پردازش فعل و متن در بیماری پارکینسون

The aim of the present study was to investigate verb and context processing in 10 individuals with Parkinson's disease (PD) and matched controls. A self-paced stop making sense judgment task was employed where participants read a sentence preceded by a context which made the thematic role of the verb plausible or implausible. Participants were required to indicate whether the sentence ceased to make sense at any point by responding yes/no at each word. PD participants were less accurate than the control participants at detecting sentence anomalies based on verb selection restrictions and previously encountered contextual elements. However, further research is required to determine the precise nature of the grammatical processing disturbance associated with PD.

A growing body of research has indicated that sentence comprehension may be impaired in individuals with Parkinson's Disease (PD) (Grossman, 1999, Grossman et al., 1992, Lieberman et al., 1992 and Ullman et al., 1997). One of the earliest studies to investigate sentence processing in PD was conducted by Natsopoulos et al. (1991) who utilised a sentence-picture matching paradigm to investigate thematic role assignment within relative clause constructions. The PD participants performed significantly poorer than normal control participants, suggesting that the PD participants were interpreting the sentences based on a restricted variety of superficial structural rules (Natsopoulos et al., 1991). Lieberman et al. (1992) employed a sentence-picture matching paradigm to investigate the comprehension of simple and complex sentences in participants with mild and moderate PD. Participants with moderate PD displayed higher error rates and longer response times than the mild PD participants, suggesting that syntactic comprehension in the PD participants was influenced by PD severity and sentence complexity. It was proposed that the comprehension difficulties reflected an impairment of syntactic rule application, rather than impaired attentional processes (Lieberman et al., 1992). Reduced attention span was discounted as a possible cause of the comprehension difficulties because short, syntactically complex sentences were associated with more errors and longer response times than longer, syntactically simple sentences. Furthermore, when sentence length was constant, error rates were higher for complex sentences than simple sentences (Lieberman et al., 1992). However, the sentences containing the complex syntactic structures may have required greater attentional resources than the simple sentences (Kemmerer, 1999). Thus, the discrepancy between the sentences may have reflected impaired language processing and/or impaired attentional processes rather than impaired language processing alone. Congruent with Lieberman et al., 1992 and Grossman et al., 1992 found a discrepancy between PD participants' comprehension of simple and complex sentences during an off-line sentence comprehension task. The researchers found that the PD participants displayed increasing sentence comprehension difficulties with increasing sentence complexity (Grossman et al., 1992). However, unlike Lieberman et al., 1992 and Grossman et al., 1992 concluded that the effects of syntactic complexity were related to limited attentional resources, rather than impaired understanding of grammatical rules. To further investigate the nature of the sentence processing impairment associated with PD, Grossman et al. (1992) examined the ability of PD participants to detect grammatical errors. Participants were required to answer questions about sentence stimuli, with 70% of the sentences containing errors. The errors included omissions of grammatical morphemes, alterations in the shape of grammatical morphemes, and modified word orders. The ability of the PD participants to detect the errors varied as a function of error type, showing that the PD group was less sensitive than the control group at detecting omitted grammatical morphemes. However, the groups did not differ in the ability to detect word order errors. It was suggested that the selective nature of the PD group's ability to detect the errors reflected impaired attentional processes (Grossman et al., 1992). However, it was acknowledged that impaired attention may not have been the sole factor influencing sentence comprehension in the PD participants, suggesting that the impaired sentence comprehension reflected an interaction between grammatical complexity and limited attentional resources (Grossman et al., 1992). However, as the experiment involved a two-stage evaluation of each sentence, unnatural demands may have been placed on the participants' attentional resources. Subsequent research has found that the comprehension accuracy of individuals with PD is sensitive to the introduction of a secondary task (Grossman et al., 2000). The role of executive functions in impaired sentence comprehension in PD was further elucidated by Lee, Grossman, Morris, Stern, and Hurtig (2003) by means of a word detection task. Word detection was selected in order to minimise the cognitive demands required by performance of the experimental task, thus allowing participants to devote a maximal amount cognitive processing to sentence comprehension. The PD participants were less sensitive to phonetic errors embedded within unbound grammatical morphemes in all sentence types and to errors in content words within object relative sentences. The authors attributed the impaired error detection rates to resource limitations in specific executive functions, notably attention and lexical retrieval associated with processing grammatically complex sentences (Lee et al., 2003). In addition to findings of impaired sentence comprehension in PD, studies have found that verb learning may be impaired in PD. Grossman, Stern, Gollomp, Vernon, and Hurtig (1994) found that PD participants had greater difficulty than control participants when responding to probe questions about grammatical and semantic information associated with a newly acquired verb. An analysis of the PD participants' individual performances indicated that a small number of the PD participants had difficulty recalling any information about the new verb. In contrast, a larger number of the PD participants had difficulties specific to sentence judgments about the usage of the new verb. It was suggested that the selective difficulties with sentence judgment may have reflected an impairment specific to grammatical processing (Grossman et al., 1994). Additional research has found that individuals with PD may have difficulty integrating previously encountered discourse elements when answering questions about stories (Murray & Stout, 1999) and individual sentences (Grossman et al., 1992), and when resolving lexical ambiguities (Copland, Chenery, & Murdoch, 2001). Difficulty with the use of syntactic-thematic rules when mapping thematic roles onto constituents of grammatically complex sentences has also been associated with PD (Geyer & Grossman, 1994). Thus, research suggests that individuals with PD may have an impaired understanding of the relationships between the constituents of individual sentences and between neighbouring sentences. In summary, previous studies have found that sentence processing is impaired in individuals with PD (Grossman et al., 1992, Lieberman et al., 1992 and Natsopoulos et al., 1991). However, the underlying cause of the impairment is unclear. Limited attentional resources and an impaired understanding of grammatical rules have been suggested as possible origins of the impairment (Grossman et al., 1992 and Lieberman et al., 1992). However, as most of the studies employed offline experimental methodologies, the results may have reflected processes other than language processing (Lee et al., 2000 and Shapiro et al., 1998). Thus, further research involving online experimental procedures is necessary to investigate the grammatical processing abilities of individuals with PD. Current lexical storage theories propose that verb representations within the lexicon contain syntactic and semantic information. This information may include specifications about the type and number of arguments permitted by the verb (Boland et al., 1995, Friederici and Frisch, 2000 and Shapiro et al., 1998). The stored information may place restrictions upon the other words in the sentence. Constraint-based models of sentence comprehension propose that when a listener perceives a verb in a sentence, all of the associated argument structures are temporarily activated in the listener's lexicon (Boland et al., 1995 and Shapiro et al., 1998). The incoming arguments perceived after the verb are checked against the syntactic and semantic restrictional information stored with the verb (Boland et al., 1995 and Friederici and Frisch, 2000). Online sentence processing tasks may provide information about which argument structures are activated for particular verbs and about the syntactic and semantic information represented with the verbs. In addition to decisions about argument acceptability, research suggests that listeners concurrently assign thematic roles to the arguments. Current thematic role assignment theories propose that listeners automatically assign grammatical roles to as many sentence components as possible (Altmann, 1999). Thus, listeners build up an online representation of the sentence which includes syntactic and thematic information (Boland et al., 1995). Altmann (1999) suggested that during online sentence processing tasks, the object role in a sentence is assigned to a previously encountered discourse element when the listener encounters the main verb. Thus, it was proposed that discourse elements outside the target sentence influence thematic role assignment within the target sentence (Altmann, 1999). This proposal is particularly pertinent to the study of PD given that PD participants may have difficulties integrating previously encountered discourse elements (Copland et al., 2001, Grossman et al., 1992 and Murray and Stout, 1999). The purpose of the present study was to investigate grammatical processing, specifically verb and context processing, in individuals with PD using a self-paced online thematic role assignment task. In order to accomplish this, a modified version of a thematic role assignment study conducted by Altmann (1999) was employed. Even though the present study aimed to investigate grammatical processing, we acknowledge that the task tapping into thematic role assignment may have involved some semantic input in addition to grammatical processing. During Altmann's study, participants were presented with sets of three sentences. In each set, the first two sentences appeared in their entirety and established a context, while the final sentence appeared word by word. The participants were required to indicate whether the final sentence ceased to make sense at any point (Altmann, 1999). Altmann manipulated both the preceding context (antecedent/no antecedent) and the main verb in the final sentence (selecting/nonselecting) (Note: Some of the sentences from Altmann's study were modified to account for Australian English). The term antecedent referred to an entity in the context sentences which could have been assigned a thematic role by the main verb in the final sentence ( Altmann, 1999). Experimental items in Altmann's study involved either antecedent (e.g. ‘A car was driving downhill when it suddenly veered out of control. In its path were some pigeons and a row of posts. It injured/missed…’) or no antecedent conditions (e.g. ‘A car was driving downhill when it suddenly veered out of control. In its path were some dustbins and a row of posts. It injured/missed…’). The term selecting referred to the main verb in the final sentence which placed restrictions upon potential discourse antecedents ( Altmann, 1999). Experimental items in Altmann's study involved either selecting (e.g. ‘It injured several posts that came close to being destroyed’) or nonselecting conditions (e.g. ‘It missed several posts that came close to being destroyed’). Altmann found that there were more ‘no’ responses for verbs that were preceded by a context that did not introduce a plausible antecedent than for the verbs that were preceded by a context that introduced a plausible antecedent. Consistent with Altmann's study, both context and the main verb in the final sentence were manipulated in the present study. Given previous findings of impaired grammatical processing (Grossman et al., 1993 and Grossman et al., 2000), discourse comprehension problems (Copland et al., 2001 and Murray and Stout, 1999), and reduced sensitivity to sentence anomalies in individuals with PD (Grossman et al., 1992 and Grossman et al., 2000), the following hypotheses were tested. Firstly, it was hypothesised that the PD participants would produce less accurate responses and longer response latencies than the control participants when deciding that a sentence had ceased to make sense. It was predicted that PD participants would produce fewer ‘no’ responses and longer response latencies than the control participants when viewing sentences that had violated verb selection restrictions. In addition, it was predicted that the control participants would produce more ‘no’ responses than the PD participants at early word positions for anomalous sentences. Secondly, it was hypothesised that the PD participants would use context less than the control participants when making sentence plausibility judgments.

Despite the limitations of the present study in terms of subject numbers, there was evidence to suggest that grammatical processing abilities differ between individuals with PD and normal controls. In terms of similarities, both the control and PD participants were more likely to decide that a sentence did not make sense if it contained a selecting rather than a nonselecting verb. For both groups, the majority of ‘no’ responses occurred after the postverbal noun. However, the PD participants were less accurate than the control participants at detecting verb selection anomalies and at using previously encountered discourse antecedents when deciding that a sentence had ceased to make sense. Although this study provides an insight into sentence processing in PD as it unfolds, the nature of the task (i.e. metalinguistic judgment) makes it susceptible to other cognitive deficits present in PD, suggesting the need to further investigate the interaction between cognitive deficits and online sentence processing.