Brain Waves

Evoked Responses

To obtain evoked or event-related responses, the EEG signal is averaged over many repeated presentations of the same stimulus. Repeated presentations are required because the activity in response to a particular event of interest or specific stimulus is difficult to measure with only one presentation due to all the background processing, or “noise,” occurring in the brain. For example, while listening to a music clip, one may also be thinking of memories associated with the music or noticing other features of the environment. Each of these activities will cause fluctuations in the EEG signal. However, each time the event of interest occurs, the evoked response to that event will be the same, but all of the other activity going on in the brain will likely be different. When all the responses to the event of interest are averaged, the other activity, being different every time the click occurs, tends to cancel itself out, while the response to the stimulus becomes clearly visible.

The evoked response will generally show positive or negative fluctuations in amplitude (size), which are called peaks, or, more formally, components. Components that occur at particular times after the stimulus and show particular amplitudes are argued to be associated with specific mental operations. For example, one commonly measured evoked response is called the “mismatch negativity,” or MMN. An MMN is elicited by first exposing a listener to repeating auditory stimuli (e.g., repeated tones of the same duration). Then, infrequently and unpredictably, some characteristic of the repeating stimulus [Page 153]is changed (e.g., the duration is unexpectedly lengthened). An MMN occurs approximately 100 to 200 milliseconds after the change and is thought to indicate preattentive brain responses to changes in auditory stimuli.

An electroencephalography machine detects a subject's brain waves at the Institute of Psychology of the University of Szeged, Hungary, October 24, 2008. In music, differences in gamma wave responses have been recorded for musicians compared to people without musical training.

Other components are frequently found in studies of music. The P600 (a positive peak occurring 600 milliseconds after the stimulus) and the early right anterior negativity (ERAN) are both elicited by unexpected musical syntax, such as an unusual chord in a harmonic chord progression. The P300 is a positive peak that occurs about 300 milliseconds after an unexpected stimulus and reflects brain response to violations of expectancy. The N400 is a negative component that occurs about 400 milliseconds after a stimulus and can be elicited by words that are incongruent with a particular musical context (e.g., hearing the word pleasant after a dissonant, strident musical context).

...