Background: It has been demonstrated that reference to the past is difficult for individuals with agrammatic aphasia, leading to the formulation of the PAst DIscourse LInking Hypothesis (PADILIH). Many of the previous studies have focused on Indo-European languages, in which time reference is expressed through verb inflection. The current study examined the PADILIH in Thai, a language that does not use verb inflection but instead uses aspectual markers to refer to time. Aims: We aimed to evaluate the pattern of impairment of time reference in Thai speakers with agrammatic aphasia, by investigating how grammatical reference to past, present, and future was processed. Methods and Procedures: A total of 15 Thai agrammatic speakers and 18 Thai non-brain-damaged (NBD) speakers participated in a sentence production task and an auditory sentence-to-picture matching comprehension task, both of which probed past, present, and future time reference. Outcomes and Results: While the NBD participants performed close to ceiling in both production and comprehension, the agrammatic speakers showed significantly more difficulty in conditions requiring reference to the future in both modalities. In production, however, the agrammatic speakers replaced the target future time reference construction with negation (a construction that can be used as an alternative means for future reference). When responses using negation were counted as correct, the individuals with agrammatic aphasia showed equal impairment across conditions. Conclusions: The results of this study were inconsistent with the PADILIH predictions: Thai agrammatic speakers experienced more vulnerability in reference to the future than the present and the past. This suggested that impairments of time reference may differ depending on the structure of the language. We hypothesized that the problems with producing future time reference in Thai may be influenced by the grammatical status of the future marker. In addition, the use of negation in place of the target word might have been because this negative construction reduces the processing load for Thai agrammatic speakers.

Background: Agrammatic speech is characterized by reduced speech rate, reduced utterance length and lack of grammatical complexity, with grammatical morphemes often omitted or substituted. At the word level, verbs have been argued to be particularly vulnerable: agrammatic speakers produce fewer verbs or use a less diverse range of verbs than unimpaired speakers and the use of finite verbs is compromised. However, this description is based on agrammatic narrative speech from languages that express Tense, Aspect and Agreement through verb inflection. To date, a few studies have described narrative speech in a language without verb inflection (e.g., Standard Indonesian). Thai is another language that does not use verb inflection but uses constructions with serial verbs. Another typical feature of Thai, which has not been investigated in agrammatic speakers, is the use of particles to express politeness, although agrammatic speakers of Standard Indonesian have an impaired use of (different) particles but produce the passive construction (which is a way to express politeness) to a normal extent. Aim: The current study aimed to characterize Thai agrammatic speech and to analyse the use of verbs and polite particles. Methods: Nine Thai agrammatic and nine non-brain-damaged (NBD) speakers participated in the study. Narrative speech was elicited in a semi-standardized interview and by picture description. First, the presence of general features of agrammatic speech were investigated in Thai: reduced speech rate and utterance length, and lack of grammatical complexity. This was followed by an in-depth analysis of verb and particle production. Results: As in other languages, Thai agrammatic speakers talked slowly and with short utterances. However, the use of embedded sentences was normal. They produced fewer verbs, more specifically, their production of the serial verb construction was seriously impaired. Remarkably, the use of polite particles was spared, in fact, the agrammatic speakers produced more of these particles than the NBD speakers, although this may have been influenced by the context. Conclusions: Thai agrammatic speech resembles that of other languages in terms of speech rate and utterance length. Interestingly, the specific features of Thai that were investigated, serial verbs and polite particles, both showed different patterns to normal, and merit further investigation in the future.

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