Bioneurofeedback (also known as neurofeedback, electroencephalography (EEG) biofeedback, or EEG operant conditioning) exploits a simple learning rule: the operant modification of signals acquired from the brain of a participant or patient. Although, advances in technology allow for more sophisticated forms of neurofeedback than was possible earlier, the basic principle has not changed over the past 40 years: A signal is acquired from the participant’s brain in the form of a recorded EEG, relevant aspects of this
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signal are extracted (e.g., power in a distinct frequency band) and fed back to the participant in real time. As soon as the signal reaches a predefined target, the participant is rewarded. It is important to note that this principle is agnostic to both the signal and the reward used.
Furthermore, there are no assumptions about the direct behavioral relevance of the signal for the patient, as, for instance, there is no direct link between a certain group of cortical oscillations and a particular disorder the patient might suffer from. Moreover, changes of the respective signals normally do not have an immediate relevance. Thus, it is vital for every successful neurofeedback training approach to increase the behavioral relevance of the signal for the patient (e.g., by choosing an appropriate reward). A further aspect to be emphasized is that the participant cannot be aware of the acquired signal without the help of a feedback, which leads to the ultimate goal of any neurofeedback approach: learning via operant modification to control a signal - putatively reflecting a distinct brain state - which is normally beyond the individual’s awareness and thereby uncontrollable.
Following the seminal work of Miller (1), demonstrating that autonomic functions can be modified through operant conditioning, Sterman and Friar not only showed that it is possible to use operant conditioning to increase sensorimotor rhythms but also that this modification lead to a decrease in the amount of seizures experienced by an epileptic patient (2). Similar encouraging results were found four years later for attention deficit hyperactive disorder (ADHD) (3), training enhancement in the alpha and reduction in the theta band. These patients successfully learned to control their EEG oscillations and to modify them into the desired direction and the ADHD symptoms improved on thirteen behavioral categories like
"Out-Of-Seat-Behavior’ and ‘sustained attention". A worsening of the symptoms was reported when the contingency of the training was reversed, resulting in a reward for decreasing alpha and increasing theta oscillations.
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These studies present important results, but they were based on single cases. Controlled studies involving groups of patients as well as control groups and / or treatments were needed to confirm these results. One of the first controlled studies concerning epilepsy and neurofeedback was conducted in 1993 (4). Twenty-five patients suffering from epilepsy learned to control their “Slow Cortical Potentials” (SCP), an event-related component indexing neuronal excitability. One year after the training 13 of 18 patients reported a significant decrease in seizure incidence. In this study, patients not only learned to reduce the SCPs, leading to lower excitability and thereby preventing seizures. The protocol involved training both directions, thus teaching the patients to actually control this aspect of their brain waves in a more complete manner. Trying to achieve a transfer from laboratory experience to real life situations, transfer trials and distraction were introduced. Transfer trials are trials in the same neurofeedback setting as used during the ‘real’ training, but without any feedback provided for the patient. Hence, the participants should be enabled to incorporate the strategy learned during the training and transfer it into their everyday routine. Distraction is used to further enhance the transfer to everyday life, as the patient is required to apply the strategy in situations outside the laboratory with some kind of distraction, such as background noise. Transfer trials and distraction ought to be considered as an important aspect in modern neurofeedback therapy.
Several controlled studies have now demonstrated promising effects of neurofeedback on epilepsy, ADHD, and other disorders such as depression (for a review see (5)).
Independent of the type of disorder, neurofeedback training always involves prior identification of an abnormal recordable signal pattern differentiating patients from healthy controls (e.g., a significant increase or decrease of power in distinct frequency bands). A
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further challenge is the demand for almost instantaneous feedback, excluding several signal-processing algorithms such as averaging data from numerous trials. Identification of abnormal signals can either be achieved by controlled studies or by using QEEG (Quantitative EEG). QEEG compresses EEG-signals acquired and processed with a standardized setting with databases obtained from either healthy individuals or patients exhibiting abnormal oscillatory activity in their EEG due to a certain defined condition. Thus, significant deviations from the standard EEG-recordings can be found and used for neurofeedback trainings (5).
The design of neurofeedback training may be regarded to be independent from the disease or the signal to be trained. It always involves the acquisition of the signal using appropriate devices which is then further processed using either proprietary software bound to the specific equipment or freely available software like ConSole (6). Although EEG signals are usually utilized for neurofeedback, today other signal sources, such as fMRI, are used as well.
The software then reduces the information of the signal to an essential minimum which is then made visible and/or audible to the patient. A common example of such a neurofeedback cue is an object moving from the left to the right side on a computer screen, whereas the information of the signal is represented by the height of the symbol on the screen. If the participant in the study is able to reach a pre-defined target (e.g., to "move the symbol" above a certain height), he/she receives a reward, which can be positive visual feedback (e.g., smiley face) appearing on the screen or, in some cases, monetary compensation as well.
Treatment of subjective tinnitus by means of neurofeedback is a relatively new application.
In the next section, we will give a short review of the identified abnormal spontaneous EEG
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patterns, followed by an overview of current neurofeedback approaches pursued in our laboratory.