Bereitschaftspotential

A pre-motor potential, the Bereitschaftspotential, BP (from german, "readiness potential"), is a manifestation of cortical contribution to the pre-motor planning of volitional movement. The BP was first recorded and reported in 1964 by Kornhuber & Deecke at the University of Freiburg in Germany. In 1965 the full publication appeared after many control experiments (Kornhuber & Deecke (1965)).

Discovery
In the spring of 1964 Hans Helmut Kornhuber (then docent and chief physician at the department of neurology, head Professor Richard Jung, university hospital Freiburg im Breisgau) and Lüder Deecke (his doctoral student) went for lunch to the 'Gasthaus zum Schwanen' at the foot of the Schlossberg hill. Sitting alone in the guest garden they discussed their frustration with research on the brain as a passive system prevailing worldwide and their desire to investigate self-initiated action (Kornhuber & Deecke 1990).

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 averager available at that time in the Freiburg laboratory. In the electroencephalogram little is to be seen preceding actions, except of an inconstant diminuition 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 (Kornhuber & Deecke 1965).

Mechanism
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 (Huckabee et al. 2003).

Outcomes
The Bereitschaftspotential was received with great interest by the scientific community as reflected by Sir John Eccles' comment: „There is a delightful parallel between these impressively simple experiments and the experiments of Galileo Galilei who investigated the laws of motion of the universe with metal balls on an inclined plane“ (Eccles & Zeier 1980). The interest was even greater in psychology and philosophy because volition is traditionally associated with human freedom (cf. Kornhuber 1984). The spirit of the time, however, was hostile to freedom in those years; it was believed that freedom is an illusion. The tradition of behaviourism and Freudism was deterministic. While will and volition were frequently leading concepts in psychological research papers before and after the first world war and even during the second war, after the end of the second world war this declined, and by the mid-sixties these key words completely disappeared and were abolished in the thesaurus of the American Psychological Association (Heckhausen 1987). The BP is an electrical sign of participation of the supplementary motor area (SMA) prior to volitional movement, which starts activity prior to the primary motor area (Deecke & Kornhuber, 1978). The BP has preciptated a worldwide discussion about free will (cf. the recent book "The Bereitschaftspotential", Jahanshahi & Hallett [eds.] 2003; incl. Deecke & Kornhuber 2003 [closing chapter]).

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.