Sensory Responses and the Expectancy Wave
The electrophysiological analysis of voluntary movement is based on evaluating changes in cerebral cortex activity. When a person passively perceives isolated stimuli (e.g., merely hears a click or sees a flash of light), sensory evoked potentials are recorded. These are short bursts of activity reflecting simple signal processing without goal formation or movement preparation (there is no slow wave between stimuli).
A completely different picture is observed when an auditory signal becomes a warning, and the subsequent visual signal requires performing an action (e.g., pressing a button). In the interval between these two events, an expectancy wave (W. Grey Walter's phenomenon), also known as contingent negative variation (Contingent Negative Variation, CNV) or the "E-wave" (Expectancy wave), forms in association areas (primarily the frontal cortex).
The dynamics of this process are as follows:
- Following the warning auditory signal, a sharp peak appears—the auditory evoked potential.
- Immediately afterward, the curve begins to rise smoothly, reflecting the slow buildup of a negative potential.
- The potential reaches its maximum strictly by the time of the trigger signal (light flash).
- After performing the required action (button press), the potential drops sharply.
The clinical significance of CNV is that the amplitude of this wave is a direct indicator of the level of attention and readiness for goal-directed action.
Movement Preparation: Readiness Potential
The next crucial stage immediately preceding the motor act itself is the generation of the readiness potential (Bereitschaftspotential). It reflects the neuronal activation required to trigger muscle contraction.
Unlike local frontal expectancy phenomena, the readiness potential is recorded across the entire scalp surface before a voluntary action begins, although its maximum expression is observed in the motor cortex. EEG dynamics show that throughout most of the resting period, the baseline remains stable. A sharp upward deflection in electrical activity begins several hundred milliseconds prior to movement initiation (e.g., before a finger flexion).
This phenomenon proves that the motor cortex begins forming the command long before actual muscle fiber contraction occurs, preparing the necessary neuronal pools in advance.
Action Initiation and the Cellular Level
The final stage of cortical electrophysiological activity is the motor potential. Unlike the diffuse readiness potential, the motor potential represents a strictly localized response. It arises in a specific region of the motor cortex (the precentral gyrus) that anatomically and precisely corresponds to the acting limb. Single-unit recording using implanted microelectrodes is frequently used for detailed study of motor cortex electrophysiology. In classic experiments, the subjects are primates (monkeys) performing voluntary arm movements, such as flexion or extension against an external load.
Graphical display of such research results correlates two key parameters:
- Mechanogram: shows the trajectory of the lever and changes in the spatial position of the limb.
- Electroneurogram: records the electrical activity of the motor cortex, where individual action potentials (spikes) appear as vertical marks.
Synchronous analysis of these indicators allows precise determination of the relationship between individual motor cortex neuron discharges and the biomechanical characteristics of the developing muscle contraction.