The possibility to do research on electrical brain potentials preceding voluntary movements came with the advent of the 'computer of average transients,' the first, primitive instrument available at that time in the Freiburg laboratory. In the electroencephalogram little is to be seen preceding actions, except of an inconstant diminution of the α- (or μ-) rhythm. The young researchers stored the electroencephalogram and electromyogram of self-initiated movements (fast finger flexions) on tape and analyzed the cerebral potentials preceding movements time-reversed with the start of the movement as the trigger, literally turning the tape over for analysis since they had no reversal playback or programmable computer. A potential preceding human voluntary movement was discovered and published in the same year (Kornhuber & Deecke 1964). After detailed investigation and control experiments such as passive finger movements the Citation Classic with the term Bereitschaftspotential was published.
Wang, Yijun; Zhang, Zhiguang; Li, Yong; Gao, Xiaorong; Gao, Shangkai; Yang, Fusheng BCI competition 2003-data set IV:An algorithm based on CSSD and FDA for classifying single-trial EEG. IEEE Transactions on Biomedical Engineering. Vol. 51, no. 6, pp. 1081-1086. June 2004.
The BP is ten to hundred times smaller than the α-rhythm of the EEG; only by averaging, relating the electrical potentials to the onset of the movement it becomes apparent. Figure shows the typical slow shifts of the cortical DC potential, called Bereitschaftspotential, preceding volitional, rapid flexions of the right index finger. The vertical line indicates the instant of triggering t = 0 (first activity in the EMG of the agonist muscle). Recording positions are left precentral (L prec, C3), right precentral (R prec, C4), mid-parietal (Pz); these are unipolar recordings with linked ears as reference. The difference between the BP in C3 and in C4 is displayed in the lowest graph (L/R prec). Superimposed are the results of eight experiments as obtained in the same subject (B.L.) on different days.
Note that the BP has two components, the early one (BP1) lasting from about –1.2 to –0.5; the late component (BP2) from 0.5 to shortly before 0 sec [From Deecke et al. 1976] The pre-motion positivity is even smaller, and the motor-potential which starts about fifty to sixty milliseconds before the onset of movement and has its maximum over the contralateral precentral hand area is still smaller. Thus, it takes great care to see these potentials: exact triggering by the real onset of movement is important, which is especially difficult preceding speech movements. Furthermore artefacts due to head-, eye-, lid-, mouth-movements and respiration have to be eliminated before averaging because such artefacts may be of a magnitude which makes it difficult to render them negligible even after hundreds of sweeps (Grözinger et al. 1980). In the case of eye movements eye muscle potentials have to be distinguished from cerebral potentials. In some cases animal experiments were necessary to clarify the origin of potentials such as the R-wave. Therefore, it took many years until some of the other laboratories were able to confirm the details of Kornhuber & Deecke's results. In addition to the finger or eye movements as mentioned above, the BP has been recorded accompanying willful movements of the wrist, arm, shoulder, hip, knee, foot and toes, it was recorded prior to speaking, writing and also swallowing.
EEGs and EMGs are used in combination with Bayesian inference to construct Bayesian networks which attempt to predict general patterns of Motor Intent Neuron Action Potentials firing. Researchers attempting to develop non-intrusive brain-machine interfaces are interested in this, as are system analysis, operations research, and epistemology (e.g. the Smith predictor has been suggested in the discussion).
The BP method has been extensively employed. In studies using conscious experience by means of introspection and the BP, a temporal order of events has been worked out in that the BP started about 0.35 sec earlier than the subject's conscious awareness that 'now he or she feels the desire to make a movement.' Libet concludes that we have no free will in the initiation of our movements, while in movement control we have, since subjects in his experiments were able to successfully pose a last minute 'veto' to the intended movement, i.e. Libet links will totally to consciousness. However, it has to be taken into account that in our brain we have conscious and unconscious matters and both are important. Furthermore, consciousness always has a delay in both the sensory system (e.g. awareness of pain) and in the motor system. In experiments like this, the question of free will cannot be tackled. The free will question has been answered already before the experiment, when the subject has agreed to the experimenter's general instruction. Thus, the free will question is solved once and not with each of the 200 or so single movements necessary for averaging the BP. During the experiment, subjects are in an experimental 'set' making the movements pretty automatically. The frontal cortex that is competent for volition and will, delegates the execution of the movements to other brain structures, such as basal ganglia and primary motor cortex (MI). The supplementary motor area (SMA) as the generator for the early BP (BP1) belongs to the frontal lobe. According to Libet's experiments, BP1 is not yet accompanied by consciousness. When the information has travelled from the SMA via the basal ganglia (motor loop) to the primary motor cortex MI (late BP, BP2), consciousness obviously is switched on in order to make changes and adaptations to the instantaneous situation with the extreme of vetoing it entirely, the most important reason for needing consciousness is to learn from the movement (see below 'further reading').
CNV is a wide and prolonged negative potential recorded during simple warned reaction time paradigms from central and parietal scalp sites. The Bereitschaftspotential or readiness potential is a slow potential which precedes a voluntary movement, and was reported for the first time by Kornhuber and Deecke. Its scalp distribution is fairly wide, always begins bilaterally, symmetrically at the midline of the precentral-parietal regions, about 1000–1500 ms before movement.
The clinical application of CNV is for the evaluation of the correlation of potential changes with changes in cognitive functions occurring in various diseases. Numerous studies have confirmed the applicability of CNV on the diagnosis of dementia, Parkinson's disease, epilepsy, schizophrenia, anxiety states, chronic pains, including migraine.
L.D. suggests that the CNV should constitute an own key word in Wikipedia 'Contingent negative variation.' It has been discovered and first reported by W. Grey Walter et al. in 1964 in Nature. William Grey Walter's biography is in Wikipedia, and nicely detailed. The biography also mentions the CNV and the Nature paper. The writer of this biography or any other of the CNV researchers should write the Contingent negative variation page.