The Defense Advanced Research Projects Agency (DARPA) is forging ahead with a four-year plan to build a sophisticated memory stimulator, as part of President Barack Obama's $100 million initiative to better understand the human brain.

The science has never been done before, and raises ethical questions about whether the human mind should be manipulated in the name of staving off war injuries or managing the aging brain.

Some say those who could benefit include the five million Americans with Alzheimer's disease and the nearly 300,000 US military men and women who have sustained traumatic brain injuries in Iraq and Afghanistan.