Contingent negative variation and activation of postural preparation before postural perturbation by backward floor translation at different initial standing positions.

We investigated the effects of balance difficulty on contingent negative variation (CNV) and postural preparation against perturbation. Thirteen subjects were perturbed by a backward floor translation (S2) after an auditory warning stimulus. To alter balance difficulty, subjects maintained standing posture from four initial positions before perturbation. The position of the center of pressure in the anteroposterior direction (CoPy) was expressed as a percentage distance of foot length (%FL) from the heel: 10%FL anterior to extreme backward leaning; quiet standing (QS); and 20%FL and 10%FL posterior to extreme forward leaning. CNV, CoPy, and electromyography (EMG) of the lower leg muscles were analyzed. Balance difficulty was represented by the relative distance of the forward peak position of CoPy after S2 from the QS position. Balance difficulty was higher with a more anterior initial position. The late CNV peaked just before S2 (latency: -76 to -306 ms), then started becoming small. CNV peak was earlier and larger with increasing balance difficulty. CoPy backward shift and a continuous EMG increase were observed as the strategy for postural preparation, and were significantly earlier (61 ms and 42 ms, respectively) than the CNV peak. CNV peak time correlated closely with onset times of CoPy backward shift (r=0.78) and continuous EMG increase (r=0.71). These findings suggest that as balance difficulty increases, attentional allocation to sensory information and/or postural preparation starts earlier just before the perturbation.