Dynamics of motor cortex activation in response selection: Activation and inhibition following subliminal response priming

Praamstra, P.1,2, and Seiss, E.1
1Behavioural Brain Sciences Centre, University of Birmingham, UK; 2Department of Clinical Neurology, Queen Elizabeth Hospital, Birmingham, UK
E-mail: p.praamstra@bham.ac.uk

Recent studies of masked response priming have shown that covert response activation, induced by subliminal visual stimuli, can be followed by an inhibition phase. Depending on the delay between priming stimulus and response signal, this activation-inhibition sequence gives rise to reversed priming effects in choice response tasks (Eimer and Schlaghecken, 1998). In movement-related EEG recordings of the lateralised readiness potential (LRP) such a sequence of events is reflected in a rapid alternation between activation states that favour correct and incorrect response alternatives. These rapid shifts in motor cortex activation and response bias were here exploited to investigate the dynamics of motor cortex activation in response choice. High-density recordings of the LRP in a masked priming experiment (participants n=9) showed a multi-phasic activation-inhibition-activation sequence. Analyses of whole-scalp voltage topography and current density distributions demonstrated that the inhibition phase was exhaustively explained by partial activation of the incorrect response, without recruitment of voluntarily controlled inhibitory mechanisms. Furthermore, activation of correct and incorrect responses were each accompanied by contralateral activation and ipsilateral inhibition. Finally, using dipole source analysis we found that partial activation of a response was adequately accounted for by an equivalent current dipole explaining movement-related activity during response execution. Together, the results reveal the operation of a potent mechanism of reciprocal inhibition, acting at a late motoric stage, that influences response choice at the level of the motor cortex. These findings refine the notion of response conflict.