Cortical responses associated with predictable and unpredictable compensatory balance reactions.

This study investigated the effects of postural set on the cortical response evoked by an external perturbation to human upright stance. Postural set was manipulated by providing either predictable or unpredictable whole body perturbations which required balance corrections to maintain upright stability. Unpredictable perturbations evoked a large negative potential (e.g., CZ: -19.9+/-5.1 microV) that was similar in timing (e.g., CZ: 98.9+/-5.5 ms) and shape to that reported in previous studies. This large negative potential was not discernable for perturbations with predictable onset timing and direction in spite of the presence of significant compensatory balance reactions. Importantly, when a surprise perturbation was presented following a series of predictable perturbations, the large negative potential occurred on this trial even though subjects expected a predictable stimulus onset. This suggests that the large negative potential was dependent on a dissociation between expected and actual stimuli rather than on a tonic central state defined by task conditions. These results suggest that cortical events may be linked to error detection that is independent of sensory or motor events associated with evoked balance reactions.