From Wired News, research into using brain-wave signatures for security access:

Their idea of utilizing brain-wave signatures as "pass-thoughts" is based on the premise that brain waves are unique to each individual. Even when thinking of the same thing, the brain's measurable electrical impulses vary slightly from person to person. Some researchers believe the difference might just be enough to create a system that allows you to log in with your thoughts.

A pass-thought could be anything from a snatch of song, the memory of your last birthday or even the image of your favorite painting. A more achievable alternative might present you with predetermined pictures, music or video clips, to which you would think "yes" or "no" while the machine monitors your brain activity.