injury. Observed neuropsychological dissociations between regular and irregular past tense performance seem to be a direct prediction of dual-mechanism theories which propose that these two verb classes are processed by completely separate mechanisms and brain systems (Clahsen, 1999; Pinker, 1991; Pinker, 1999; Ullman, Corkin, Coppola, Hickok, Growdon, Koroshetz, & Pinker, 1997). Although double dissociations may be transparently in line with a dual-mechanism approach, they have also been demonstrated to be equally compatible with and predictable from connectionist, single system models (Plaut, 1995; Plunkett & Juola, 1999). The Rumelhart & McClelland (1986) parallel-distributed processing approach to past tense production proposes a single system drawing on phonological and semantic knowledge to process both regular and irregular verbs. In this approach, the prediction of a double dissociation in impaired past tense processing follows from the combination of greater phonological complexity of the regular than the irregular past tense, plus a hypothesised greater reliance of irregular forms on a contribution from word meaning. In this account, therefore, the pattern of irregular > regular should be associated with a general phonological impairment, while the pattern of regular > irregular is predicted

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from a general semantic deficit (Joanisse & Seidenberg, 1999; Patterson, Lambon Ralph, Hodges, & McClelland, 2001). Regular English verbs make up approximately 86% of verb vocabulary (Plunkett & Nakisa, 1997). The present tense or stem is transformed into the past tense by the addition of –ed to the orthographic form, but the resulting phonological form is dependent on the final phoneme of the stem. The final alveolar stop in the regular past tense is consistent with the voicing of the preceding phoneme. Thus stems ending with a voiced phoneme (stab, love, raise) add the voiced allophone[d] in the past tense, those ending with unvoiced phonemes (stop, laugh, race) add the unvoiced version [t], and those already ending with either the voiced or unvoiced alveolar stop (fade, hate) acquire an extra syllable [ d]. The remaining 14% irregular verbs form their past tenses in a variety of ways: a) no change (hit, spread) b) change final [d] to [t] (lend -> lent, send -> sent) c) simple vowel change (lead -> led, choose -> chose) d) complex changes involving vowel change plus addition of final alveolar (think -> thought, say -> said) e) few very high frequency suppletives (go -> went, am -> was)

Note that there are several factors which keep ‘irregular’ in this context from being completely unpredictable. One is that, within the vowel change sets (c) and (d), there exist ‘families’ of verbs such as ring -> rang, sing -> sang and weep -> wept, creep -> crept. Another is that the majority of irregular past tense forms, like all regular past tense forms, end with an alveolar stop; but a major distinction between regular and irregular is that, while regular past tense forms are always produced by the addition of extra phonemes at offset, this is not the case with many irregular verbs. The dual mechanism account of Ullman et al. (1997) states that irregular verbs and their past tense forms are stored in a lexicon which is part of the declarative memory system.

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If a learned irregular past tense form is found in the lexicon, a signal is sent to the rulegoverned procedural system to block the use of the ‘add –ed’ rule for past tense production. A refined version of the dual mechanism account, the procedural/declarative model (Ullman, 2001), differs from the traditional view in that rote memory for transformations of irregular verbs is replaced by an associative memory of distributed representations, which learns mappings of different forms and may then generalise to new forms (Pinker, 1991; Pinker, 1999). As Ullman (2001) states, this system is very similar to that modelled by connectionist networks (Joanisse & Seidenberg, 1999; Plaut, McClelland, Seidenberg, & Patterson, 1996). The associative memory also plays no part in regular past tense formation, and the activation of a stored irregular past tense form still inhibits the use of the ‘add –ed’ rule in the procedural mechanism. The most important feature of the single mechanism account, which distinguishes it from all variants of dual mechanism approaches, is that all verbs are dealt with in a single system. Consider the constraint-satisfaction PDP model described by Joanisse & Seidenberg (1999), in which the production of a past tense form involves the simultaneous use of both phonological and semantic information in processing verbs of all kinds. The network is sensitive to systematicity between present and past tense forms across the language, and the predominance of the simple addition of an alveolar stop, consistent in voicing with the final phoneme of the present tense form, results in a strong tendency to compute this phonological change. As the network is continuously and bi-directionally interactive, semantic knowledge plays a part for all words; this is particularly salient for irregular verbs, which must oppose the strong predisposition towards a simple phonological transformation. As learning is modulated by frequency of experience, the lower frequency irregular verbs depend upon this semantic constraint to an exaggerated degree relative to regular and higher frequency verbs. Thus a frequency by regularity interaction results, with low frequency, irregular forms causing relatively selective difficulty. This is precisely the pattern found in a study of the generation and recognition of past tense verbs in semantic dementia (Patterson et al., 2001). These patients were selected because of their specific and progressive degradation of semantic

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knowledge, with spared phonological processing. They showed the predicted frequency by regularity interaction in accuracy of past tense transformations, together with a highly significant correlation between their ability to produce and recognise the correct past tense forms of irregular verbs and the integrity of their semantic knowledge of the same verbs (tested by synonym judgement). Dual mechanism proponents would argue that these patients with semantic dementia have a specific impairment of the lexical associative mechanism, which would also result in the preponderance of over-regularisation errors (digged) observed in their performance. Other error types were produced, however, and those which indicate partial preservation of knowledge about particular types of irregular transformations (slit -> slat, grind -> grind) might seem to be more compatible with the single system account. Nevertheless, the declarative/procedural version of a dual-mechanism model, which essentially employs the same system as connectionist models within the declarative memory responsible for the irregular past tense, may explain such errors on the basis of its pattern association learning. More of a problem for any dual mechanism account seem to be errors which combine regular and irregular elements in a single response (tored, frozed). If access to the correct past tense form (here tore and froze) is supposed to block application of the regular rule in the procedural system, then such errors should not occur; but nearly 10% of the irregular past tense errors in Patterson et al. (2001) were of this form. On balance, although we would interpret the data from semantically impaired patients with a regular > irregular advantage in past tense processing as slightly favouring the singlemechanism account, these results do not provide any definitive resolution of the debate. More critical to this debate, and in any case de rigueur for a fuller theoretical account, is to address the other side of the dissociation. That is the purpose of this article. Recall that, within the PDP/constraint satisfaction model of past tense processing, the explanation for patients with an irregular > regular advantage hinges on one fact and a related hypothesis. The important fact is that, with certain notable exceptions that will play an important role in our stimulus materials, the past tense forms of regular versus irregular verbs differ in phonological

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complexity, with most regular forms (such as talked or stopped) containing terminal consonant clusters that are largely absent from irregular forms (such as spoke or fought). The related hypothesis is that patients displaying the irregular > regular pattern, being nonfluent aphasics of the Broca type, have phonological impairments that result, amongst other deficits, in relative difficulty with phonologically complex forms. The critical prediction arising from this account is that, if the phonological complexity of regular and irregular past tense forms were matched, then an irregular > regular dissociation should be transformed into irregular = regular performance. Accordingly, the logic of this study was to screen nonfluent aphasic patients for cases who showed irregular > regular on test materials like those employed by Ullman et al. (1997) and Ullman, Izvorski, Love, Yee, Swinney, & Hickok (in press). If this pattern is attributable to disruption of a procedural rule system, it should be maintained on phonologically controlled materials. If, on the other hand, the disadvantage for regular past tense verbs derives from their phonological complexity, it should be eliminated by phonological matching of items.

Patient testing The patient group of interest was that described by Ullman et al. (1997) and Ullman et al. (in press) as agrammatic nonfluent aphasics with a significant advantage for irregular > regular in past tense processing. In order to identify similar patients, we used two sets of regular and irregular verbs (one set of our own devising and one set taken from Ullman et al., in press) and administered the materials in three production tasks: sentence completion, single word reading and repetition. We screened 50 individuals who were clinically described as nonfluent aphasics following a left-lateralised cerebrovascular accident (stroke). On the combined screening tests, 10 of these 50 cases exhibited an irregular > regular advantage across the three production tasks, and were willing to undertake extensive testing. The results of these screening tests will be presented and discussed after some basic information about the patient sample has been provided. Patient details and aetiology are shown in Table 1.

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Table 1 about here.

All the patients completed a battery of assessments to give a profile of general language and semantic capabilities, and these results are shown in Table 2. Both spoken and pointing digit spans are indicated: the latter was tested in order to assess the contribution of purely speech output problems to the poor spoken digit spans of most of the patients. Pointing span was comparable to the spoken span in all cases, suggesting that output problems are not their sole source of this deficit. The individual patients’ results are displayed here and throughout the remainder of the paper in ascending order of combined spoken and pointing span. Spontaneous speech fluency was assessed by calculating words produced per minute during a description of the Boston Cookie Theft picture (Goodglass & Kaplan, 1983). By this measure, all patients were substantially dysfluent relative to normal controls (lowest control = 96 wpm). All patients were impaired at receptive syntax as measured by the number of blocks passed (max = 20) in the Test for the Reception of Grammar (TROG; Bishop, 1989). There was considerable variability in reading and naming success, but all 10 patients were impaired on both. In PALPA test 31 (Kay, Lesser, & Coltheart, 1992), which assesses single word reading aloud with manipulations of word frequency and imageability, all patients performed better at the higher imageability items, and this difference was significant for BB, IJ, IB, AB and GN. In picture naming and spoken word to picture matching the same 64 item target set was used (Bozeat, Lambon Ralph, Patterson, Garrard, & Hodges, 2000). Each target in the word to picture matching test was accompanied by nine distracters from the same semantic category. As shown in Table 2, all patients performed reasonably well on word to picture matching and also on the three-picture version of the Pyramids and Palm Trees Test (PPT; Howard & Patterson, 1992), although as the lowest control score on PPT is 49/52, some of the patients apparently had a mild semantic impairment.

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Table 2 about here

Table 3 shows the results of three ‘metalinguistic’ phonological assessments (from Patterson & Marcel, 1992): rhyme judgement, rhyme production and segmentation (addition and subtraction of initial phonemes). While there was considerable variation in performance within the group, these results establish that all the patients had some impairment in these kinds of explicit phonological tasks on which control subjects perform at ceiling. Half of the patients were unable to make an attempt at the adding and subtracting the initial phonemes of words.

Table 3 about here

Verb screening assessments Method We used a screening test comprising 24 regular and 24 irregular verbs matched for frequency and imageability which were administered to patients across three tasks, sentence completion, repetition and reading aloud. The sentence completion task had simultaneous visual and auditory presentation, and used a cloze paradigm in which the present tense was presented within a sentence context, followed by a sentence demanding the past tense with the verb omitted (e.g. ‘Today I dig the garden. Yesterday I ____ the garden’). Subjects were asked to produce only the missing word, and were told it should be the same verb but in its past tense form. The same items were presented singly for immediate repetition and reading aloud of both present and past tense forms. We also assessed the patients with lists previously used by Ullman et al. (1997; in press). These comprised (a) the 40 verbs (half regular, half irregular) that formed Ullman et al.’s main past tense production task (this set did not control for any of the relevant variables), which we used in both sentence completion and repetition; and (b) the 34 verbs (again half regular and half irregular) for which Ullman et al. controlled offset consonant cluster size (their so-called anterior aphasic reading list), which we also used

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to test reading only. Appendix 1 shows the items in our screening set. The past tense forms of our regular verbs had a mean of 4.50 phonemes, compared with 3.58 phonemes for the irregular past tense forms. This is very similar to the Ullman et al. 40 item set (regular 4.55 phonemes; irregular 3.65 phonemes). In contrast, the regular verbs in the Ullman et al. reading set had a mean of 3.71 phonemes, and the irregular items 4.06 phonemes.

Results The combined proportions of correct responses to the two sets of items for each task are shown for each patient in Figure 1. The advantage for irregular > regular verbs in sentence completion was significant1 for DE (

2

= 4.24, p = .039), RT (

2

2

= 8.45, p = .004) and GN (

= 5.60, p = .018); using a 1-tailed test, this difference was also reliable for IJ and MB. In the repetition assessment, significant effects were found for IB ( 22.57, p < .001), RT (

2

= 4.73, p = .030), GN (

2

2

= 5.34, p = .021), DE (

= 4.16, p = .041) and JL (

2

2

=

=11.00, p =

.001), with1-tailed significance for BB and IJ. In reading, the advantage for irregular past tenses reached significance for IJ (Fisher’s Exact z = 2.59, p = .010), DE ( .001), AB (

2

= 10.69, p = .001), RT (

2

= 8.37, p = .004) and JL (

2

2

= 6.61, p
regular past tense verb production in all tasks. The mean proportions of correct responses for irregular and regular verbs were .37 and .20 respectively in sentence completion (t (df 9) = 6.24, p < .001), .68 and .47 in repetition (t (df 9) = 5.51, p < .001) and .44 versus .24 in reading (t (df 9) = 5.61, p < .001).

Figure 1 about here.

1

All results are reported with correction and are two-tailed unless otherwise stated.

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In a statistical comparison between the patients’ performance on our own screens and on the Ullman et al. sets, there was no interaction between source of materials and verb regularity in the patients’ performance on the sentence completion and repetition screens (see Figure 2), but there was a significant source by regularity interaction associated with their performance in reading (F (1,9) = 13.90, p = .005). Recall that the regular items in the Ullman et al. reading set had fewer phonemes than the irregular items; in our screens the opposite was true. While the majority of our patients did show an irregular > regular numerical advantage in both reading sets, this did not reach significance for any individual on the Ullman et al. set. In their study of past tense production in nonfluent aphasia, Ullman et al. (in press) identified only one patient, FCL, who was able to do the sentence completion task and showed a significant advantage for generating irregular past tense verbs. FCL was also one of the patients tested on Ullman et al.’s phonologically controlled reading set, but on these items he did not show a significant irregular > regular advantage either (regular 7/17, irregular 9/16). The only other nonfluent aphasic patient who provided data from the sentence completion task was RBA, whose performance on regular and irregular verbs did not significantly differ (and his oral reading was not assessed). We will return later, following the results of our main experiment on past tense production, to the issue of similarities/differences between the patients in our study and those of Ullman et al. (1997; in press).

Figure 2 about here

Summary We have established that the individuals in this patient group experience particular difficulties in the production of regular past tense verbs relative to irregular past forms, and also that they all have a phonological impairment. As indicated at the end of the Introduction, the question with which we are concerned is whether the phonological impairment might be entirely or largely responsible for the past tense production deficit, and whether controlling for phonological complexity would therefore essentially eliminate the advantage for irregular

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verbs. We also wanted to examine the factors affecting performance in the production tasks, as this might also have a bearing on the differential success with regular and irregular verbs. We have already shown that, in reading, imageability is a crucial predictor of the performance of all the patients in this group.

Main verb production experiment Method In order to facilitate the examination of psycholinguistic variables and different types of irregular verbs, we chose a large set of 126 irregular single syllable verbs. All had consistently irregular past tense forms; that is, we excluded verbs which can be optionally treated as regular, such as dive -> dove or dived. A further 126 regular single syllable verbs were selected; and this 252 item set included a subset of 34 regular and 34 irregular past tense forms matched pairwise for word frequency (Baayen, Piepenbrock, & Gulikers, 1995), imageability (Bird, Franklin, & Howard, 2001; Coltheart, 1981) and CV structure. Also included was a subset of 84 regular verbs, the past tense forms comprising 28 of each of the three allophones [d], [t] and [ d] controlled for frequency and imageability and CV structure of the stem (those stems with a final alveolar stop necessarily became two-syllable words in the past tense). The motivation for this manipulation was to assess the effect of the increase in number of syllables with the accompanying reduction of final consonant cluster size. These sets of controlled materials are shown in Appendix 2. We also attempted to control as closely as possible across different types of irregular past tense forms; but the ‘families’ vary considerably in frequency of use, and in some cases there was no overlap in frequency that could successfully be exploited. In addition to the 252 real verbs, a pseudo-word was created for each verb. Where permitting, this entailed the alteration of one phoneme / letter. The nonfluent Broca’s patients were assessed on the production of these pseudo-words randomised among the real verbs. In sentence completion, the same cloze paradigm was employed as described above. The

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sentences used for real verbs and pseudo-words were identical, but the phonologically similar pseudo-words were embedded in sentences created for verbs other than those from which they were derived. This was to avoid the pseudo-words’ sounding like a real verb that might plausibly fit into the sentence. There were therefore a total of 504 sentences for completion, split across eight blocks of 63. This is an extremely difficult task for these patients. Due to very slow rates of responses and intolerance to this form of assessment, AB, BB and IJ completed only the controlled subset of 34 each of regular and irregular real verbs; and unfortunately VC was not available to take part in further sentence completion tasks. The remaining six patients completed the full set of 252 real and 252 pseudo-verb stimuli in the sentence completion paradigm. For the repetition and reading assessments, the same set of items was used, with the addition of the production of all present tense forms. Thus the total number of stimuli in each of these tests was 1008; these were again divided for presentation into eight blocks of 126 items each. Administration of the blocks was arranged such that patients never encountered the same stimulus item more than once within a testing session. All ten patients completed the repetition and reading assessments. We present here only the results of the two types of realword past tense forms, since the other materials do not bear on the main issues presently under consideration.

Results Figure 3 shows the patients’ performance on the subset of 34 regular and 34 irregular verbs controlled pair-wise for CV structure as well as imageability and word frequency. The irregular > regular advantage observed in the screening assessments in both sentence completion and repetition (Figure 1) was no longer apparent for any individual patient, nor was there any significant regularity effect for the group as a whole; in fact five of the nine patients showed a (non-significant) advantage for regular past tense forms in sentence completion on the matched sets. The mean proportions of correct responses to irregular and regular verbs in sentence completion were .26 and .29 respectively, and .65 versus .64 in

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repetition. The absence of any selective difficulty with regular verbs on the complexitycontrolled materials lends strong support to the hypothesis that the previously seen disadvantage was due to the greater phonological complexity of regular past tense forms. In the reading task, however, there was still a significant, though reduced disadvantage for regular forms across the group (.39 versus .28, t (df 9) = 3.09, p = .012), although this effect reached significance at an individual level only for AB ( for RT (

2

2

= 4.91, p = .027) and marginally

= 3.68, p = .055).

Figure 3 about here.

The residual advantage for the irregular past tense in reading might be due to the visual representations and/or alternative meanings of these items. It is well established that agrammatic Broca’s patients have particular difficulties reading abstract words, as shown in this patient group by their performance on the PALPA items (Table 2). There are many irregular past tense forms that are homographs of other, more concrete words. Out of the set of 34 irregular past tense forms, both AB and RT correctly read six which are homographs of concrete nouns and adjectives, or nouns derived from, and more imageable than, the verb (these include ground, wound, left, stole, spoke and bit). Another factor is that the past tense of some irregular verbs is identical to the present tense or stem (let, split and cast are the only such verbs in this subset). All the patients were very much better at reading present tense forms compared to the past tense of the same irregular verbs, so it is reasonable to assume that some or all ‘no change’ verbs were read as if they were present tense forms (read was always pronounced [rid], for example). If all such ‘problem’ items are removed, along with their regular counterparts (pair-wise matching means the variables remain controlled), AB’s scores are 9/23 for irregular and 4/23 for regular past tense reading, and RT’s are 12/23 and 6/23 respectively, neither of which represents a significant effect, even on a one-tailed test (Fisher’s Exact AB z = 1.30, 1 tailed p = 0.10; RT z = 1.49, 1 tailed p = .067). The set used by

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(Ullman et al., in press) for reading assessment also included some irregular past tense forms which were homographs (felt, left, spoke and lent). The Ullman et al. patients achieved a success rate of 60% in reading these items, compared to 48% on the non-homograph items. Finally, as so many nonfluent patients produce visual errors in reading, it is also conceivable that regular past tense forms are particularly vulnerable to the error of suffix deletion because they are so visually similar to their present tense forms. By definition, the past tense differs from the present only by a suffix of between one and three letters. All patients’ scores were combined for all 252 past tense verbs, and backwards stepwise regression analyses run for each production task, entering the following variables: number of phonemes, number of syllables, number of consonants at onset, number of consonants of offset, imageability, frequency of the past tense form and whether the past tense form is a homograph (see Table 4). In sentence completion, the only significant predictor of performance was the number of phonemes in the target past tense form. In repetition, fewer consonants both at the beginning and at the end of a target resulted in significantly better performance. This combination of results lends support to the idea that these patients’ difficulty with past tense regular verbs is due to the phonological complexity of such items, perhaps especially the addition of a final phoneme. The only other significant predictor of performance in repetition was the frequency of the past tense word form.

Table 4 about here.

In reading, however, the results were rather different. The significant predictors of reading success were the frequency of the past tense word form, the number of consonants at onset (but not at offset), imageability and whether the past tense form of the verb was a homograph. This latter effect supports the assertion that the remaining irregular > regular advantage in some patients’ reading, even when CV structure is controlled, is at least partially attributable to the existence of (relatively high imageability) homographs for many irregular past tense forms.

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Finally, we examined the patients’ performance on the different types of regular verbs. In none of the three production tasks was there any difference in accuracy between items such as stabbed and stopped, for which the difference was the presence or absence of voicing on the final consonant; neither did the addition of an extra syllable on stems with a final alveolar stop (e.g. faded, hated) have any effect across the group in sentence completion or reading. In repetition, however, performance was significantly better for past tense forms with two syllables when compared with one syllable past tense items matched for frequency and imageability (mean proportions correct .68 and .42 respectively; t (df 28) = 2.53, p = .017). A numerical advantage in repetition for two-syllable past tense forms was apparent for all patients and reached significance for the following individuals: IJ (Fishers Exact z = 2.25, p = .025), IB ( (

2

2

= 4.43, p = .035), DE (

= 6.03, p = .014), RT (

2

2

= 18.05, p < .001.), AB (

= 13.13, p < .001) and GN (

2

2

= 4.73, p = .030), MB

= 8.72, p = .003). That this effect

was robust only in repetition suggests that it might be largely receptive in origin.

Comparability of our patient sample to that of Ullman et al. (1997; in press) We have demonstrated (a) that the patients in this study displayed a significant irregular > regular advantage for producing the past tense of verbs on a screening test with materials similar to (in fact incorporating) the items used by Ullman et al., and (b) that this differential irregular > regular success was subsequently abolished in two of the three production tasks and reduced to insignificance in the third task when the two verb classes were matched for phonological structure. The implication of these results is that we might expect the significant irregular > regular effect reported by Ullman et al. (1997; in press) likewise to be reduced or indeed eliminated on stimulus materials like our controlled subsets. This prediction, however, raises an issue universally encountered in comparisons across different neuropsychological studies: the comparability of the patients. Even though the patients tested by us and by Ullman et al. are all described as nonfluent aphasics, they would almost inevitably be non-identical in lesion size, precise lesion location, and the

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behavioural/linguistic consequences of the lesions. How confident can we be in generalising our results to a prediction about the performance of a different patient sample? The first point to note is that even the nonfluent patients described by Ullman et al. (in press) were a heterogeneous set, and furthermore were not all tested on the same tasks. This renders even more difficult a direct comparison with our patient sample. Examination of the error types made by the two groups can, however, be illuminating in such circumstances. Ullman et al. (in press) listed their patients’ different error types in considerable detail, and for ease of comparison we have amalgamated them into unmarked forms (repetition of the present tense or stem of the target verb), incorrect morphological affixation (usually –ing, but this also includes –s and –en), and ‘other’. The latter includes all other categories of error, but the majority of these were substitutions, distortions and failures to respond. We compared these error types in sentence completion for the two patients reported by Ullman et al. (in press) who could accomplish this task to those of the ten patients in our study on exactly the same items (the Ullman et al. word set). The distribution of correct responses and error types for regular and irregular verbs is shown in Figure 4. The error distributions for FCL and RBA are by no means identical: apart from unmarked productions, FCL’s main error type was morphological, while RBA mostly produced ‘other’ responses. The error patterns in our sample were also quite heterogeneous: some patients produced many phonological distortions, while for others unmarked and morphological errors predominated. On the whole, the sentence completion error pattern of the patients included here perhaps resembled that of RBA more than FCL; but half of these cases as individuals and all ten taken as a group did, like FCL (but not RBA), have a reliable advantage for irregular > regular verbs in past tense generation in sentences. Figure 4 about here.

We also compared reading error types on the items used by Ullman et al. (in press) in their reading assessment, between our patient sample (N=10) and their patient sample (N=9), see Figure 5. As noted earlier (see Figure 2), our patients did not show as large an irregular >

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regualr advantage in reading on this screening test (in which the complexity of offset was controlled) as on our own uncontrolled reading materials; as Figure 5 indicates, the irregularity advantage for our patients in reading these materials was also smaller than that for the cases of Ullman et al. The pattern of error types, however, was fairly similar across the two patient groups. The reading performance of most nonfluent aphasic patients can be characterised as phonological or deep dyslexia, in which patients produce a large proportion of visual and/or semantic errors (categorised as ‘other’ in our error analysis). This was certainly the case in our patient group (for example, several patients read flowed as flower). Note that this pattern of reading disorder, which has been attributed to a general phonological deficit, is a direct prediction of the Plaut et al. (1996) model of reading. We have also reported repetition performance on our set of items used both for sentence completion and reading. Errors in repetition were almost entirely phonological: phonemes were usually deleted or substituted. Although unmarked forms were also produced for regular past tense verb stimuli, for most regular past tense forms, this is the inevitable result of the omission of the final phoneme. There is no completely satisfactory way to resolve the problem of comparison across studies of different patients. Nevertheless, and despite the fact that none of the individual patients in our sample produced quite as large a discrepancy as patient FCL (from Ullman et al. 1997; in press) between irregular and regular verbs on past tense sentence completion, we see no basis for assuming qualitative differences between our cases and theirs. This is especially so in light of the facts (a) that FCL failed to show a significant advantage for irregular past tense in reading, and (b) that their case RBA failed to show a significant advantage for irregular past tense in sentence completion. One patient in our study, GN, demonstrated performance similar to that of FCL: in the sentence completion screens, GN's advantage for irregular > regular past tense was substantial (though not quite as large as FCL's); while in the reading screens GN, like FCL, had no significant difference between the two verb types. We were able to obtain a structural MRI for GN, and Figure X demonstrates that GN’' s lesion was also very similar to that of FCL: both had damage to Broca’s area, including the frontal operculum, middle and inferior frontal gyri and

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insula. FCL had additional damage to the basal ganglia (putamen, globus pallidus, but not the caudate nucleus). In both patients’, parietal and temporal lobes remained essentially intact. On the basis of these neuroradiological similarities, and especially the behavioural results, we therefore offer the following tentative conclusion or rather prediction: the elimination of the irregular advantage observed in our cases when stimulus words were controlled for phonological structure would also characterise the patients assessed by Ullman et al. if tested on the same materials.

Figure 6 about here

Receptive phonology We have established that these ten patients all had some degree of phonological impairment, and their poor performance on rhyme judgement indicated that this might not be limited to phonological processing for speech production. We used sentence completion, repetition and reading assessments to demonstrate the impact of this deficit on the processing of the regular past tense, but these paradigms all involve speech production. Ullman et al. (in press) also showed that when their nonfluent aphasic patients were not required to produce the past tense but merely to listen to correct or incorrect alternatives and make acceptability judgements, performance was worse for the past tense of regular than irregular verbs. In order to determine whether the impaired phonological processes of the patients in our study might affect receptive as well as productive phonology, we conducted an auditory same/different judgement assessment. The experiment was designed to answer three questions: 1. Do these patients have difficulty differentiating the regular past tense from its present tense form (e.g. press versus pressed)? 2. If so, do they have similar problems differentiating word pairs for which the phonological difference is identical to present/past tense regular verbs (e.g. cress versus crest)?

19

3. Is any difficulty in detecting the addition or deletion of final alveolar stops mediated by the consistency of voicing of the preceding phoneme (e.g. is it easier to discriminate an from ant, he from heat or heed from heat, than it is he from heed or an from and)? Question 1 speaks to the issue of whether these patients’ deficits in the production of past tense regular verbs might extend to their speech perception and comprehension. Their performance on the phonologically controlled sets indicated that the apparently specific deficit in generating the regular past tense could be explained simply by a difficulty in the production of consonant clusters. Question 2 addresses the hypothesis that any difficulties in detecting the presence of the final alveolar stop on a regular past tense form might again be due to a general phonological deficit rather than a specific morphological impairment. Question 3 has implications for irregular verbs such as send and bend. The past tense forms of such verbs also have final alveolar stops, but unlike all regular past tense forms, the final /t/ is unvoiced, even though the preceding nasal consonant is voiced. This difference might make it easier to discriminate between the present and past tense of irregular verbs such as send and sent than between present and past tense forms of regular verbs.

Method A set of materials was constructed incorporating 38 pairs which were present and past tense forms of regular verbs (e.g. press/pressed) pair-matched to word pairs which were semantically distinct but which had the same phonological distinction between the pair (e.g. cress/crest). These were matched as closely as possible for word frequency and imageability. Also included were 125 word pairs (which were not verbs) in which the final alveolar could be manipulated but still provide real English words (e.g. an/and, an/ant, and/ant). Each word in the set was recorded individually onto digital tape in a sound proof studio by a male and a female speaker. Male and female voices were used so that the subjects would not be able to discriminate merely on the basis of acoustic similarity. The digitised words were transformed into sound files for manipulation using Syntrillium CoolEdit software. The experiment was designed so that each ‘different’ pair would be presented twice, once each way round (e.g.

20

press/pressed and pressed/press). Each ‘same’ pair was presented once (e.g. press/press and pressed/pressed). This would have provided an equal number of ‘same’ and ‘different’ pairs in the set, but the inclusion of triads of items such as the an/and/ant manipulation, which provided six ‘different’ pairs and only three ‘same’ pairs, meant that the complete set comprised 60% ‘different’ pairs and 40% ‘same’ pairs. For all pairs, the female voice preceded the male voice version. The list was pseudo-randomised and divided into four blocks for presentation. The same word pair (albeit in reverse order) was never presented twice in the same block. Each sound file was copied with a one second pause between members of the same pair and a three second pause after each word pair. Six practice pairs were also recorded, using words that did not appear in the stimulus set. Subjects listened to the stimuli over headphones and responded by pointing to the words ‘same’ or ‘different’ to indicate whether they thought the two words spoken by the female and male voice were the same or not. All the patients in this study had been given audiograms, and some were found to have significant age-related sensory hearing impairments; in one case (MB) this was quite severe. Control subjects from the MRC Cognition and Brain Sciences Unit volunteer panel were selected and audiograms were conducted prior to testing. Ten control subjects were chosen to match the patients as closely as possible for both age and hearing acuity.

Results The overall performance on this task is shown in Table 5. Control subjects, regardless of hearing impairment, performed essentially at ceiling. The slight imbalance in the relative proportions of ‘same’ and ‘different’ pairs was thus not considered a problem: if control subjects had given equal numbers of same and different responses, we would have seen poorer performance on the ‘different’ pairs. All of the patients’ scores on the ‘different’ pairs fell below the control range.

21

Table 5 about here

The following analyses pertain to the patients’ responses to ‘different’ stimuli. Figure 7 shows that, with the exception of JL, these patients were impaired at discriminating regular present from past tense, but they were equally poor at detecting the difference between pairs such as cress and crest. The mean proportions correct were .54 and .55 respectively (t (df 9) = -.677, ns). This pair of findings provides the answers to questions 1 and 2, by suggesting that impairment to phonological processes is responsible for the difficulty in discriminating these fine phonemic contrasts, a difficulty that is clearly not limited to morphological contrasts.

Figure 7 about here

Figure 8 shows the results for the different types of contrasts. In this analysis the results of the present/past verb pairs have been combined with those of their phonologically matched morphologically distinct pairs, as there was no difference in performance between the two conditions. We can see from Figure 8 that the pairs for which voicing is consistent across final and penultimate phoneme (the light grey bars) were those with which the patients had the greatest difficulty. This is true, whether or not the pairs are present and past tense of the same verb (such as press/pressed; own/owned) or two distinct words (such as cress/crest; an/and). Detection of a difference was much better for word pairs that also differ by only the final phoneme, but for which the final phoneme is unvoiced and the penultimate phoneme is voiced, such as an/ant (the black bars)2. This difference in accuracy between the grey-bar and black-bar conditions is highly significant (t (df 88) = -8.01, p < .001). The patients seemed to have greater difficulty discriminating pairs with unvoiced than voiced final alveolars, but there were only six possible manipulations of the unvoiced type, so the data set is very small, and there was no pair-wise matching across the voicing contrast.

22

Figure 8 about here

Even though control subjects made very few errors, the differences they failed to detect were also significantly more likely to be those within word pairs in which the word with the extra phoneme had consistency of voicing (t (df 88) = -2.526, p = .013). There was no difference in accuracy between verb pairs and their phonologically controlled partners. The pattern observed in the patient group is therefore an exaggerated normal pattern: it is easier to perceive the difference between he and heat than that between he and heed. Figure 9 shows the acoustic difference between the words he, heed and heat by means of spectrograms of these words spoken by the same male speaker. Not only is the vowel in heat shorter, but there is an approximately 14ms period between the offset of voicing and the final stop. This contrast has been noted previously with regard to final fricatives in English by Massaro (1987).

Figure 9 about here.

To summarise, the results provide a positive answer to all three questions posed by this experiment: these patients do have difficulty differentiating the spoken form of the regular past tense from its present tense form; they have an identical degree of difficulty differentiating morphologically unrelated spoken word pairs for which the phonological difference is identical to present/past tense regular verbs; and their success in detecting the addition or deletion of final alveolar stops is significantly predicted by the consistency of voicing of the preceding phoneme. In other words, the patients do have particular problems in distinguishing between past and present forms of regular verbs; but the source of this deficit is phonological, not morphological.

2

The reverse is of course not possible in English.

23

A brief addendum on the other side of the double dissociation Five patients with semantic dementia were tested on the set of verb materials employed in this study, in order to establish that they would exhibit the same regular > irregular advantage as previously shown by Patterson et al. (2001). Two of these patients (AT and WM) had taken part in the previous study. Table 6 displays their performance broken down by word frequency, and also on the planned subsets of items. The data clearly establish the same regularity by frequency interaction as was previously documented, with a strong disadvantage for irregular verbs, in particular those of low frequency. This regularity effect is maintained in the subsets of regular and irregular verbs matched for frequency, imageability and CV structure. As in the Patterson et al. (2001) study, errors in generating the past tense were predominantly over-regularisations of irregular verbs (e.g. grinded). This error type accounted for 66% of all five patients’ errors combined. The most severely impaired patients (MK, AT and WM) also tended to repeat the stem or present tense form. This kind of response accounted for 19% of the total errors. Errors on verbs containing [

], a

considerable quasi-regular subset of irregular verbs, comprised 41% of the other error types (6% of the total errors). These were all instances either of incorrect application of the ‘singsang’ transformation (e.g. swing-swang), or errors of the reverse transformation (e.g. bringbrung). Even more interesting are the errors on regular verbs in which phonological transformations drawn from irregular verbs are used. Examples from these five SD patients include blink-blank, reap-rept and creak-croak (compare drink-drank, weep-wept and speakspoke). Such errors provide testimony that these patients apply their remaining knowledge of the way in which past tenses of verbs are formed through phonological transformations, and that this knowledge base includes irregular forms as well as the overwhelmingly common -ed.

Table 6 about here.

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General discussion Neuropsychological dissociations between regular and irregular past tense verb processing have been explained in two basic ways: (a) separate mechanisms comprising a rule-governed process for regular verbs and a lexical-associative process for irregular verbs, which can be independently disrupted by brain damage; (b) a single system for all verbs in which damage to the phonological or semantic component of the language system has differential consequences for regular versus irregular verbs. In the Joanisse & Seidenberg (1999) model, a single network employed phonological and semantic representations interactively to process all verbs, and deficits for regular and irregular forms were caused by damage to phonological and semantic representations respectively. A phonological impairment predicts poorer performance in the production of regular than irregular past tense verbs due to the greater phonological complexity of the regular forms. To date, there has been a relative paucity of evidence for a reliable impairment on regular relative to irregular past tense verbs. In the sphere of past tense production, the majority of such reported effects have been limited to reading tasks, although deficits in the production of the regular past tense have been noted in writing and repetition; see Ullman et al. (in press) for a summary. While Ullman et al. (in press) demonstrated a significant advantage for the irregular past tense in the reading performance of five out of the nine nonfluent patients who could complete the task, they reported only two patients (FCL and RBA) who could perform the sentence completion task at all, and only FCL showed a reliable effect. FCL, however, did not show a significant irregular > regular advantage in the reading task, and RBA’s oral reading was not tested. The other major source of evidence has come from two receptive tasks. Marslen-Wilson & Tyler (1997) employed a task of primed auditory lexical decision with nonfluent aphasic patients and normal controls. The controls showed significant facilitation (faster RTs) when a present tense verb was primed by its past tense for both regular and irregular conditions (saved-save, gave-give); by contrast, the performance of aphasic patients was facilitated to a normal degree in the irregular condition but not at all

25

(indeed RTs were prolonged) in the regular condition. The performance of our patients on the same-different receptive task demonstrated their particular difficulty in the discrimination of word pairs differing only in the addition of an alveolar stop, including the past and present tense of regular verbs (e.g. pressed-press). One might expect, therefore, that when such patients are presented with the prime pressed and asked to make a lexical decision on press, the two stimuli might be perceived as identical and hence result in repetition priming. It is therefore particularly puzzling that, in this condition of Marslen-Wilson & Tyler’s (1997) experiment, the nonfluent patients’ RTs were slowed. In another receptive task, Ullman et al (in press) reported an advantage for irregular past tense verbs when nonfluent aphasic patients were asked to judge the acceptability of correct and incorrect forms in sentences. We will discuss this result further below. In the current study, our goal was to identify as many patients as possible who (a) were capable of generating at least some past tense verbs in the sentence completion task, and (b) showed superior performance on the irregular past tense, not only in sentence completion, but also in repetition and reading. Following the premise of the single mechanism account (Joanisse & Seidenberg, 1999; Patterson et al., 2001), we expected these patients to have phonological deficits across a variety of tasks, and indeed all ten patients meeting our verb criteria on screening assessments were impaired on tests of both receptive and productive phonology: digit span, rhyme judgement, rhyme production and phoneme segmentation. We proposed that, if their especial difficulty with the regular past tense were due to disruption, not to a rule based system, but to phonological processing, the disadvantage for regular verbs should be eliminated when phonological variables such as complexity of CV structure were controlled. On a set of materials designed to match CV structure (as well as word frequency and imageability) across regular and irregular past tense forms, the previously measured disadvantage for the regular past tense in sentence completion and repetition did indeed disappear. Multiple regression analyses on the whole set of 252 items revealed that, in both of these tasks, measures of phonological complexity were significant predictors of performance.

26

In reading, the task for which an advantage for the irregular past tense had been most often reported and most robust in previous studies, the effect was reduced on our phonologically matched materials, but still significant in two out of ten cases. It is our contention, however, that reading is subject to several other influences that do not affect past tense generation or repetition and that might at least in part account for the reduced but not wholly eliminated irregular > regular advantage. First, many irregular past tense forms are homographs of other more concrete words (e.g. ground, the past tense of grind, is also a noun referring to the surface of the earth). The oral reading performance of all of the patients studied here, in keeping with other nonfluent agrammatic patients reported in the literature, was characterised by a strong advantage for high > low imageability words. Removal of the irregular past tense forms that are homographs of concrete words from the stimulus set rendered the irregular advantage in reading insignificant. Moreover, on the complete set of 252 items, multiple regression analysis revealed that homograph status was a significant predictor of performance in reading, but not in the other two production tasks. A second reason for the remaining trend towards a disadvantage for regular past tense forms in reading is the visual similarity of these to their present tense counterparts. Many of the oral reading errors of these patients consist of producing another word that is orthographically similar to the target, for example ‘flowed’ -> flower; rather than a morphological error, responding to a regular past tense stimulus with its present tense form could equally be construed as one of these visual/orthographic errors. It may be noteworthy in this regard that regular past tense forms are orthographically identical to their present counterparts up to the very end of the word, which of course is rarely true of irregular verbs (e.g. compare blinked-blink with thought-think). This is a matter for further investigation. How might our account explain the result of Ullman et al (in press), mentioned above, on a past tense judgement task where the nonfluent aphasic patients were not required to speak? In that experiment, patients received simultaneous written and spoken presentation of the same sentences as had been used in the sentence completion paradigm, but the verb was either the correct past tense form or an incorrect alternative. For regular verbs, the incorrect

27

form was the present tense of the verb; for irregular verbs, the incorrect form was a regularised form (e.g. digged). Patients were asked to rate how good or bad the verb form was, or say whether or not the target was acceptable. Two of the three patients tested (FCL and BMC) chose more correct irregular than correct regular forms, and also incorrectly accepted as an appropriate past tense more regular present tense forms than regularised irregular verbs. The third patient (RBA) showed these effects only in RT data, not in accuracy. Germane to this result is our demonstration that, in a receptive phonological task, all 10 of the nonfluent patients tested here were impaired at judging spoken regular present and past tense verbs like press and pressed to be different. We therefore suggest that, in the Ullman et al study, the higher error rate or slowed responses to correct past tense forms and to incorrect present tense forms of regular verbs might have arisen because the patients found it difficult to distinguish between the two. For irregular verbs, the correct past tense forms were phonologically (and orthographically) much more distinct from their present tense counterparts. Thus one might expect the patients to be more often sure of what they had heard and accept the correct forms more readily. The incorrect ‘past tense’ offered for the irregular verbs in the Ullman et al. experiment was a regularisation: patients were therefore asked to assess the acceptability of non-words such as digged. Many studies in the literature on nonfluent aphasia have employed visual and/or auditory lexical decision tasks; and while there is some debate about speed of processing, it is clear that patients of this type are able to distinguish quite reliably been realand pseudo-words (Hagoort, 1997; Prather, Zurif, Love, & Brownell, 1997; Prather, Zurif, Stern, & Rosen, 1992; Tyler, Ostrin, Cooke, & Moss, 1995). It is possible, in light of the results of our same-different task, that the patients in the Ullman et al. study might have perceived digged as dig. It is worth noting, however, that while all 20 of the regular past tense verb stimuli in the past tense judgement experiment were single-syllable words (e.g. slammed), 5 out of the 16 irregular verbs, when regularised, formed two-syllable words (e.g. standed). When the addition of –ed to a regular verb resulted in an extra syllable, the patients

28

in our study were consistently more successful in the repetition of the regular past tense, which we interpreted as indicative of better perception of these items. This is yet another possible non-morphological factor that might account for the finding by Ullman et al. (in press) of more successful (or quicker) rejections of the incorrect alternatives for irregular than verbs. We have proposed that deficits in the processing of regular past tense verbs arise from a phonological deficit. It is important to note in this regard that we screened many patients (N = 50), clinically diagnosed as ‘nonfluent’, all of whom presumably had phonological deficits, but most of whom did not show a consistent advantage for the irregular past tense across the three production tasks. Why might this be? One reason is that many of these patients had such severe expressive aphasia that they were unable to cope with the sentence completion task. Some of these patients also had varying degrees of accompanying oral dyspraxia, which clearly will affect articulation. More interesting is the question of exactly what is meant by a ‘phonological deficit’: we have said that our patient group exhibited difficulties with receptive as well as productive phonology, but this is still a very broad description. We do not yet know precisely how, or to what degree, their phonological processes were impaired: this must be the subject of further investigation. We can assume, however, that the answer to this question will not be the same for all patients; as a result their performance on many tasks, particularly those involving the subtle phonological distinctions embodied in the regular past tense, might vary considerably. Therefore we should not expect to find a simple measure of ‘phonological deficit’ that can be correlated straightforwardly with degree of difficulty in regular past tense processing. Our production materials were designed to equate the number of consonants within clusters across regular and irregular items. Our same-different phonological judgement experiment, however, demonstrated clearly that the kinds of phonemes that cluster together are vitally important in perception. It is more difficult to perceive the presence or absence of a final consonant when it shares voicing with the preceding phoneme. This is due to differences in vowel length and voice offset times. For some irregular past tense forms (e.g. meant),

29

voicing ceases prior to the final alveolar stop; this never occurs in the regular past tense. It is possible that for some patients, this factor might affect repetition performance, even on our CV-controlled materials, due to the increased perceptual salience in the offset of such irregular past tense forms. The manipulation of voicing required in the production of these items might also be relevant, although it is not obvious whether voicing consistency renders articulation easier. There is yet another phonological difference between the regular and irregular past tense in English, which we have not considered here: the types of vowels that they include. In the regular past tense, the phoneme preceding the final alveolar stop is always either another consonant (slipped) or a long vowel (booed) or a diphthong (showed). While this is also true of many irregular past tense forms (kept, bought, strode), there are some irregular past forms whose final consonant is preceded by a short vowel. For irregular verbs the final consonant is not necessarily a stop (slid, fled, had, but also ran, fell, clung). This apparent anomaly is clarified by the observation that there are no present tense forms (and indeed no words at all) that end with a short vowel. One cannot remove the alveolar stop from words such as slid or fled and arrive at a phonotactically legal word of English. While the patients in our study appeared to be unaffected by vowel length, it is feasible that this might be a predictor of performance in other cases. As correctly pointed out by Ullman et al. (in press), our phonological deficit theory predicts that, just as final stops are often omitted from the past tense of regular verbs by these patients, they should also be omitted from phonologically similar irregular past tense forms. The nonfluent patients in their study, however, never produced responses such as [k p] as the past tense form of keep. Some of the patients did omit final alveolar stops when reading present tense forms of irregular verbs (e.g. send -> [s n]). One could argue that such omission errors to irregular targets might occur less often than to regular past tense forms, due to the resulting production of a non-word. It cannot be denied, however, that the phonological errors made by nonfluent patients do frequently result in non-words. On the irregular verbs in

30

our phonologically controlled subset (N = 34), the patients in this study made very few errors that consisted solely of the omission of a final alveolar stop; but there were some. DE gave [h l] in response to held in reading and repetition; both DE and IJ produced [so l] as the past tense of sell in sentence completion. These omissions do, however, result in real words. Similar errors that did not result in real words numbered only thirteen in total, and eight of these were made during the repetition task, in which the patient might have reasonably assumed that the target was indeed a non-word, and they were perhaps repeating the item as they had perceived it. In summary, despite several unresolved issues concerning the precise nature of the phonological deficits critical to our account, we contend that the data presented here provide novel and compelling evidence that deficits on regular past tense verbs can be explained by phonological factors and without recourse to a separate rule-based mechanism. The two critical results on N = 10 patients are (a) the demonstration that a significant irregular > regular advantage across three production tasks (sentence completion, repetition, oral reading) was eliminated by phonological matching of regular and irregular past tense forms; and (b) the demonstration that a significant deficit in judging as different the spoken forms of present and past tense regular verbs (press/pressed) applied equally to phonologically identical but non-morphological contrasts (cress/crest). Our interpretation invokes phonological impairment as the basis of poorer performance for regular than irregular past tense forms, due to the greater phonological complexity of the regular past tense, and our neuropsychological data have demonstrated the link between these phenomena. Together with the evidence from Patterson et al. (2001) for an account of selective impairment to irregular past tense forms in terms of a semantic deficit, these data support a model in which regular and irregular past tense verbs are processed within a single system drawing on semantic and phonological knowledge.

31

Acknowledgements The authors would like to thank the patients for their tireless participation in this research, and the control subjects who supplied normal control data. We are grateful to Catrin Blank for analysis of GN’s lesion data. Support for this research was provided by a Leverhulme Trust grant to MALR and KP and by a grant from the National Institute of Mental Health, RO1MH58734 to Mark S. Seidenberg.

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Appendix 1: Items used in screening assessment Regular verbs

Irregular verbs

step

fall

rock

eat

pass

fight

space

lead

march

rise

heat

sit

wheel

read

dust

stand

voice

speak

test

sleep

clear

cut

play

write

fork

weep

tempt

flee

graze

grind

tune

froze

tap

leap

roar

sting

dent

lend

lace

swim

rake

slit

chew

stride

beg

steal

pour

tear

33

Appendix 2a: Subset of verbs controlled for frequency, imageability and CV structure Regular verbs

Irregular verbs Frequency

Image-

Past CV

ability

structure

seem live name try guess use lay show train tow gain fail die pull fill flow view weigh call sigh chew sew stew play spray pay drop pray cry slap wipe howl tie fry

2.9479 2.7180 1.7628 2.8680 1.8873 2.9018 2.0821 2.6702 1.7711 0.5723 1.9413 2.1625 2.3775 2.2686 2.1377 1.5358 1.4778 1.4770 2.8406 1.4834 1.3037 1.0494 0.6154 2.6062 1.0015 2.5433 2.1529 1.4495 2.0803 1.2160 1.5710 0.9942 1.7876 1.3283

249 291 348 362 330 336 405 375 406 406 346 379 395 405 450 417 413 411 421 418 448 450 469 409 522 491 414 464 523 531 529 507 551 562

Mean

1.8701

425

SD

0.6657

74.25

CVCC CVCC CVCC CCVC CVCC CVCC CVC CVC CCVCC CVC CVCC CVCC CVC CVCC CVCC CCVC CCVC CVC CVCC CVC CVC CVC CCCVC CCVC CCCVC CVC CCVCC CCVC CCVC CCVCC CVCC CVCC CVC CCVC

Frequency

Image-

Past CV

ability

structure

mean keep lend bring cast leave win let grind bid bend sell buy build deal slide flee ride hold shake dig ring split swing stride teach sleep steal speak sweep weep wind bite freeze

2.8643 2.8179 1.4355 2.7089 1.6137 2.8936 2.1874 2.6343 1.4373 0.6756 1.8177 2.1605 2.4084 2.3833 2.0183 1.5428 1.4292 1.7544 2.6665 2.1189 1.6011 1.9571 1.5236 1.7466 1.0984 2.1541 2.1091 1.7198 2.5693 1.7076 1.4443 1.2131 1.4382 1.6487

207 286 327 346 367 373 382 383 386 394 395 397 397 399 421 427 431 432 445 450 455 455 459 459 462 473 477 486 488 510 523 535 553 585

Mean

1.9265

428

SD

0.5530

75.44

CVCC CVCC CVCC CCVC CVCC CVCC CVC CVC CCVCC CVC CVCC CVCC CVC CVCC CVCC CCVC CCVC CVC CVCC CVC CVC CVC CCCVC CCVC CCCVC CVC CCVCC CCVC CCVC CCVCC CVCC CVCC CVC CCVC

34

Appendix 2b: Controlled subsets of regular verbs manipulating –ed allomorphs [d]

Frequency

Image-

[t]

Frequency

ability

Image-

[ d]

Frequency

ability

blame gain name love solve soothe beg rob train save groan fill kill file sail wail crawl frown prune bowl howl jog wave scrub breathe plunge comb grill

1.6674 1.9413 1.7628 2.3773 1.7184 0.8601 1.4834 1.1645 1.7711 2.0862 1.0472 2.1377 2.3209 1.3167 1.3013 0.9715 1.4033 1.2175 0.3994 1.1060 0.9942 0.6600 1.6563 1.0204 1.6444 1.3001 0.9476 0.7632

309 346 348 355 360 379 390 400 406 408 428 450 450 464 468 471 482 486 490 494 507 510 514 523 525 548 552 558

guess place hope like pop creak mix switch check reap shock reach face chop crush wrap hop bake mop wreck smoke smack splash chase blink slap shop sketch

Mean

1.3943

451 Mean

SD

0.5075

70.42 SD

ability

1.8873 2.0020 2.3569 2.5137 1.2777 0.9076 1.7052 1.6383 1.9188 0.6383 1.4959 2.4068 2.1332 1.2890 1.3167 1.5435 0.9337 1.3725 0.7548 1.0087 1.6471 0.7756 1.0015 1.3061 1.1371 1.2160 0.9106 0.7836

330 340 362 363 382 393 395 397 401 408 413 436 442 462 480 482 486 495 502 508 509 510 512 529 531 531 557 566

1.4242

454 Mean

0.5349

Image-

grant hate head tend tempt fade treat sift rent grunt lift wait shout melt pat mend float glide greet knead hunt chat wound twist sprint plant skate roast

68.70 SD

1.6411 1.9618 1.6694 2.1147 1.2712 1.5544 1.9927 0.5975 1.4622 0.9610 1.9443 2.5036 1.9234 1.3857 1.2262 0.9252 1.5134 0.8107 1.4936 0.4622 1.4572 1.0663 1.3936 1.5888 0.4364 1.4770 0.4274 0.9637

329 336 340 340 345 383 389 397 405 409 414 440 455 461 473 483 486 487 494 500 502 508 512 521 523 542 562 569

1.3652

450

0.5329

72.85

35

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Ullman. (2001). The declarative/procedural model of lexicon and grammar. Journal of Psycholinguistic Research, 30, 37-69. Ullman, M. T. (1999). Acceptability ratings of regular and irregular past-tense forms: evidence for a dual-system model of language from word frequency and phonological neighbourhood effects. Language and Cognitive Processes, 14, 47-67. Ullman, M. T., Corkin, S., Coppola, M., Hickok, G., Growdon, J. H., Koroshetz, W. J., & Pinker, S. (1997). A neural dissociation within language: Evidence that the mental dictionary is part of declarative memory, and that grammatical rules are processed by the procedural system. Journal of Cognitive Neuroscience, 9, 266-276. Ullman, M. T., & Gopnik, M. (1999). Inflectional morphology in a family with inherited specific language impairment. Applied Psycholinguistics, 20, 51-117. Ullman, M. T., Izvorski, R., Love, T., Yee, E., Swinney, D. A., & Hickok, G. (in press). Neural correlates of lexicon and grammar: evidence from the production, reading, and judgment of inflection in aphasia. Brain and Language.

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Figure Captions 1. Proportion of correct responses in screening tasks of past tense production 2. Extent of irregular > regular advantage in the two sets of screens across all three tasks 3. Proportion of correct responses in past tense production of CV controlled sets of verbs 4. Distribution of correct responses and error types in sentence completion (Ullman et al. items) 5. Distribution of correct responses and error types in reading (Ullman et al. items) 6. Structural MRI of patient GN: transverse and coronal views 7. Same/different performance on pair matched items: note that each proportion is out of a total of 76, because the 38 pairs in each condition were presented twice – once in each order (e.g. cress before and after crest) 8. Proportion of correct same/different judgements by item type

9. Spectrographs of he, heed and heat

40

Fig.1 sentence completion screens 1

proportion correct

0.8

0.6 irregular regular 0.4

0.2

0 BB

IJ

IB

DE

AB

MB

RT

VC

GN

JL

repetition screens 1

proportion correct

0.8

0.6 irregular regular 0.4

0.2

0 BB

IJ

IB

DE

AB

MB

RT

VC

GN

JL

reading screens 1

proportion correct

0.8

0.6 irregular regular 0.4

0.2

0 BB

IJ

IB

DE

AB

MB

RT

VC

GN

JL

41

Fig.2 sentence completion 0.8

mean proportion correct

0.7 0.6 0.5 Ullman et al. set

0.4

Bird et al. set

0.3 0.2 0.1 0 irregular

regular

repetition 0.8

mean proportion correct

0.7 0.6 0.5 Ullman et al. set

0.4

Bird et al. set

0.3 0.2 0.1 0 irregular

regular

reading 0.8

mean proportion correct

0.7 0.6 0.5 Ullman et al. set

0.4

Bird et al. set

0.3 0.2 0.1 0 irregular

regular

42

Fig. 3 sentence completion with CV structure controlled 1

proportion correct

0.8

0.6 irregular regular

0.4

0.2

0 BB

IJ

IB

DE

AB

MB

RT

GN

JL

repetition with CV structure controlled 1

proportion correct

0.8

0.6 irregular regular

0.4

0.2

0 BB

IJ

IB

DE

AB

MB

RT

VC

GN

JL

reading with CV structure controlled 1

proportion correct

0.8

0.6 irregular regular

0.4

0.2

0 BB

IJ

IB

DE

AB

MB

RT

VC

GN

JL

43

Fig. 4 FCL

proportion of responses

1 0.8 0.6

irregular regular

0.4 0.2 0 correct

unmarked

morphological

other

RBA

proportion of responses

1 0.8 0.6

irregular regular

0.4 0.2 0 correct

unmarked

morphological

other

Mean across patient group in present study

proportion of responses

1 0.8 0.6

irregular regular

0.4 0.2 0 correct

unmarked

morphological

other

44

Fig. 5

Ullman patients

proportion of responses

1 0.8 0.6

irregular regular

0.4 0.2 0 correct

unmarked

morphological

other

Present study patients

proportion of responses

1 0.8 0.6

irregular regular

0.4 0.2 0 correct

unmarked

morphological

other

45

Fig. 6

R

L

R

L

46

Fig. 7

same/different performance on pair matched items

proportion correct

1

0.8

0.6

regular present/ past tense verbs e.g.press/pressed

0.4

phonologically matched nonmorphological pairs e.g. cress/crest

0.2

0 BB

IJ

IB

DE

AB

MB

VC

GN

RT

JL

patient

47

Fig. 8

proportion correct for 'different' pairs

1

0.8

0.6

0.4

0.2

0 press/pressed, cress/crest

own/owned, an/and

show/showed, he/heed

he/heat

heed/heat

an/ant

48

Fig. 9

49

Table 1: Patient details

Patient

Age

Sex

Aetiology / CT report

BB

60

F

1996 L CVA during operation for R acoustic neuroma

IJ

61

M

1995 low attenuation L frontal, temporal & parietal lobes, MCA ischaemic infarct

IB

53

F

1990 subarachnoid haemorrhage from L MCA aneurysm & infarct

DE

64

M

1997 non-haemorrhagic infarct L MCA territory

AB

61

F

1996 embolic CVA L MCA territory

MB

83

F

1998 non-haemorrhagic L frontoparietal infarct (5 months previously 2 small R frontal infarcts)

RT

58

M

1987 L CVA

VC

65

F

1998 low attenuation L frontal & parietal lobes, ischaemic infarct

GN

72

M

1999 L frontal infarct in MCA territory, also established low density lesion in L occipital lobe

JL

46

F

1996 L CVA involving temporal, parietal and frontal lobe

50

Table 2: General language assessments

Patient

Digit span spoken

Spontaneous speech:

(pointing)

words per minute

BB

1 (2)

36

IJ

2 (2)

IB

TROG /20 Reading /80

Naming

Word-picture

PPT

/64

matching /64

/52

10 14 (HI 12, LI 2)

7

64

48

9

5 15 (HI 13, LI 2)

21

63

49

2 (3)

22

9 32 (HI 27, LI 5)

33

62

49

DE

3 (2)

10

6 33 (HI 20, LI 13)

34

64

50

AB

3 (3)

5

8 32 (HI 29, LI 3)

42

62

47

MB

3 (3)

27

10 28 (HI 17, LI 11)

35

60

47

RT

3 (4)

39

15 76 (HI 32, LI 24)

33

63

43

VC

4 (4)

18

7 59 (HI 32, LI 27)

42

56

43

GN

4 (4)

27

10 40 (HI 29, LI 11)

48

60

45

JL

5 (5)

47

19 72 (HI 39, LI 33)

54

64

51

(HI /40 LI /40)

51

Table 3: Phonological assessments

Patient

Rhyme judgement /48

Rhyme production /24

Segmentation /96

BB

14

0

unable

IJ

23

10

3

IB

22

0

unable

DE

18

2

unable

AB

22

7

unable

MB

16

0

unable

RT

48

7

22

VC

23

4

14

GN

21

8

34

JL

48

16

61

52

Table 4: Results of regression analyses Coefficient Unstandardised Task

Independent variable

Sentence completion

Phonemes

Repetition

Reading

Standardised

B

SE

Beta

t

-.375

.066

-.350

-5.686**

Initial consonants

-1.058

.177

-.330

-5.984**

Final consonants

-1.391

.157

-4.466

-8.852**

Frequency of past tense

.466

.133

.192

3.494**

Frequency of past tense

.991

.144

.433

6.874**

-.695

.174

-.230

-3.989**

Imageability

.003

.001

.133

2.172*

Homograph

.841

.245

.190

3.433**

Initial consonants

* p < .05 ** p < .01

53

Table 5: Results of same/different judgement task

Mean age (range, SD)

Proportion correct (SD) Same

Different

Control subjects

64.0 (53-83, 8.8)

0.97 (0.02)

0.99 (0.01)

Patients

62.3 (46-83, 10.1)

0.91 (0.07)

0.62 (0.16)

54

Table 6: Proportion correct responses obtained from SD patients in sentence completion task AN

EK

AT

WM

MK

Mean

High Frequency Regular Verbs (N=63)

1.0

1.0

.97

1.0

.94

.98

Low Frequency Regular verbs (N=63)

.98

.92

.97

1.0

.87

.95

High Frequency Irregular verbs (N=63)

.95

.94

.86

.89

.33

.79

Low Frequency Irregular verbs (N=63)

.83

.60

.48

.32

.16

.48

Regular verbs matched for CV structure (N=34)

1.0

1.0

1.0

1.0

.88

.98

Irregular verbs matched for CV structure (N=34)

.97

.74

.53

.59

.09

.58

55