The number of studies that investigate verbal morphology is rather limited, but their results are sound. The interest focuses on the asymmetry between regular and irregular
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verbal morphology, in accordance to the hypothesis advanced by Ullman (2001) of a dual-system for language processing.
In Ullman’s (2001) model, language processing counts on two systems that replicate the dissociation between the declarative and the procedural memory: speakers count on a mental lexicon and on a mental grammar. The former belongs to the declarative system because it stores lexical entries; the latter is procedural in nature because it allows for the rule-driven derivation of different forms. In many languages, the verbal system offers the opportunity to test the status of the two components, because it includes both regular and irregular forms. According to Ullman (2001), the procedural system is responsible for producing regular forms, while irregular forms are retrieved from the mental lexicon, i.e. the declarative memory.
Walenski et al. (2009) test Italian PADs with a sentence completion task that elicits the production of regular and irregular present-tense and past-participle forms. The study aims at verifying the status of the procedural and declarative linguistic components in patients with Alzheimer.
Italian verbs distribute across three different inflectional classes. These can be recognized by the thematic vowels in the infinitive forms. Class I mainly includes regular verbs, which take the morpheme –are at the infinitive and –ato at the past-tense (ballare 'to dance' ballato). This class is still very productive, in the sense that neologisms assume its inflectional paradigm. Class II is characterized by the –ere marker at the infinitive. Many verbs in this class are irregular and change stem at the past (e.g. prendere 'to take' preso). Finally, Class III includes verbs that end in –ire at the infinitive. The class includes both regular (sentire 'to hear', sentito) and irregular past-tense forms (aprire, 'to open' aperto).
The sentence in (2) is an example of the experimental trials in use in Walenski et al. (2009):
(2) A Giovanni piace ballare il tango. Allora ieri Giovanni ha _______ il tango 'Giovanni likes dancing tango. So yesterday Giovanni has _______ tango.'
Results from the experimental task show that PADs are spared at producing the regular forms, while they are impaired at retrieving the irregular ones.
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In order to test patients’ ability to use their procedural knowledge productively and to recognize and exploit morphological information, the task also includes some novel verbs. Authors make up regular verbs for Class I (carlare) and irregular ones for Class II (schidere), depending on the thematic vowels (-are for the regular Class I and –ere for the irregular Class II). Patients are consistent in their behaviour in the sense that they manage to produce correct forms for novel regular verbs, while they are impaired with novel irregular verbs. In other words, patients recognize the target inflectional class by the thematic vowel in the infinitive and perform accordingly. They produce regular forms for novel verbs in –are, while they hesitate with novel verbs in –ere, thus recognizing that these belong to the irregular class. Their mistakes are crucially not of the regularization kind though. They try to produce irregular forms, but their guesses do not meet those provided by healthy controls8.
Patients’ ability to recognize and take advantage of morphological cues is further confirmed by a similar task conducted by Colombo, Fonti & Stracciari (2009). Authors design two different conditions for the task. In the first condition, verb production is elicited with infinitive forms as in (2); in the second condition, participants see the first person singular of the target verb (3).
(3) Io ballo tutti i giorni. Quindi ieri ho ________
'I dance everyday. So yesterday I have ______'
The first person singular form is characterized by a –o final morpheme, which does not allow for the recognition of the inflectional class, as the three verbal classes all share the morpheme –o for the first person singular. The performance of PADs in the second condition is poorer in comparison to the first condition, in which morphological cues about the inflectional paradigm are overtly realized on the infinitive. This means that patients’ impairment becomes more evident when no morphological cue is at hand. In contrast, if any morphological cue is available, they use it to perform accordingly.
Colombo et al. (2009) do not completely agree on the hypothesis of a strict dual system, though; they claim that the declarative and the procedural linguistic components rather
8 Target forms of novel irregular verbs are determined according to the intuitions of healthy speakers, based on phonological similarities with real irregular verbs.
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distribute along a continuum, and contribute to language processing according to variable ratios.
Cameli et al. (2005) present similar results from two bilingual English-French PADs. The pattern described above is replicated only for the first language though. In the weaker language (acquired as L2 during puberty and then used throughout adult life), the performance resembles the one on irregular verbs for the native language. This reveals that verbal forms of the L2 are all stored in the mental lexicon, and they are not produced by the mental grammar.
Fyndanis et al. (2013) investigate Greek verbal morphology with rather different results. Authors design sentence completion and grammaticality judgement tasks in order to sample the status of verbal morphology, with respect to agreement, tense and aspect.
The Greek verbal system is very rich and counts on morphological markers for the three categories. Greek patients perform with good accuracy on items that concern subject/verb agreement, while they make many mistakes on items that concern tense and, most of all, aspect. Fyndanis et al. (2013) deduce that agreement is spared because it only involves syntactic information. In contrast, tense and aspect correspond to interpretable features at LF, their implementation requires the integration of morphological information as well as the integration of discourse/semantic information, thus resulting in a higher level of complexity for PADs.
The data interpretation offered by Fyndanis et al. (2013) sounds rather plausible.
However, it is at odds with the results from the studies reviewed above, in which patients do not have a specific impairment at realizing past-tense forms, which entail both a tense feature (past) and an aspectual feature (perfective).
In my opinion, the asymmetry in results is due to differences in administration procedures. In the studies by Walenski et al. (2009) and by Colombo et al. (2009), the tasks in use include only one kind of manipulation at the time, such that participants see sentences with verbs at the infinitive form and have to produce the equivalent past form.
Authors assume that the task directly tackles verbal morphology, because it keeps interpretative and post-interpretative demands at a minimum. In contrast, Fyndanis et al.
(2013) test agreement, tense and aspect within the same task. Consequently, patients constantly have to shift the focus of their attention and to process sentences fully in order to provide a congruent answer. The execution of this kind of task is therefore much more
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demanding. Finally, as the authors themselves point out, tense and aspect are rather subjective information, because they depend on the speakers’ perspective and attitude towards the sentence content, which are rather difficult information to convey in out-of-the-blue sentences.
In sum, PADs seem to be spared at producing regular verbal morphology, while they are impaired with irregular verbal morphology. Data from studies with Italian speakers (Colombo et al., 2009; Walenski et al., 2009) show that their morphological competence is active and productive as they can actively use morphological cues and they inflect novel verbs accordingly. Together with studies on anomia (see above), results from production of irregular verbs further confirm that the mental lexicon of PADs is disrupted.
Finally, these conclusions hold true for verb production as a purely morphological phenomenon. When the task in use demands the integration of semantic information and interpretable features at LF, the overall performance of PADs is not accurate anymore, as suggested by the results in Fyndanis (2013).