Introduction

Memory - the ability to store and to recall events, actions and new information is clearly extremely important for all aspects of children’s development. Memory is the storage system for all information and new learning. The human memory system is not fully understood and researchers have described and divided memory in a number of ways, sometimes based on whether the memories are being stored for seconds rather than days (short-term versus long-term memory systems) and sometimes based on the type of information being learned or stored (implicit/explicit, procedural/declarative - implicit or procedural memories are usually memories for skills and habits such as riding a bicycle or playing the piano and explicit or declarative memories are the recall of episodes, events or facts, typically involving verbal recall).

Research into the memory abilities of individuals with Down syndrome has mainly focused on short-term memory. There has been very little research into the long-term memory abilities of individuals with Down syndrome. A limited number of studies suggest that their implicit memory functions better than their explicit memory. [@vicari_implicit_2001] This could be because explicit memory usually involves language to a greater extent than implicit memory. Observational reports from parents and teachers suggest that the long-term memory abilities of individuals with Down syndrome, for skills learned and for events recalled, are good. However, this is an area needing more research.

The majority of the research into the memory skills of individuals with Down syndrome has focused on short-term memory abilities and the learning of new tasks. Much of the most recent work published has been based on the working memory model of short-term memory [@baddeley_working_1974] [TODO: 3] and this work is the focus of this module because of its practical importance. The short-term memory and processing skills described as working memory have been shown to be important for learning to talk, for processing spoken language, for reading and mental arithmetic, and for the support of everyday tasks such as remembering a telephone number while dialling or a postcode while writing it.[TODO: see 4] Working memory measures are also associated with measures of general intelligence.[TODO: 5]

Aspects of the working memory system seem to be specifically impaired in children and adults with Down syndrome [@jarrold_down_1999] and these impairments could explain much of the difficulty that the children have in learning to talk, in general cognitive development and in learning in some classroom situations. It is possible that, conversely, over time, the speech and language difficulties of many children with Down syndrome limit the development of working memory capacities. It follows, then, that if it is possible to improve the functioning of working memory, this would have a major effect on the speech, language and cognitive difficulties associated with Down syndrome. For this reason, the working memory system is described in some detail in this module.

Drawing on research into the development of working memory in typically developing children, possible ways of improving basic memory function are discussed. Some of these have not yet been tried or evaluated. There have been a number of memory training studies with children, teenagers and adults with Down syndrome that have been published and the outcomes from these are discussed.

In the last section of this module, practical games and activities which may help memory development are described. It is not necessary to understand the research into working memory discussed in the next sections in order to use the practical activities, therefore some readers may wish to go straight to [the practical section].

The working memory system

The working memory system, as conceived by Baddeley and Hitch in Cambridge in 1974, [@baddeley_working_1974] [TODO: 3] is illustrated in [Figure 1]. This model identifies three components, a central executive - the part of the system which processes information, and two temporary stores which are specialised for holding information - the phonological loop for holding verbal information and the visuo-spatial scratch pad for holding visual and spatial information.

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Figure 1. The working memory model [@baddeley_working_1974]

The working memory model has been widely used and has stimulated much research into the development and nature of short-term memory and cognitive processing. Working memory can be thought of as a mental workspace [@gathercole_working_2000] in which information can be temporarily stored and manipulated during mental activities such as ‘listening to another speaker; decoding an unfamiliar word whilst holding the meaning of the previously decoded text in mind; and engaging in mental arithmetic’.[TODO: 9:p95] The working memory skills of an individual will, therefore, be very important for supporting such mental activities, for cognitive development and for learning.

In addition, the phonological loop is thought to play an important role in vocabulary learning and may also influence the learning of sentence structures and grammar in early childhood. [@gathercole_working_1993]

The working memory system can be thought of as a child or adult’s ‘gateway’ to the world. The working memory system is in use throughout an individual’s waking hours and the functioning of this system will influence an individual’s ability to make sense of all daily experiences, to understand and remember what is being said to them, to understand and remember what they see, to reason and to learn in the classroom.

The central executive

The central executive is a limited capacity system which is thought to be responsible for a range of cognitive (mental) activities, including the processing and storage of incoming information, the retrieval of long-term knowledge, the control of concurrent processing in the system, and the control of action and planning [@baddeley_working_1986] [TODO: 11] The central executive is the part of the system responsible for processing information, while the phonological loop and the visual-spatial scratchpad hold information in temporary store, making it available for processing. They have therefore been described as ‘slave’ systems. It is measures of central executive function which have been shown to predict children’s educational progress in the classroom[TODO: 8,9] and their general intelligence.[TODO: 5]

The phonological loop

The phonological loop is a limited capacity system which is specialised for the temporary storage of verbal information. Research indicates that verbal information is stored in the phonological loop in its spoken (or phonological) form [@baddeley_working_1986] [TODO: 4] and that from about 7 years of age children begin to rehearse the information (silently or aloud) to prevent it from being forgotten (i.e. repeating a telephone number while going to dial it; repeating a post code or the spelling of an unfamiliar name while writing it down). This rehearsal process is influenced by the speech rate - faster speakers can rehearse information faster, whether silently (sub-vocal rehearsal) or aloud.

The visuo-spatial scratchpad

The visuo-spatial scratchpad is a limited capacity system specialised for the temporary storage of visual and spatial information. It has not received as much research attention as the central executive or the phonological loop and is, therefore, less well understood. Recent evidence suggests that it may consist of two systems, one for visual and one for spatial information. [@gathercole_assessment_2000] However, tasks used to measure the capacity of this system to date have usually required the individual to remember visual items and their spatial locations.

Measuring working memory skills

The capacities of all the components of working memory increase during childhood, and if they do not do so at the expected rate, then this will limit the amount of information that a child or adult can store or process. The measures used to assess the function of each part of the system are described below. The measures described for the phonological loop (verbal short-term spans) and visuo-spatial scratchpad (visuo-spatial short-term spans) have been used in research with children with Down syndrome. To our knowledge, there have not yet been any studies of central executive function in children with Down syndrome.

Measuring verbal short-term memory

Figure 2. Examples of measures of verbal short-term memory (STM)

The capacity of the phonological loop is measured by digit span tasks. In a standard digit span task a child or adult is required to listen to a random list of digits spoken at the rate of one per second, and the individual’s digit span is the longest set of digits that they can recall accurately, in the order in which they were spoken. In childhood, digit spans typically increase from a digit span of 3 at 4-5 years of age to a digit span of 7-8 at 16 years. [@chi_age_1977]

The capacity of the phonological loop can also be measured with word lists. The purest measure of phonological loop function may be obtained by using a non-word repetition test designed by Sue Gathercole and colleagues in 1994. [@gathercole_childrens_1994] This test requires the child to repeat nonwords of varying lengths such as ‘nate’, ‘diller’ or ‘bannifer’ and as these words will not have been heard before, the ability to repeat them will not be influenced by familiarity with the words.

Measuring visuo-spatial memory

The capacity of the visuo-spatial scratchpad is measured by testing the individual’s ability to accurately recall patterns, for example, which squares are filled in a checkerboard or the order in which blocks on a board were touched (the Corsi blocks task) - [see ###### Figure 3.]

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Figure 3. Examples of measures of visuo-spatial short-term memory (STM)

Measuring central executive function

Type of short-term memory (STM)	Method	Examples of stimuli	Correct response
Working memory/executive processes	Listening span	Apples play football Cats have ears	"no" "yes"
	Counting span		"3, 5, 4"
	Backward digit span	8…3…5	"5…3…8"

Figure 4. Examples of measures of central executive function

Tasks used to measure central executive capacity usually require both storage and manipulation of information. One such task is the backward digit span task. In this task the child or adult is required to listen to digits as in the digit span task, but to repeat them in the reverse order from the order in which they were spoken. The individual has to be able to store them and reverse the order. Another task used to measure central executive function is a listening span task in which the child may be asked to decide if sentences are correct or not, then to recall the last words of the sentences (see [Figure 4]).

A test designed to measure all three aspects of working memory function for children is about to be published. [TODO: references 15]

Articles about memory

• Memory development for children with Down syndrome
• Intervention strategies