Children with limited ability to keep auditory information in mind (auditory working memory) may appear to have "ADD" although they really have limited space to keep auditory information 'in mind'. The study below shows the extra demands on auditory working memory when sentence structure becomes more complex. Children with this difficulty present with attention problems in upper elementary, middle, or high school because the language demands of classroom lectures have increased. Often these children can improve their performance dramatically with Teachers Notes and being allowed to bring textbooks home.
Neural Correlates of Syntactic Ambiguity in Sentence Comprehension for Low and High Span Readers -- Fiebach et al. 16 (9): 1562 -- The Journal of Cognitive Neuroscience