Meta-analysis of explicit memory studies in populations with intellectual disability
Memory, which is defined as the capacity to acquire, retain and retrieve experiences and/or information, is no longer considered a unitary function. Functional dissociation in participants without brain damage and neuropsychological dissociation in braindamaged patients suggest that memory is composed of a series of functionally independent, but interacting, systems. Memory is not a unitary system. The short-term memory (STM), or working memory, is the system in which essential temporary information is stored for a short period of time. Some researchers claim that STM and longterm memory (LTM) represent the same system, but that the information found in the STM can be used under very special conditions, and it can almost never be stored for a long time (Craik and Lockhart 1972; Ranganath and Blumenfeld 2005). Others claim that these are two separate systems that act together in an integrative manner (Atkinson and Shiffrin 1971). Nonetheless, there is agreement that STM enables us to maintain or perform manipulations on a limited amount of information (see review in Baddeley ). LTM is the system in which information which we receive from the environment is stored for long periods of time. Information transferred to the LTM can also be forgotten, but at a much slower rate than the information in the STM. One of the fundamental dissociations in LTM is between explicit and implicit memory. Schacter and Buckner’s (1998, 284-285) definition of these two components of LTM is:
The present meta-analysis focuses on explicit memory. Forness and Kavale (1993) carried out a meta-analysis of 268 memory studies
among populations with mild to moderate intellectual disability (ID) that were performed between 1960 and the mid-1980s. Of those studies, 172 focused on memory performance and 96 on learning strategies for improving memory. Approximately 45% of the studies were carried out between 1961 and 1970, 50% between 1971 and 1980 and only 5% after 1980, indicating that interest in memory studies decreased. The findings indicated that ‘primary memory’ (STM) in populations with ID does not differ from that of their peers with typical development (TD). However, differences in secondary memory (LTM) were obvious. Forness and Kavale (1968) postulated that secondary memory has a more limited capacity among people with ID. Another possibility is that the mechanism which transfers items from the primary to the secondary memory is impaired. The aforementioned meta-analysis contributed to understanding memory processes in populations with ID. However, it only covered memory studies until the mid-1980s.