Alzheimer's dementia produces a loss of discrimination but no increase in rate of memory decay in delayed matching to sample.

Patients diagnosed with senile dementia of the Alzheimer type (SDAT) were compared with control subjects on a computerized delayed matching to sample task. Performance was assessed with a measure of discriminability at zero delay and a measure of rate of forgetting--these are the two parameters of an exponential function derived from extensive animal testing. The SDAT group showed significantly poorer discriminability at zero delay than controls but equivalent rates of forgetting over a 32-sec delay. These data suggest that SDAT may have little effect on the decay rate of the short term (or 'primary') memory trace and may instead affect encoding, initial storage or retrieval mechanisms. The effects of SDAT on this task are consistent with previous results with anticholinergic agents in rats.