What is the Bereitschaftspotential?

Abstract

Since discovery of the slow negative electroencephalographic (EEG) activity preceding self-initiated movement by Kornhuber and Deecke [Kornhuber HH, Deecke L. Hirnpotentialänderungen bei Willkurbewegungen und passiven Bewegungen des Menschen: Bereitschaftspotential und reafferente Potentiale. Pflugers Archiv 1965;284:1–17], various source localization techniques in normal subjects and epicortical recording in epilepsy patients have disclosed the generator mechanisms of each identifiable component of movement-related cortical potentials (MRCPs) to some extent. The initial slow segment of BP, called ‘early BP’ in this article, begins about 2 s before the movement onset in the pre-supplementary motor area (pre-SMA) with no site-specificity and in the SMA proper according to the somatotopic organization, and shortly thereafter in the lateral premotor cortex bilaterally with relatively clear somatotopy. About 400 ms before the movement onset, the steeper negative slope, called ‘late BP’ in this article (also referred to as NS′), occurs in the contralateral primary motor cortex (M1) and lateral premotor cortex with precise somatotopy. These two phases of BP are differentially influenced by various factors, especially by complexity of the movement which enhances only the late BP. Event-related desynchronization (ERD) of beta frequency EEG band before self-initiated movements shows a different temporospatial pattern from that of the BP, suggesting different neuronal mechanisms for the two. BP has been applied for investigating pathophysiology of various movement disorders. Volitional motor inhibition or muscle relaxation is preceded by BP quite similar to that preceding voluntary muscle contraction. Since BP of typical waveforms and temporospatial pattern does not occur before organic involuntary movements, BP is used for detecting the participation of the ‘voluntary motor system’ in the generation of apparently involuntary movements in patients with psychogenic movement disorders. In view of Libet et al.’s report [Libet B, Gleason CA, Wright EW, Pearl DK. Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential). The unconscious initiation of a freely voluntary act. Brain 1983;106:623–642] that the awareness of intention to move occurred much later than the onset of BP, the early BP might reflect, physiologically, slowly increasing cortical excitability and, behaviorally, subconscious readiness for the forthcoming movement. Whether the late BP reflects conscious preparation for intended movement or not remains to be clarified.

Introduction

Kornhuber and Deecke (1964) made the first report of electroencephalographic (EEG) activity preceding volitional movement in humans. Prior to that, Bates (1951) attempted to record the movement-related activity by photographic superimposition of multiple single-sweep EEG traces, but he could identify only the post-movement activity probably due to low signal-to-noise ratio. In the 1960’s, no computer software for making on-line back averaging was available. Therefore, Kornhuber and Deecke (1964) recorded EEG and electromyogram (EMG) simultaneously while the subjects were repeating movements at a self-paced rate, and stored all the data on magnetic tape. Then they made an off-line averaging of the EEG segment prior to the EMG onset by playing the tape backward. By using this chronologically reversed averaging technique, they successfully identified two components, one each before and after the EMG onset. Those were the Bereitschaftspotential (BP) or readiness potential (RP), and reafferente Potentiale (Kornhuber and Deecke, 1964, Kornhuber and Deecke, 1965). Later they found two more components just before the movement onset: pre-motion positivity (PMP) and motor potential (MP) (Deecke et al., 1969). Since then, a number of studies on the movement-related cortical potentials (MRCP) have been reported both in terms of physiological findings and clinical application, but the physiological significance of each identifiable component, among others that of BP, has not been fully clarified yet. Libet et al. (1983), by employing their novel technique in which the subjects were requested to remember the time of their actual awareness of intention to move by watching a clock, reported that the intention to move occurred much later than the onset of BP. Their report has brought up a continuing question as to the physiological implication of the BP (Klein, 2002, Eagleman, 2004). In 2003, a comprehensive book entirely devoted to “The Bereitschaftspotential” was published (Jahanshahi and Hallett, 2003). Since, in that book, each specific aspect of BP was discussed in detail, it seems now appropriate to integrate various aspects of the BP by further updating more recent findings with special emphasis placed on the information obtained by epicortical recording, the issue of voluntary motor inhibition, praxis movement, and the physiological implication of the BP.