The model rats start accumulating oligomeric intracellular Abeta in the cortex and the hippocampus as soon as 1 week postnatally, leading to the accumulation of extracellular Abeta and eventually the formation of neuritic plaques at the age of 6 months (in homozygous animals only), with progressive cognitive decline already apparent at the age of 3 months (prior to plaque formation). This evidence concerns the gene APP and Mental deterioration.