Demonstrating the effects of phonological similarity and frequency on item and order memory in Down syndrome using process dissociation.

It is important to distinguish between memory for item information and memory for order information when considering the nature of verbal short-term memory (vSTM) performance. Although other researchers have attempted to make this distinction between item and order memory in children, none has done so using process dissociation. This study shows that such an approach can be particularly useful and informative. Individuals with Down syndrome (DS) tend to experience a vSTM deficit. These two experiments explored whether phonological similarity (Experiment 1) and item frequency (Experiment 2) affected vSTM for item and order information in a group of individuals with DS compared with typically developing (TD) vocabulary-matched children. Process dissociation was used to obtain measures of item and order memory via Nairne and Kelley's procedure (Journal of Memory and Language, 50 (2004) 113-133). Those with DS were poorer than the matched TD group for recall of both item and order information. However, in both populations, phonologically similar items reduced order memory but enhanced item memory, whereas high-frequency items resulted in improvements in both item and order memory-effects that are in line with previous research in the adult literature. These results indicate that, despite poorer vSTM performance in DS, individuals experience phonological coding of verbal input and a contribution of long-term memory knowledge to recall. These findings inform routes for interventions for those with DS, highlighting the need to enhance both item and order memory. Moreover, this work demonstrates that process dissociation is applicable and informative for studying special populations and children.