The puzzle of working memory for sign language

Why is immediate-serial-recall (short-term memory) span consistently shorter for sign language than it is for speech? A new study by Boutla et al. shows that neither the length of signs, nor the formational similarity of signed digits, can account for the difference. Their results suggest instead that the answer lies in differences between the auditory and visual systems. At the same time, however, their results show that sign language and spoken language yield equivalent processing spans, suggesting that reliance on immediate-serial-recall measures in clinical and educational testing is misplaced.