Somewhere around three hundred milliseconds after something rare happens, the brain produces a positive deflection known as the P300: the electrical trace of noticing. It does not mark the stimulus itself, but the moment it was evaluated as relevant.
The examination below follows the oddball paradigm. Most of what you will see is ordinary. Rarely, it is not. Respond only to the rare event. Your reaction time will be used to extrapolate how long it takes you to recognise that something matters.
The observer does not care how you feel about the result. Only how long it took.