Worldwide, 100,000 people have electrical implants in their brains to treat the involuntary movements associated with Parkinson’s disease, and scientists are experimenting with the technique for depression and other disorders.

But today’s so-called deep brain stimulation only treats — it does not monitor its own effectiveness, partly because complex ailments like depression do not have defined biological signatures.

The federal Defense Advanced Research Projects Agency, known as Darpa, announced Thursday that it intended to spend more than $70 million over five years to jump to the next level of brain implants, either by improving deep brain stimulation or by developing new technology.

Justin Sanchez, Darpa program manager, said that for scientists now, “there is no technology that can acquire signals that can tell them precisely what is going on with the brain.”