Dissociation of functional categories in the syntax of Persian speaking deaf individuals

An interesting area of research in deaf studies concerns the idea that various language components and particularly different functional categories such as tense, mood, and agreement are not impaired to the same extent. The present study aimed to explore the performance of Persian speaking deaf individuals on tests dealing with five functional categories, namely complementizer/Wh-words, tense, aspect, mood, and agreement.
This research was a cross-sectional study with two groups, first of which included 11 (4 boys and 7 girls) profoundly deaf students with hearing loss above 90 dB for both ears, aged between 14 and 22; and second group of 15 students with normal hearing with mean (SD) age of 14 (2) years. In addition to interviews, we also conducted sentence-completion and grammaticality judgment tasks to explore their performance in each category.
The deaf group performed significantly worse than hearing group in all the tests. Our results also demonstrated a significant numerical gap between all five categories in the deaf group, beginning from the lowest least impaired category, which is in agreement, and ending up to the most impaired category, that is, complementizer.
We found a dissociation of functional categories in deaf individuals. Also, higher nodes are more vulnerable to impairments than lower nodes.