Differential rates of forgetting from long-term memory in Alzheimer's and multi-infarct dementia

In this study we explored the rate of forgetting from long-term memory in Alzheimer's (AD) and multi-infarct (MID) dementia. For this purpose, we administered to 15 AD, 15 MID, and 22 control subjects two tasks exploring, respectively, long-term verbal and long-term visuo-spatial memory. The absolute rate of forgetting in both tasks was computed as the difference between immediate and delayed recall of memorandum. Since level of immediate recall was significantly different between groups, a proportional rate of forgetting (percentage of memorandum lost passing from immediate to delayed recall) was computed for each patient. In the verbal task (Rey's 15 words) AD patients displayed significantly larger absolute and proportional rates of forgetting than MID and control subjects. In the spatial task (Corsi block supraspan), the absolute rate of forgetting was only marginally different between groups. Nevertheless, AD patients demonstrated a larger proportional rate of forgetting than MID and normal subjects. These results point out an exalted decay of information from long-term memory store in AD patients. In the light of previous data (Corkin et al., 1984; Kopelman, 1985) we propose that long-term memory deficits in AD is due, at least in part, to an abnormal forgetting of information within the first few minutes following acquisition. Information still present in the subsequent period (10 min to several days) is retained normally. The normal rate of forgetting in MID patients, further, suggests different mechanisms underlying memory disorders in vascular and degenerative dementias.