A person is the sum of their memories, so what happens when our personal histories can be deleted at will? That's the ethical dilemma facing researchers over at the University of California, San Diego, who have found that it's possible to delete and recover memories created in the minds of genetically engineered rats in the same way MIT scientists did with mice last year. In a slightly gruesome series of experiments, the rodents were given electric shocks while their neurons were bombarded by light pulses delivered by an optical fiber implanted into their brains. After a while, the shocks stopped, but whenever their brains were stimulated, the rats continued to feel fear, since they were drawing on memories.Subsequently, the team weakened the rats synapses and found that they could break the link between the feelings of fear and the optical stimulation. Then, to cap the process off, resumed the original series of pulses and reactivated the feelings of fear and apprehension without any additional electric shocks. The head of the project, Professor Robert Malinow, said that "We can form a memory, erase that memory and we can reactivate it, at will," but don't worry, he's not got any plans for world domination just yet. Instead, he's hoping that his findings will help to counteract the symptoms of Alzheimer's disease -- although we're going to keep a tinfoil hat on standby, just in case.