Sentry duty is a tough assignment. Most of the time there's nothing to see, and when a threat does pop up, it can be hard to spot. In some military studies, humans are shown to detect only 47 percent of visible dangers.
A project run by the Defense Advanced Research Projects Agency (DARPA) suggests that combining the abilities of human sentries with those of machine-vision systems could be a better way to identify danger. It also uses electroencephalography to identify spikes in brain activity that can correspond to subconscious recognition of an object.