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  • 1. Brain Research Bulletin 74 (2007) 1–13 Review Are regular and irregular verbs dissociated in non-fluent aphasia? A meta-analysis Yasmeen Faroqi-Shah∗ Department of Hearing and Speech Sciences, University of Maryland, 0100, Lefrak Hall, College Park, MD 20742, United States Received 10 February 2007; received in revised form 7 June 2007; accepted 12 June 2007 Available online 5 July 2007 Abstract The cognitive mechanisms and neuroantomical substrates used by the brain to effortlessly generate morphologically complex words (write + ing → writing) are little understood. The left inferior frontal gyrus (LIFG, including Broca’s area) is often implicated as being involved, although its specific role is unclear. Data from brain damaged individuals, particularly those with Broca’s aphasia, are often used as evidence to support or refute various theoretical perspectives. Typically, performance on two types of morphologically complex verbs, regulars (walk-walked, slip-slipped) and irregulars (sing-sang, sleep-slept) is contrasted for evidence of single or double dissociations. The question of how Broca’s aphasic individuals dissociate in their production of inflectional morphology is important to our understanding of how the brain is organized to compute morphologically complex words. This article is a synthesis of research studies investigating the production of morphologically complex regular and irregular verbs in individuals with Broca’s aphasia. The question being asked is if there is a robust and consistent dissociation, and if this dissociation can be tied to lesions of the left frontal lobe. This meta-analysis of 75 patients failed to show a single consistent dissociation pattern and over half the datasets had no significant difference between regulars and irregulars. There was also no relationship of any performance pattern to frontal lobe lesions, highlighting the difficulty of identifying any single neuroanatomical lesion for regular–irregular verb production deficits. The implications for various theoretical and neuroanatomical hypotheses are discussed. The role of neuropsychological dissociations in constraining hypothesis of normal neuroanatomical organization is evaluated. © 2007 Elsevier Inc. All rights reserved. Keywords: Morphology; Broca’s aphasia; Language production; Sentence completion; Repetition; Verb Contents 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Neuroanatomical predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2. Neuropsychological dissociations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3. Purpose of this meta-analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Coding and data analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Sentence production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Repetition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.3. Lesion information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.4. Inter-test consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1. Is there a dissociation between regular and irregular verbs? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Are specific lesions sites associated with specific dissociation patterns?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 ∗ Tel.: +1 301 405 4229; fax: +1 301 314 2023. E-mail address: yshah@hesp.umd.edu. 0361-9230/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.brainresbull.2007.06.007
  • 2. 2 Y. Faroqi-Shah / Brain Research Bulletin 74 (2007) 1–13 4.3. Can dissociations be manipulated by experimental variables? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5. Conclusions and future research considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Conflict of interest. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction Few questions about the nature and neural organization of the human language faculty have generated the kind of intense debate that word inflections have in the past two decades. The controversy over regular and irregular word inflec- tions, often called the “English past tense debate” [46], has spanned cognitive neuroscience, linguistics, cognitive psychol- ogy, neuropsychology, language development, and artificial intelligence alike. Many languages have two types of inflected words (although this dichotomy may be simplistic [9]): regu- lar inflections, such as past tense -ed in English (walk-walked, slip-slipped) and the participle -t in German (tanzen-getanzt [dance] [45]) and irregular inflections with non-default changes, such as sing-sang, sleep-slept in English, or the participle -n in German (trinken-getrunken [drink]). Differing opinions pre- vail regarding the nature of linguistic and cognitive differences between regular and irregular words. Some believe that compre- hending and producing regular inflections involves affixation (walk + ed → walked), while irregular inflections are retrieved from memory (dual mechanism account) [12,41,47,59]. Others believe that all inflections are retrieved from memory (single route, connectionistic accounts) [10,31,48], or are affixed (dis- tributed morphology) [26,54], or that there is a competition between both memory and affixation routes [2]. Linguistic and neurocognitive distinctions between regulars and irregulars have assumed enormous importance among lan- guage researchers because these succinctly embody a broader question about human language: whether we have two distinct language modules, a mental grammar and a mental lexicon. The mental grammar is a system of combinatorial rules that can be productively applied to generate sentences as well as complex words. Regular inflections are considered to be archetypes of these combinatorial rules. The mental lexicon is essentially a listing of all known words, including irregular inflections [46]. Recently, the question is not so much whether the two words differ in psycholinguistic aspects, but whether these differences have neuroanatomical validity. 1.1. Neuroanatomical predictions The Declarative/Procedural hypothesis first made distinct predictions about the neural correlates of regular and irreg- ular words [59]. Based on the assumption that grammatical combinatorial rules operate for the affixation of regulars, the left frontal cortex, particularly Broca’s area, was implicated in the processing and production of regular words. Basal ganglia circuits were also identified in this fronto-striatal procedural network. In contrast, irregular verbs were assumed to be housed in the mental lexicon and retrieved from declarative memory, and linked to the left temporo-parietal cortex (Fig. 1a). Func- tional neuroimaging studies have tested these neuroanatomical predictions using a variety of techniques, such as positron emission tomography, functional magnetic resonance imag- ing (FMRI), and magnetoencephalography. Some studies have found supportive evidence for the declarative/procedural model with greater left frontal activation for regulars and greater left temporal activity for irregulars [17,30,44,58]. However, other neuroimaging studies have found the reverse pattern [28], or no remarkable differences in the neural processing of reg- ulars and irregulars [49,54]. Hence, neuroimaging evidence for the declarative/procedural model has been inconclusive [5]. Fig. 1. Predictions of the various neuroanatomical models: (a) declarative- procedural model [59]; (b) phonology-semantics model [57]; (c) two-stage verb generation model [5], not shown are: left insula and hippocampus, also impli- cated for regular verbs and right DLPFC, also implicated for irregular verbs. See text (Section 1.1.) for details.
  • 3. Y. Faroqi-Shah / Brain Research Bulletin 74 (2007) 1–13 3 A more recent neuroanatomical hypothesis was proposed on the basis of auditory input processing deficits of regulars and irregulars [56,57] (hereafter called the semantics-phonology model). Examples of input processing tasks include deciding if auditorily presented stimuli are real words (pless, vaim) and same-different judgments with word pairs (sprayed-spray, trade-tray, taught-teach). The dorsal–ventral speech process- ing framework [27] was used to propose that the presence of regular inflectional affixes engages the dorsal (fronto-temporal) network to greater extent than irregular inflections [56]. This fronto-temporal network, which includes bilateral superior tem- poral gyri, left inferior frontal gyrus (LIFG), and left anterior cingulate, is implicated for “morphophonological parsing pro- cesses that segment morphologically complex spoken forms into stems and affixes” [56, p. 8380] (sprayed → spray + ed) (Fig. 1b). Explicit statements about the neuroanatomical cor- relates of irregulars are not made in earlier versions [56,57], although a recent version [42] allocates left posterior superior temporal gyrus and middle temporal gyrus for semantic access of irregular verbs and monomorphemic words. In other words, this model claims that phonological demands are greater for regulars while semantic access is more crucial for processing irregular verbs and monomorphemic words. An FMRI study using same-different word pair judgments by the same authors revealed that regulars and irregulars activated the same left supe- rior temporal–frontal network and only varied in the extent of activation (greater activity for regulars) [57]. Another recent data-driven proposal is based on FMRI acti- vation patterns for covert generation of Spanish regular and irregular verbs [5] (referred to as the two-stage verb genera- tion model hereafter). Both regular and irregular verbs activated the left opercular LIFG, regular verbs showed increased acti- vation of left anterior superior temporal gyrus-insula and hippocampus, while irregular verbs showed increased activa- tion bilaterally in the dorsolateral–prefrontal cortex (DLPFC). The authors proposed a two-stage model for the generation of verb inflections, the first stage is language and verb type spe- cific, and recruits the DLPFC for lexical access of irregulars, and grammatical–phonological network of left anterior superior temporal gyrus, insula and hippocampus for regular verbs. The second step is common to all verb types, involves retrieval of grammatical information, such as tense and recruits the LIFG (Fig. 1c). To summarize, the declarative/procedural model proposes left fronto-striatal involvement for all regulars, and left temporo- parietal involvement for all irregulars, irrespective of task demands [59]. The semantics-phonology model claims greater fronto-temporal (dorsal) activity for regulars compared to irregulars, but does not make any specific statements about neuroanatomical correlates for irregulars, or for production tasks [56,57]. Finally, the two-stage verb generation model [5] suggests that LIFG activity is common to both verb types, DLPFC is increasingly activated for irregulars and a temporal-insular-hippocampal network is used for regulars. These predictions are illustrated in Fig. 1. Neuroimaging studies have produced inconsistent results and are hence inconclusive regarding the neuroanatomical correlates of regulars and irregu- lars [16,17,28,30,49,54,56]. In the next section, another source of evidence for neural differences between regulars and irregu- lars is evaluated. 1.2. Neuropsychological dissociations A widely cited substantiation for the neuroanatomical pre- dictions of the Declarative/Procedural hypothesis comes from single and double dissociations in brain damaged populations [41,59]. Although a variety of conditions that predominantly affect left frontal–striatal structures [Broca’s aphasia, Parkin- son’s disease, Huntington’s disease] have been compared with conditions that predominantly affect posterior language areas (anomic aphasia, Wernicke’s aphasia, Alzheimer’s dementia) [58,29,60], Broca’s aphasia has received particular attention as the poster-child in the debate between regular and irregular inflections [8,21,41,43]. Broca’s aphasia typically results from focal brain damage to left hemisphere frontal regions [15] and is characterized by reduced speech rate, inaccurate grammar, and incorrect usage of verb inflections [50]. Using data from sin- gle case studies, it was originally proposed that Broca’s aphasic individuals, owing to frontal lobe damage, show dissociation between regulars and irregulars, with worse performance on regulars [41,59]. However, an accumulating body of evidence suggests that the relationship between regular verb impairment and Broca’s aphasia is not straightforward because some studies have found the reverse pattern of worse performance on irreg- ulars compared to regulars [8,6,45,29,43], while others have found no difference between regulars and irregulars [8,15], or a variety of patterns [22]. Task differences, language differences, and variations in the number of patients tested are only a few factors that have obscured the relation between regular verb production and Broca’s aphasia. For example, it can be questioned if disso- ciation patterns from cognitively diverse tasks, such as single word reading, lexical priming, and sentence completion, can be pooled together. Is it reasonable to assume that the putative neural differences between regulars and irregulars will surpass neuroanatomical differences from inherently dissimilar tasks, such as reading and sentence completion? Further, unlike irreg- ulars in English (swam, kept, etc.), irregulars in languages, such as Spanish and German contain non-default (albeit quasiregu- lar) affixes, and it is unclear if the brain makes the same binary distinction between regulars and irregulars in such languages [45]. In addition to the extensive methodological diversity across studies, there are conceptual and statistical concerns that cast doubt on the robustness of regular–irregular dissocia- tions in aphasic individuals [7,13].1 Neuroanatomical models of regular–irregular verb processing imply normal versus impaired performance on irregular and regular verbs, respectively, in indi- viduals with frontal lobe damage [41,59]. However, operational definitions of normal and impaired performance or what con- 1 These conceptual and statistical limitations apply to other areas of neuropsy- chological research as well [36].
  • 4. 4 Y. Faroqi-Shah / Brain Research Bulletin 74 (2007) 1–13 stitutes a dissociation are not provided in most studies, a point that has been made about most research on neuropsychologi- cal dissociations [14]. It has also been emphasized elsewhere that reliable inferences about normal neurocognitive architec- ture can be made from neuropsychological dissociations only upon statistical comparisons with a normal control group [14]. Most studies of regular–irregular dissociations in aphasia have used intra-individual comparisons, without reference to a con- trol group. These so called trend dissociations are considered to be weakest form of neuropsychological evidence [52,38]. Fur- ther, it has been demonstrated that tasks that are likely to produce ceiling effects in normal controls (100% accuracy) significantly inflate the occurrence of dissociations (high false positives-Type I error] in brain damaged individuals [38]. Finally, conclu- sions about neuroanatomical localization of function are limited when patients with similar lesions exhibit reverse dissociation patterns. In other words, neuropsychological dissociations are informative only if we can clearly demonstrate that the dissoci- ation exists. 1.3. Purpose of this meta-analysis In the presence of mixed evidence and methodological con- founds, it is unclear if a single dissociation pattern between regular and irregular verbs exists in Broca’s aphasia. This ques- tion is important because different neural regions are identified for regular–irregular processing by the declarative/procedural, two-stage verb generation, and auditory input models (Fig. 1), and it is unclear if existing data favor any one of these models. This scenario warrants a systematic synthesis of research find- ings to ascertain the weight of neuropsychological evidence. The question being evaluated in this meta-analysis is not whether regulars and irregulars can dissociate, rather if they consistently dissociate in a single direction (regulars worse than irregulars) in patients with Broca’s aphasia, and whether this dissociation is consistently associated with left frontal lobe lesions. A sec- ondary purpose was to explore variables that are likely to result in dissociations. Such a synthesis is likely to yield fruitful results if patient performance is compared across similar tasks. Since a majority of studies have used production tasks to compare reg- ular and irregular verbs in Broca’s aphasia, this meta-analysis focuses on two production tasks: sentence production and word repetition. 2. Materials and methods Published articles or abstracts that compared the production of regular and irregular verbs in English peer-reviewed journals were identified by searches through electronic databases. The electronic databases included Science Cita- tion Index, Medline (Pubmed), PsycInfo, and CINAHL. Combinations of the following keywords were used to conduct the search: aphasia, verbs, language production, morphology, dissociation, regular, and irregular. The search was restricted to research studies published between 1980 and October 2006. The search resulted in a total of 56 reports. The search was further narrowed down to 25 articles after reading the abstracts for relevance. Reports that duplicated data, such as conference presentations and full articles, were included only once. The entire text of these final 25 articles was read and the following inclusionary criteria were used: (1) the study reported original data obtained from patient(s) described as non-fluent, agrammatic or Broca’s aphasic by the author(s); (2) individual subject data were reported; (3) regular and irregular verb produc- tion was tested and the scores of regular and irregular verbs were separately reported2; (4) either sentence completion, and/or single word repetition tasks were used. Fifteen studies met these criteria. Of these, three studies reported both sentence production and word repetition data, 10 studies reported only sen- tence production data and two had only single word repetition. Multiple data sets from individual patients were included in the meta-analysis only if each data set was original to the study. 2.1. Coding and data analyses All studies were coded for a variety of variables including, language of testing, the number of stimuli used, raw scores, proportion accuracy, description of aphasia profile, description of lesion information, phonological and frequency matching of stimuli, and the overall conclusions of the authors. For sentence production studies, three task variations have been used, these were separately coded as A, B, and C. Task A referred to a fill in the blank task where the present tense verb was provided in a preceding context (Everyday I rob a bank. Yesterday I ), Task B corresponded to fill in the blank that required selection from multiple options (Yesterday I a bank: rob, robbing, robbed), and Task C elicited the sentence in response to a question (The man is stealing the silverware. What did he do yesterday?). For studies where raw data were not reported, these were computed from the relevant figures or percentage scores. The authors of one study provided raw scores via personal communication [8]. Raw scores were statistically compared using the Fisher-exact test since this test considers the number of stimuli in calculation of p-values [40,51]. The one-tailed p-values thus derived were used to code if performance of regulars was worse than irregulars since this was the predicted direction of dissociation for Broca’s aphasia [41,59]. 3. Results A total of 110 different datasets were obtained across all tasks from 75 patients. Nineteen individuals contributed more than one dataset and 16 patients contributed to both sentence pro- duction and word repetition data. A complete list of included studies and individual subject data are provided in Table 1 for sentence completion and Table 2 for word repetition. The find- ings of the two tasks are described in separate sections below. This is followed by the results of the lesion analysis. 3.1. Sentence production A total of 78 separate patient data sets, representing 66 dif- ferent aphasic patients were obtained for sentence production (Table 1). The number of data sets in which regular verbs were significantly lower in accuracy compared to irregular verbs (one- tailed p < 0.05) is 10 (9 unique patients). In 26 data sets (23 unique patients), regular verbs were significantly better than irregulars in accuracy. In the remaining 42 data sets, there were no significant differences between regular and irregular verbs. These proportions are summarized in Fig. 2. Overall, there was a high positive correlation between the accuracies of regulars and irregulars, when computed across all 78 datasets (rs = 0.64, p < 000). Other variables. Forty-nine (out of total 78) data sets came from English speakers (33 unique patients). Interestingly, 9 out of the 10 patients who had poorer accuracy for regulars were 2 Authors definitions of and decisions about what constitutes “regularity” were used for the purpose of this analysis.
  • 5. Y.Faroqi-Shah/BrainResearchBulletin74(2007)1–135 Table 1 Studies of sentence production sorted by language and chronological order Reference Language Task/stimuli Patient Lesion # Stimuli Accuracy p One-tail R < I? p < 0.05 R I R I Ullman et al., p. 271 (Table 3) [59] English B FCL L frontal, insula, white matter 20 20 0.20 0.69 0.00 Yes Bird et al., p. 507 (Fig. 1), p. 509 (Fig. 2) [8] English B F BB L 44 44 0.00 0.08 0.06 No IJ L 44 44 0.07 0.23 0.03 Yes IB L frontal temporal parietal 44 44 0.09 0.14 0.37 No DE L mca 44 44 0.20 0.43 0.02 Yes AB L mca 44 44 0.24 0.36 0.18 No MB L frontoparietal, R frontal 44 44 0.18 0.39 0.03 Yes RT L 44 44 0.07 0.34 0.00 Yes VC L frontoparietal 44 44 0.20 0.36 0.08 No GN L mca incl. Broca’s, occipital 44 44 0.28 0.51 0.02 Yes JL L frontal temporal parietal 44 44 0.69 0.84 0.07 No English B P F BB 34 34 0.18 0.12 0.85 No IJ 34 34 0.06 0.06 0.69 No IB 34 34 0.26 0.09 0.98 No DE 34 34 0.21 0.26 0.39 No AB 34 34 0.24 0.18 0.82 No MB 34 34 0.24 0.26 0.53 No RT 34 34 0.09 0.24 0.09 No GN 34 34 0.59 0.05 1.00 No JL 34 34 0.79 0.62 0.97 No Shapiro and Caramazza, p. 1193 [53] English B RC L frontal incl. Broca’s, insula, internal capsule, putamen 112 154 0.43 0.18 1.00 No B F RC 26 26 0.46 0.08 1.00 No Lambon Ralph et al., p. 110 (Table 4) [36] English B P F GD L 44 44 0.67 0.75 0.24 No PG L mca, parietal 44 44 0.54 0.29 1.09 No DC L parietal 44 44 0.83 0.75 0.91 No JS L 44 44 0.75 0.54 0.99 No DM L subarachanoid hemorrhage 44 44 0.96 0.67 1.00 No AB 44 44 0.25 0.33 0.24 No GN 44 44 0.33 0.63 0.01 Yes JL 44 44 0.71 0.83 0.10 No Ullman et al., p. 202 (Table 5) [60] English B (F) RBA L Broca’s, temporal isthmus 20 16 0.20 0.25 0.51 No Druks p. 1006 [19] English B F MC L 36 36 0.42 0.36 0.77 No Fix and Thompson, p. 167 (Table 1)* [22] English B F P1/B6 21 39 0.43 0.49 0.44 No P2 21 39 0.71 0.05 1.00 No P3/B8 21 39 0.71 0.31 1.00 No P5 21 39 0.29 0.62 0.02 Yes P6 21 39 0.60 0.16 1.00 No P7/B1 21 39 0.57 0.49 0.81 No P8 21 39 0.38 0.41 0.52 No Faroqi-Shah and Thompson (Fig. 2)* [21] English A F B1/P7 L mca 15 15 0.50 0.60 0.43 No B2 L mca 15 15 0.53 0.67 0.36 No B3 L mca 15 15 0.53 0.53 0.64 No B4 L mca 15 15 0.67 0.53 0.87 No
  • 6. 6Y.Faroqi-Shah/BrainResearchBulletin74(2007)1–13 Table 1 (Continued ) Reference Language Task/stimuli Patient Lesion # Stimuli Accuracy p One-tail R < I? p < 0.05 R I R I B5 L mca 15 15 0.40 0.53 0.36 No B6/P1 L mca 15 15 0.33 0.33 0.65 No B7 L mca 15 15 0.47 0.33 0.87 No B8/P3 L mca 15 15 0.47 0.93 0.01 Yes B9 L mca 15 15 0.40 0.53 0.43 No B10 L mca, basal ganglia 15 15 0.87 0.80 0.83 No Penke and Westerman, p. 568 (Fig. 2) [43] Dutch B MG L 67 51 1.00 0.60 1.00 No CO L frontal 30 30 1.00 0.72 1.00 No HU L dorsal mca 30 30 1.00 0.82 1.00 No Z 67 51 0.92 0.58 1.00 No KE L internal capsule 30 30 0.97 0.74 1.00 No AR L fronto temporal 30 30 0.93 0.74 0.99 No JW L 67 51 0.98 0.82 1.00 No KL L mca 30 30 0.97 0.90 0.94 No NI L mca 30 30 0.97 0.93 0.88 No AD L mca 67 51 0.92 0.91 0.67 No AN L 67 51 0.93 0.96 0.35 No RE 30 30 0.81 0.81 0.63 No Penke et al., p. 228 (Fig. 1) [45] German B MT L frontal area of mca 39 39 1.00 0.35 1.00 No PB L mca 39 39 1.00 0.50 1.00 No MB L frontal area of mca 39 39 1.00 0.65 1.00 No KH L mca 39 39 1.00 0.91 1.00 No WW L mca 39 39 0.99 0.90 1.00 No HR L mca 39 39 0.99 0.71 1.00 No JZ L mca 39 39 0.99 0.57 1.00 No AH L frontal area of mca 39 39 0.91 0.57 1.00 No GB L mca incl. Broca’s 39 39 0.85 0.80 0.81 No MJ L 39 39 0.89 0.98 0.18 No FW L 39 39 0.60 0.65 0.41 No Tsapkini et al., p. 276 [55] Greek C SK L temporal, internal capsule 11 8 0.27 0.625 0.14 No Laiacona and Caramazza, p. 116 (Fig. 5), p. 117 (Fig. 6) [35] Italian B MR Frontoparietal, ant. and sup. temporal 204 156 0.67 0.52 1.00 Yes Balaguer, p. 217–218, p. 220 (Fig. 4) [6]$ Spanish B JM L fronto temporal subcortical, Rt. periventricular 34 30 0.824 0.437 1.00 No MP L mca 28 25 0.90 0.524 1.00 No Catalan B JM 34 39 0.912 0.713 1.00 No MP 35 18 0.80 0.417 1.00 No Mean 0.59 0.51 Standard deviation 0.31 0.26 Standard error of mean 0.04 0.03 Abbreviations: A, fill blank sentence completion; ant., anterior; B, multiple choice sentence completion; C, response to question; F, frequency matching of regular–irregular; I = irregular; L, left hemisphere; MCA, middle cerebral artery; P, phonological matching of regular–irregulars (syllable structure); post., posterior; R, regular; R < I, accuracy of regulars is less than irregulars; Rt., right; sup., superior; $, past and present elicited; (*) three participants overlapped in these two studies and their corresponding codes are listed. Disclaimer: In some cases, p-values may vary marginally from those reported by authors since raw data were computed from figures.
  • 7. Y.Faroqi-Shah/BrainResearchBulletin74(2007)1–137 Table 2 Studies of word repetition sorted by language and chronological order Author/data location Language stimuli Patient Lesion # Stimuli Accuracy % p One-tail R<I? p < 0.05 R I R I Badeckar and Caramazza, p. 295 (Table 9) [3] English F FM L post. inf. frontal, inf. parietal, ant. temporal 50 50 0.56 0.74 0.00 Yes Kohn and Melvold, p. 337 [33] English BD L temporo-parietal, R lacunar 10 10 0.60 0.90 0.15 No JW L frontal–parietal temporal 10 10 0.50 0.90 0.07 No Tsapkini et al., p. 278 [55] Greek SK L temporal, internal capsule 43 43 1.00 0.98 1.00 No Bird et al., p. 507 (Fig. 1), p. 509 (Fig. 2) [8] English F BB L 44 44 0.13 0.32 0.03 Yes IJ L 44 44 0.15 0.35 0.04 Yes IB L frontal temporal parietal 44 44 0.56 0.82 0.00 Yes DE L mca 44 44 0.15 0.65 0.00 Yes AB L mca 44 44 0.58 0.82 0.01 Yes MB L frontoparietal, R frontal 44 44 0.08 0.20 0.00 Yes RT L 44 44 0.75 0.95 0.30 No VC L frontoparietal 44 44 0.74 0.82 0.01 Yes GN L frontal including Broca’s, occipital 44 44 0.76 0.94 0.00 Yes JL L frontal temporal parietal 44 44 0.66 0.96 0.11 No English P F BB 34 34 0.25 0.30 0.47 No IJ 34 34 0.25 0.18 0.87 No IB 34 34 0.90 0.89 0.79 No DE 34 34 0.59 0.50 0.83 No AB 34 34 0.70 0.80 0.29 No MB 34 34 0.20 0.25 0.41 No RT 34 34 0.91 0.91 0.66 No VC 34 34 0.91 0.85 0.87 No GN 34 34 0.81 0.85 0.50 No JL 34 34 0.98 0.92 0.94 No Lambon Ralph et al., p. 112 (Table 6) [36] English F AB L 34 34 0.85 0.97 0.10 No GN L mca, parietal 34 34 0.82 0.94 0.13 No JL 34 34 0.91 0.91 0.66 No GD L 34 34 0.97 0.97 0.75 No PG L subarachanoid hem. 34 34 0.97 0.94 0.88 No DC 34 34 1.00 0.97 1.00 No JS 34 34 0.94 0.88 0.90 No DM 34 34 1.00 0.91 1.00 No Mean 0.66 0.76 Standard deviation 0.30 0.26 Standard error of mean 0.05 0.04 Abbreviations: ant., anterior; F, frequency matching of regular–irregular; I = irregular; L, left hemisphere; mca, middle cerebral artery; P, phonological matching of regular–irregulars (syllable structure); post., posterior; R, regular; R < I, accuracy of regulars is less than irregulars; Rt., right; sup., superior. Disclaimer: In some cases, p-values may vary marginally from those reported by authors since raw data were computed from figures.
  • 8. 8 Y. Faroqi-Shah / Brain Research Bulletin 74 (2007) 1–13 Fig. 2. Meta-analysis of sentence completion data showing proportion of patients with: significantly lower accuracy of regulars compared to irregulars (R < I), irregulars compared to regulars (R > I), and no significant differences between accuracies of regulars and irregulars (R–I ns) (Fisher-exact test, one- tailed p < 0.05) (N = 78). English speakers. The number of English speakers who showed poorer accuracy for irregulars is also 9 (out of a total of 26 who showed this pattern). Seven (out of a total of 13) reports matched regulars and irregulars for frequency of occurrence, at least for a subset of the stimuli, and only two studies had matched phonological aspects of regular and irregular verbs, such as syllable structure. The number of stimuli used ranged from 8 to 204 [35,55]. To summarize, over half of the sen- tence production datasets are from English speakers. An equal number of English speaking patients showed opposite patterns (regulars < irregularsandirregular < regulars).Ifallpatientswho were worse on regulars were considered, a majority (9/10) were English speakers. Further, only half of the studies matched stim- uli for frequency and an even smaller number matched stimuli for phonological complexity. 3.2. Repetition Word repetition included 32 data sets from 26 different apha- sic patients (see Table 2). A majority of these data sets (N = 28) came from the same group of authors [8,36] whose focus was to demonstrate that variations in stimulus characteristics can influence the existence of a dissociation. Nine datasets showed significantly lower accuracy of regular verbs compared to irregu- larverbs.Theremaining23datasetshadnosignificantdifference between regulars and irregulars as shown in Fig. 3. No patients with significantly better performance on irregulars compared Fig. 3. Meta-analysis of repetition data showing proportion of patients with: significantly lower accuracy of regulars compared to irregulars (R < I), irregulars comparedtoregulars(R > I),andnosignificantdifferencesbetweenaccuraciesof regulars and irregulars (R–I ns) (Fisher-exact test, one-tailed p < 0.05) (N = 22). to regulars were reported. There was a high positive correla- tion between the accuracies of regulars and irregulars, when computed across all datasets (rs = 0.8, p < 000). Other variables. Word repetition data are primarily from English speakers, and only one of the 32 datasets came from a non-English speaker [55]. Four out of six reports had some frequency matching between regulars and irregular verb stim- uli, while only one study reported matching of phonological variables. 3.3. Lesion information The results of the lesion analysis are reported in Table 3 and Fig. 4. As is evident from Table 3, lesion data from only 31 patients could be entered into the analysis of dissociations because lesion information was either not reported (N = 9) or described in non-specific terms, such as left hemisphere or left middle cerebral artery (N = 35). Of particular interest to the neu- roanatomical hypotheses being evaluated are patients whose lesions were exclusively frontal (or frontoparietal) without any temporal lobe involvement and vice versa (N = 15 datasets). Table 3 reveals that no specific dissociation patterns were asso- ciated with either of these lesion sites. 3.4. Inter-test consistency As mentioned earlier, 19 individuals contributed more than one dataset. A comparison of regular–irregular performance Table 3 Regular–irregular verb performance for various lesions # Patients # Datasets R < I R > I R = I, ns Patients Frontal excl. temporal 9 12 5 6 1 FCL, MB, VC, RC, PG, CO, MT, AH, GN Temporal excl. frontal 2 3 3 SK, BD Frontal + parietal + temporal 5 12 3 9 FM, JW, IB, JL, MR Frontal + temporal 3 3 1 2 RBA, AR, JM Other (subcortical, subarachnoid, etc.) 12 Non-specific description (L mca) 35 No lesion information 9 P1-P7, RE, Z
  • 9. Y. Faroqi-Shah / Brain Research Bulletin 74 (2007) 1–13 9 Fig. 4. Proportion of patients with various lesion sites. across the different datasets of each of these patients revealed intra-individual variations in dissociation patterns. In 13 cases, the profile changed from regular < irregular to no significant dif- ference between regulars and irregulars. A majority of these patients were reported by the same group of authors whose primary focus was to demonstrate how dissociation patterns are vulnerable to stimulus manipulations [8,36]. A patient (B8/P3) who participated in studies by two different authors [21,22], was significantly worse on regulars in one study while being significantly worse on irregulars in another study (Table 1). These two studies used different task variations. The performance of five other patients remained unchanged across multiple datasets. To summarize, these 19 individuals who contributed multiple datasets, demonstrate poor inter- test consistency. This indicates that dissociation patterns are susceptible change across testing sessions and experimental variations. 4. Discussion This meta-analysis of published research on regular–irregular verb production in individuals with non-fluent/Broca’s aphasia was conducted to address three questions. (1) Is there a con- sistent pattern of dissociation between regulars and irregulars? (2) Are specific dissociation patterns associated with specific lesions? In particular, is the pattern of worse performance on regulars associated with frontal lobe lesions? More specifically, we compared the predictions of the declarative-procedural [59], semantic-phonology [57] and two-stage [5] models, and (3) is it possible to delineate any variables that influence dissocia- tion patterns? These questions are addressed in the following sections. 4.1. Is there a dissociation between regular and irregular verbs? The most evident aspect of the data presented is the great variability across patients in the production of regular and irreg- ular verbs, although all patients were described as individuals with non-fluent/Broca’s aphasia. Combined across the 110 data sets of sentence production and word repetition, all three pos- sible patterns were observed: no difference between regulars and irregulars (65 datasets), worse performance on irregulars (26 datasets) and worse performance on regulars (9 datasets). The most frequent pattern in both sentence production and word repetition tasks was that of no significant difference between regulars and irregulars (Figs. 2 and 3). It is also noteworthy that only five datasets out of a total of 110 had a difference of greater than 50% between regulars and irregulars (sentences: FCL, GN, MT, PB; repetition: DE). Among these five datasets, there was no consistent pattern. In other words, classical dissoci- ations, the strongest form of neuropsychological evidence [52], were hardly found. Moreover, a significantly high positive cor- relation was found between regular and irregular verb accuracy. The observed heterogeneity, predominant pattern of no differ- ence, and high positive correlation, argue against the presence of a clear and consistent pattern of dissociation between regulars and irregulars in Broca’s aphasia. It is evident that the declarative/procedural [59] and seman- tic/phonology [31,32,42,56,57] models, both of which predict worse performance on regular compared to irregular verbs in Broca’s aphasia, are not supported. The semantics/phonology [31,32] model, assumes that individuals with Broca’s aphasia have phonological deficits and hence difficulty with regular verb production. Since the presence of phonological deficits was not documented by most authors, it may be preliminary to reject its claims on the basis of lack of evidence. In a study of two fluent and two non-fluent aphasic individuals with con- firmed phonological deficits, repetition of regular verbs was worse than irregulars in all four patients [33]. In another study, controlling for phonological difficulty eliminated prior disso- ciations between regulars and irregulars in non-fluent aphasic patients [8]. Hence, it appears that phonological complexity is a stimulus artifact that needs to be controlled in future stud- ies of regular–irregular verb processing. Given that a majority of data sets found no regular–irregular differences, the data are best accommodated by viewpoints that assume more similarities rather than differences between regular and irregular verb pro- duction, such as the two-stage model [5,6,37]. The second stage of the two-stage model is assumed to be common to both irreg- ular and regular verbs by involving retrieval of morphosyntactic information, such as tense [5,6]. Further, this second stage is claimed to be processed by the LIFG, a brain area whose lesions are commonly associated with Broca’s aphasia. The neuroanatomical models that were developed on the idea that regulars are impaired in Broca’s aphasia have largely ignored the fact that some aspects of regular morphology are relatively spared in Broca’s aphasia [41,57]. For example, pro- gressive forms of verbs (kicking, sleeping), verbs that inflect to agree with the preceding noun (The cat sleeps versus The cats sleep), and plural noun morphology (dogs) are all regular and are produced with relatively high accuracy [23,24]. This means that models that make a binary distinction between regulars and irregulars while failing to incorporate the whole spectrum of regular morphological operations are inadequate. Any model of the neurocognitive differences between regular and irregular morphology is incomplete if it does not account for the relative differences among these various regular morphological forms [35,53].
  • 10. 10 Y. Faroqi-Shah / Brain Research Bulletin 74 (2007) 1–13 4.2. Are specific lesions sites associated with specific dissociation patterns? Not surprisingly, lesions of the left hemisphere were reported for all the participants. A variety of lesions, including cor- tical and subcortical regions, were reported. Given that the LIFG is frequently credited with the mental grammar and hence production of regular verbs [41,59], the most interest- ing group constitutes patients with exclusively frontal lobe (or frontal–parietal) lesions. We are making the liberal assump- tion that the lesion was inclusive of the LIFG in all patients with reported frontal lobe lesions. Considerable variability in regular–irregular performance is observed across the 17 patients who had some form of frontal lobe lesion (see Table 3), with approximately equal numbers of datasets showing regu- lars worse and better than irregulars. Hence, the data do not support a special connection between LIFG lesions and a reg- ular verb deficit. The three patients who had temporal lobe lesions with the sparing of frontal lobe showed no signifi- cant differences between regulars and irregulars. Recall that the declarative/procedural and semantic/phonology models predict poor performance on irregulars in temporal lobe lesions, while the two-stage model predicts poor regular verb performance for such patients. To summarize, there seems to be no evident cor- relation between any pattern of performance and specific frontal and temporal lesion sites. These complex lesion-deficit findings are actually con- sistent with neuroimaging studies of regular–irregular verbs that have found a wide variety of activation patterns [16,17,28,30,49,54,56]. No two-brain imaging studies agree on the regions activated for regular and irregular verbs, perhaps due to methodological variations. In other words, neuropsycholog- ical and neuroimaging data suggest that the neural generation of regulars and irregulars is far more complex than that pre- dicted by any current neuroanatomical model. One similarity across most brain imaging studies is the reported activation of frontal regions for both regular and irregular verbs. As for left frontal lobe involvement, the two-stage model [5] is the only neuroanatomical model that specifically implicates both regular and irregular verb processing for this region. Hence, one may conclude (though not without reservations) that the finding of high correlation between regular and irregular verb scores in Broca’s aphasia fits best with the two-stage model. A cautionary note about the present lesion analysis is that less than half of all patients could contribute to the lesion anal- ysis because lesion data in other studies were not reported in terms that were specific enough for inclusion. It is quite possi- ble that the null result may be because the lesion descriptions are too vague and non-specific to reveal any correlations. Perhaps this question needs to be addressed using a more fine-grained approach, such as voxel-based lesion analysis [56]. 4.3. Can dissociations be manipulated by experimental variables? At least three distinct points can be made regarding this question. First, a non-trivial finding of this meta-analysis was that a majority of patients who contributed more than one dataset showed variations in the relative performance of reg- ulars and irregulars across the datasets (Section 3.4). This intra-individual variability questions the robustness of previ- ously reported “regular–irregular dissociations” and highlights the importance of demonstrating test–retest consistency in neu- ropsychological data, especially when such data are used to frame theories about the normal neural processing of regular and irregular verbs [56,59]. Secondly, the data suggest that at least some reported disso- ciations may have been an artifact of stimulus manipulations. This point is especially made by a few studies in which apparent dissociations between regular and irregular verb pro- duction were eliminated when verbs with matched phonological complexity (the number of syllables, the presence of con- sonant clusters) were used [8,36]. It was emphasized that, because a majority of regularly inflected verbs are phonolog- ically complex in English, failure to control for phonological complexity disproportionately affects the production of regular verbs. Phonological factors seem to influence the occurrence of regular–irregular dissociations in another manner. Aphasic individuals with phonological deficits have a greater difficulty producing regular verb inflections in English, even when they do not have Broca’s aphasia [33]. Hence, the pattern of worse per- formance on regulars is not invariably linked to Broca’s aphasia, but may be influenced by phonological deficits [57] and stimulus characteristics [8,36]. There may also be an interaction between modality (reading versus repetition) for phonological variables. In a previous study, matching phonological complexity in a read- ing task (links versus lynx) still showed a worse performance for the morphologically complex word (links) [3,4]. A variety of frequency measures including affix frequency [11] lexeme fre- quency [20], and stem frequency [45] have also been shown to influence accuracy of verb morphology. As Tables 1 and 2 show, a majority of the studies did not control frequency mea- sures between regular and irregular stimuli, thereby weakening the findings. Task variations are another factor that need careful consideration as is evident with patient B6/P1 [21,22] in Table 1 and also in other studies [8,34]. Finally, the reviewed studies had at least two methodolog- ical weaknesses that are likely to inflate the occurrence of dissociations: use of experimental tasks that produced ceiling performance in unimpaired controls, and the failure to use a matched control group [13,14,38,52]. 5. Conclusions and future research considerations The following statements can be made on the basis of the 110 datasets reviewed: (1) there was no consistent pattern of disso- ciation between regular and irregular verbs in persons described as non-fluent or Broca’s aphasia, at least for sentence and word repetition tasks reviewed in this meta-analysis3; (2) the most fre- quent pattern was that of no difference in performance; (3) the 3 The semantics/phonology model is primarily based on auditory same differ- ent discrimination and lexical priming [56].
  • 11. Y. Faroqi-Shah / Brain Research Bulletin 74 (2007) 1–13 11 magnitude of difference between regular and irregular produc- tion accuracy was minimal, with only five individuals showing a difference of greater than 50%; (4) a wide variety of lesions were reported; (5) lesion sites were not associated with specific performance patterns in any consistent manner; and (6) inter-test variability was found in nearly two-thirds of patients who had multiple datasets. The absence of a consistent dissociation and lesion-deficit correlation can be interpreted in different ways. First, the results may be taken at face value to mean that there is indeed no dissociation between regulars and irregulars in Broca’s apha- sia. Regular and irregular verbs may fail to dissociate because a complex network of multiple, overlapping brain regions are involved in producing both verb types, and this was demon- strated by brain imaging research [16,17,28,30,32,49,54,56]. Alternatively, these verbs may fail to dissociate because the psycholinguistic dichotomy between regulars and irregulars assumed by aphasiologists is overly simplistic [26,54,22,19]. Arguments for a gradation between regulars and irregulars include the presence of subregularities among irregulars (sang, rang, drank versus slept, kept, wept) [9], affix-like patterns in some irregulars (kept → kep + t) [22,26], presence of affixes in irregular verbs of other languages [6,43], and “morphologi- cally simple” behavior of high frequency regulars [1]. As some authors have previously pointed out, regular and irregular past tense verbs have more commonalities (morphosemantic notion of past tense, syntactic structure of the sentence, experimen- tal task demands, etc.) than the one morphological difference [5,6,37]. As for the three neuroanatomical viewpoints presented earlier, the existing data are incompatible with the declarative- procedural model [41], but fairly consistent with the two-stage model [5]. The two-stage model assumes some degree of neu- roanatomical overlap between regulars and irregulars and hence the failure of regulars and irregulars to completely dissoci- ate. Some aspects of the results may be consistent with the semantic-phonology model, especially if one makes the con- nection between regulars and phonological skills rather than between regulars and frontal lobe. Another neuroanatomical model that is a viable alternative is the memory, unification, control (MUC) framework [25], which allocates the role of unification to the LIFG. Unification is defined as combining linguistic elements into larger units and it is recognized that uni- fication operations occur in parallel in semantic, syntactic, and phonological domains. Although the author does not explicitly talk about morphology, it can be seen how both regulars and irregulars involve the same semantic and syntactic unification operations, differing only in morphological unification. There- fore LIFG lesions would impair both regular and irregular verb “unification” operations.4 There are several important considerations for future research. First, the analysis underlines the relevance of metic- ulous control and documentation of confounding variables, 4 In its present form, the MUC framework [61] is drawn from comprehension data. number of stimuli, task demands, inter-session consistency, and associated deficits when using neuropsychological data to for- mulate theories of the neural organization of language. Secondly, a frequent logical fallacy when drawing interpre- tations about normal language function from studies of aphasic individuals is the over-simplified assumption that all Broca’s aphasic individuals have a lesion of the Broca’s area, and there- fore observed deficits reflect functions of a normal Broca’s area. A fine-grained documentation of the extent of lesion is war- ranted in every study that makes specific claims about normal neural architecture. There are previous instances when a study of Broca’s aphasic individuals is cited elsewhere as a study of patients with lesions of LIFG, even though the original authors did not report LIFG lesions (e.g. [5, p. 883]). In fact, only 50–60% of patients with lesions to Broca’s area have persistent Broca’s aphasia and about 15% of right-handed Broca’s aphasic individuals do not have lesions of Broca’s area [18]. Also given the heterogeneity of the definition of Broca’s area across studies [39], neuroanatomical models that are based on patient perfor- mance need to explicitly define their “Broca’s area” and provide fine-grained lesion information. Third, future models also need to incorporate a more com- plete range of morphological operations, such as noun plurals, subject verb agreement, and non-finite verbs. This is war- ranted because recent research suggests that the trouble in Broca’s aphasia may not be morphological regularity per se, but rather an effect of grammatical class [35,53], or mor- phosemantic aspects, such as tense marking [21,23,61]. Finally, language specific effects need to be considered since this meta- analysis revealed a high proportion of English speakers in the group that performed worse on regular verbs. Currently, only the two-stage model accommodates language specific patterns. It is noteworthy that recent neuroanatomical hypotheses are moving in the right direction and becoming increasingly sophis- ticated in their empirically driven descriptions of processes and neural regions involved in the generation of regular and irregular verbs [5,33,57]. To conclude, this meta-analysis does not claim that regular and irregular verbs cannot dissociate in non-fluent/Broca’s aphasia. Rather, the finding is that non- fluent/Broca’s aphasia is not associated with a consistent pattern or direction of dissociation. Conflict of interest None. Acknowledgements The author wishes to thank Rachel Mont, Adrianna Naim and Jennifer Maultasch for help with data coding. I also thank two anonymous reviewers for their helpful comments on an earlier version of this article. References [1] M. Alegre, P. Gordon, Frequency effects and the representational status of regular inflections, J. Mem. Lang. 40 (1999) 41–61.
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