There are lots of ways that testers can manipulate humans to degrade their performance, way way back my dad designed this sort of stuff for Army experiments. The best way is to have the subject really tired, that is about the same as getting them drunk. Then play loud startling noises at them while they try to perform the task. Flashing lights are also good. If you wanted to be really cruel you could add some physical discomfort, maybe have them hold some object that unexpectedly starts to vibrate...