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.