Processing of time reference in agrammatic speakers of Akan: a language with grammatical tone

Background: Languages of the world have several ways of expressing time reference. Many languages such as those in the Indo-European group express time reference through tense. Languages such as Chinese and Standard Indonesian express time reference through aspectual adverbs, while Akan does so through grammatical tone. Previous studies have found that time reference is selectively impaired, with reference to the past being more impaired than reference to the non-past. The PAst DIscourse LInking Hypothesis (PADILIH) posits that pastime reference is difficult because it requires discourse linking. Aims: The goal of this study was first to examine whether pastime reference is impaired also in languages that do not use grammatical affixes but rather tone, to make time reference. Second, this study aims to decouple the effect of tone from the effect of temporal reference on Akan verbs. Method and Procedures: Ten Akan agrammatic speakers and 10 non-brain-damaged speakers (NBDs) participated in this study. An Akan adapted version of the Test for Assessing Reference of Time (African TART), for both production and comprehension was used. The TART focuses on the future, present (habitual) and the pastime frames. Additionally, five of the agrammatic speakers performed two tonal discrimination tasks: a non-linguistic and a linguistic (lexical) one. Outcomes and Results: While the NBDs scored at ceiling, the agrammatic speakers made errors, and these affected past more than present and the future time references, in both comprehension and production tasks. However, the comprehension data showed a dissociation between the present habitual and the future. The substitution error analysis revealed a preference for the present. The five agrammatic speakers showed an intact performance on non-linguistic tonal discrimination task. Conclusion: The conclusion is that regardless of how time reference is expressed, whether through inflectional morphology or grammatical tone, reference to the past is problematic for individuals with agrammatic aphasia. The fact that the agrammatic speakers could perceive the non-linguistic tonal differences demonstrates that it is not tone in general that is disrupted, but rather time reference, particularly reference to the past, as predicted by the PADILIH.