An investigation of time reference in production and comprehension in Thai speakers with agrammatic aphasia

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.