The brain can generate ERP’s in response to stimuli in any sensory system, or channel. In most ERP experiments, subjects are exposed to stimuli in the visual or auditory channels (or both). Visual stimuli, such as shapes or letters, can be presented on a computer screen; auditory stimuli, such as tones or clicks, can be presented through headphones. The subject may be asked to attend to some stimuli while ignoring others. Generally the subject is asked to make a behavioral response to a certain stimulus. For example, he or she may be asked to press a button whenever the stimulus is detected. Stimuli that require a response are called targets. In some tasks, the subject must attend or respond to a specific stimulus in one sensory channel while ignoring all stimuli in another sensory channel; the sensory channel containing the target stimulus is called the relevant channel.
