Short-term memory binding deficits in Alzheimers disease

Alzheimers disease impairs long term memories for related events (e.g. faces with names) more than for single events (e.g. list of faces or names). Whether or not this associative or binding deficit is also found in short-term memory has not yet been explored. In two experiments we investigated binding deficits in verbal short-term memory in Alzheimers disease. Experiment 1 : 23 patients with Alzheimers disease and 23 age and education matched healthy elderly were recruited. Participants studied visual arrays of objects (six for healthy elderly and four for Alzheimers disease patients), colours (six for healthy elderly and four for Alzheimers disease patients), unbound objects and colours (three for healthy elderly and two for Alzheimers disease patients in each of the two categories), or objects bound with colours (three for healthy elderly and two for Alzheimers disease patients). They were then asked to recall the items verbally. The memory of patients with Alzheimers disease for objects bound with colours was significantly worse than for single or unbound features whereas healthy elderlys memory for bound and unbound features did not differ. Experiment 2 : 21 Alzheimers disease patients and 20 matched healthy elderly were recruited. Memory load was increased for the healthy elderly group to eight items in the conditions assessing memory for single or unbound features and to four items in the condition assessing memory for the binding of these features. For Alzheimers disease patients the task remained the same. This manipulation permitted the performance to be equated across groups in the conditions assessing memory for single or unbound features. The impairment in Alzheimers disease patients in recalling bound objects reported in Experiment 1 was replicated. The binding cost was greater than that observed in the healthy elderly group, who did not differ in their performance for bound and unbound features. Alzheimers disease grossly impairs the mechanisms responsible for holding integrated objects in verbal short-term memory.