research

Memory problems are the central symptom of Alzheimer’s disease. Diagnosing Alzheimer’s disease on the basis of memory difficulties alone, however, is problematic. This is because memory difficulties are not specific to Alzheimer’s disease. They are also a common feature of healthy ageing and can be caused by a number of disorders.

Research by our group and colleagues has shown that people with Alzheimer’s disease have significant difficulties doing two things at once: ‘dual-tasking’. Healthy older adults do not demonstrate any difficulty dual-tasking. This dual-task impairment, therefore, may be specific to Alzheimer’s disease.

We are now trying to convert our research tools into a clinical assessment, and ensure that these results are sensitive and specific to Alzheimer’s disease. After this, we hope that this tool will be used by GPs, psychiatrists, psychologists and neurologists to assist diagnosis of Alzheimer’s disease and allow follow up studies to check whether treatments are effective.

what is dual-tasking?
It is thought that dual-tasking, the ability to do two things at once, is a product of the brain’s ability to coordinate. We understand this dual-task coordination to be part of ‘working memory’. In 1974, scientists Alan Baddeley and Graham Hitch proposed that short-term memory should be thought of as ‘working memory’, which can store and maintain a certain amount of information temporarily. Information may be phonological (sounds/language) or visuo-spatial (vision/space). Phonological information, such as a friend’s telephone number, is handled by the ‘phonological loop’ and visuo-spatial information, such as the route you would plan to take to go to your friend’s house, is handled by the ‘visuo-spatial sketchpad’.

Phonological and visuo-spatial information can be integrated to form a memory. This is done by the ‘episodic buffer’. These three ‘slave’ systems: the phonological loop, the visuo-spatial sketchpad and the episodic buffer, are co-ordinated by the ‘central executive’:

Baddeley’s (2000) Model of Working Memory

When we do two things at once, such as walking and talking, Baddeley’s (2000) model of working memory suggests that the central executive has to coordinate the phonological loop and visuo-spatial sketchpad slave systems for successful performance of these two concurrent activities. It is therefore proposed that central executive dysfunction can cause dual-task impairment, as seen in people with Alzheimer’s disease.

what happens to dual-tasking ability in alzheimer’s disease?
In 1986, Baddeley, Logie, Bressi, Della Sala and Spinnler reported that people with Alzheimer’s disease (AD) demonstrate a selective impairment in dual-tasking. They based their conclusions on studies comparing people with AD with healthy young and older adults, performing two tasks at once: a tracking task as well as an articulatory suppression, simple reaction time to tone or auditory digit span task. They found that when tracking was paired with simple reaction time or digit span task (but not articulatory suppression), the people with AD performed dramatically lower than the healthy young or older adults. This effect remained even when the difficulty of the tracking task, and the length of digit span were adjusted, so as to equate performance across the three groups when the tasks were performed alone. The group hypothesised that this effect was not seen when the tracking task was paired with the articulatory suppression task because it was not sufficiently demanding to require the participant to dual-task.

Some suggested, however, that the findings in the Baddeley et al. (1986) study could have been caused by information overload. Thus, Baddeley, Bressi, Della Sala, Logie and Spinnler (1991) did a follow-up study of the same participants, assessing their ability to dual task when the level of difficulty of the two tasks was adjusted. They suggested that should the cause of the impairment be due to overload, there should be greater difficulty across all tasks: single or dual, when task difficulty was increased. They found, however, that there was further decrement in performance in the dual-task condition only. Furthermore, they found that there was no tendency for more difficult single tasks to show greater sensitivity to the progression of the disease. 

These experiments and findings suggest that that AD features a specific impairment in the central executive. Further research has found that dual-task performance correlates with the presence of behavioural problems (Baddeley, Della Sala, Papagno & Spinnler, 1997b) and difficulties in people with AD performing everyday tasks that require dual-tasking, such as keeping track of conversations (Alberoni, Baddeley, Della Sala, Logie & Spinnler, 1992) or talking while walking (Cocchini, Della Sala, Logie, Pagani, Sacco & Spinnler, 2004).

how can this be developed into a clinical tool?
Currently, assessments of memory functioning are thought to be the most useful tests to detect AD. These tests are very sensitive to AD, but unfortunately as memory difficulties can be present in many other types of disorders and even in normal ageing, these tests are not specific to AD. This can lead to diagnostic uncertainty, worrying for both patient and significant others, or worse, incorrect diagnosis of AD. These tests, therefore, require further components to improve their specificity.

Previous research revealed that when single task difficulty was equated across groups, people with AD have difficulty dual-tasking, but younger and older healthy adults do not (Logie, Cocchini, Della Sala & Baddeley, 2004). Moreover, difficulty dual-tasking worsens as the disease progresses; whereas the ability to do either of the two tasks alone deteriorates much less dramatically (Baddeley, Bressi, Della Sala, Logie & Spinnler, 1991). Furthermore, this dual-tasking impairment has been found to exist across a range of different combinations of tasks (Logie et al., 2004). The assessment of dual-task ability, therefore, may be an excellent way of improving the specificity of AD diagnosis.

A paper and pencil version was developed, which has been piloted and found to generate the same results as the laboratory version (Baddeley, Della Sala, Gray, Papagno & Spinnler,1997a; Della Sala, Baddeley, Papagno & Spinnler, 1995), but this version is not yet ready for widespread clinical use. Firstly, a sensitive assessment procedure must be established; secondly, a larger study assessing group differences must be conducted; and thirdly its specificity across a wider range of disorders must be assessed. This current project, funded by the Alzheimer’s Society, aims to develop a valid and reliable test of dual-tasking ability, which is sensitive and specific to Alzheimer’s disease.

references

	Alberoni, M., Baddeley, A., Della Sala, S., Logie, R. H. & Spinnler, H. (1992). Keeping track of a conversation: Impairments in Alzheimer's disease. International Journal of Geriatric Psychiatry, 7, 639-646.
	Baddeley, A. D. & Hitch, G. J. (1974). Working memory. In G. H. Bower (Ed.), The psychology of learning and motivation (Vol. 8; pp 47 - 89). London: Academic Press.
	Baddeley, A. D, Bressi, S., Della Sala, S., Logie, R. H. & Spinnler, H. (1991). The decline in working memory in  Alzheimer’s disease: A longitudinal study. Brain, 114, 2521 – 2542.
	Baddeley, A., Della Sala, S., Gray, C., Papagno, C., & Spinnler, H. (1997a). Testing central executive functioning with a pencil-and-paper test. In P. Rabbitt (Ed.), Methodology of frontal and executive function (pp. 61 - 80). Hove: Psychology Press.
	Baddeley, A., Della Sala, S., Papagno, C., & Spinnler, H. (1997b). Dual-task performance in dysexecutive and nondysexecutive patients with a frontal lesion. Neuropsychology, 11, 187-194.
	Baddeley, A. D., Logie, R., Bressi, S., Della Sala, S., & Spinnler, H. (1986). Dementia and working memory. Quarterly Journal of Experimental Psychology, 38A, 603-618.
	Clare, L., Woods, B., Moniz-Cook, E., Orrell, M. & Spector, A. (2005). Cognitive rehabilitation interventions  targeting memory functioning in early-stage Alzheimer's disease and vascular dementia. Systematic review. Cochrane Library.
	Cocchini, G, Della Sala, S., Logie, R. H., Pagani, R., Sacco, L. & Spinnler, H. (2004). Dual task effects of walking while talking in alkzheimer disease. Revue Neurologique, 160, 74-80.
	Della Sala, S., Baddeley, A., Papagno, C. & Spinnler, H. (1995). Dual task paradigm: A means to examine the central executive. In J. Grafman, K. J. Holyoak, F. Boller (Eds.). Structure and functions of the human prefrontal cortex. Vol. 769. New York: Annals of the New York Academy of Sciences, pp. 161-171.
	Giacobini, E. (2000). Cholinesterase inhibitors stabilize Alzheimer’s disease. Annals of the New York Academy of Sciences, 920, 321 – 327.
	Logie, R.H., Cocchini, G., Della Sala, S. & Baddeley, A.D. (2004). Is there a specific executive capacity for dual task co-ordination? Evidence from Alzheimer’s Disease. Neuropsychology 18, 504-513.