Steep early negative slopes can be demonstrated in pre-movement bereitschaftspotential

The study presents data suggesting that the classic bereitschaftspotential hides in its early component (BP1) steep increases of negativity which precede the movement at varying intervals in repeated trials.

In 12 volunteers, surface EEG from Fz, Cz, C3, C4, and Pz electrodes and EMG from the flexor digitorum communis were recorded during self-paced wrist flexions. Two hundred trials were collected from each subject. The single trials were grouped for averaging in two different ways. In the first one, single trials for each subject were used to create individual averages. In the second, all single trials were divided into groups according to the point in time of small steep negative shift from the baseline detected on sweeps before the movement and time group averages were created. The identification of small shifts was based on the comparison of calculated mean amplitudes in the first and the second half of the 1 s time window moved along the time axis.

The small negative shifts were identified in 97.2% of analyzed records. In each subject, their position on the time axis varied considerably. Individual averages exhibited the characteristics of classical readiness potential, i.e. slow early component, steep late component, laterality over motor cortices. On the other hand, all time group averages (26) displayed an early steep negative shift followed by plateau which, about 0.5 s before the movement, gave rise to the second, late steep negative shift. The slope values calculated in the definite segments of averaged curves were used as a measure of the amplitude of these shifts over various brain areas. MANOVA showed a significant effect of the electrode position both in the case of early slopes (F(4,115)=9.7; P<0.000) and in the case of late slopes (F(4,115)=22.5; P<0.000). In both cases, the largest value was demonstrated under Cz electrode. In contrast to the late slopes, the values of early slopes did not exhibit laterality and suggested greater importance of pre-frontal regions in their formation.

We have suggested that the formation of steep early negative shifts from the baseline in time group averages was due to synchronization of a mental process which, under classical averaging procedure, was dispersed uniformly throughout the pre-movement period.