THE FORCE CONSTRAINT STRATEGY FOR STANCE IS INDEPENDENT OF PRIOR EXPERIENCE

The purpose of this study was to examine the effect of prior experienc e concerning direction of a postural perturbation on the balance respo nse of cats to translations of their support surface. Previous work ha s shown that, when cats are translated in many directions in the horiz ontal plane, they respond by exerting active forces with each paw in o nly two directions, termed the force constraint strategy. This study e xamined whether the force constraint strategy could be modified based on predictability of the direction of translation and whether this str ategy is used by the naive animal with no prior experience of platform translation. Four cats were trained to stand quietly on the force pla tform using positive reinforcement, and then were implanted with chron ically indwelling electrodes for recording electromyographic (EMG) act ivity. The first experiment concerned the response of the naive cats t o their first exposure to platform translation and consisted of transl ations presented randomly in four different directions in the horizont al plane. The second experiment consisted of two complete sets of 16 d irections of translation (15 trials per direction), with the direction of translation randomized in one set and seriary ordered in the other , to make the direction of translation unpredictable or predictable, r espectively. Forces exerted by the cat, EMG activity, and platform pos ition were recorded during the 1-s trials. The use of the force constr aint strategy was independent of prior experience with direction of tr anslation, as was the amplitude of the response. Moreover, this strate gy was observed in the naive cat. These findings suggest that the forc e constraint is a robust and consistent response to translational pert urbations of stance in the cat and is part of its natural behavioral r epertoire. The accuracy in specification of the direction of a postura l response must be based on the sensory information that is obtained w ithin a very short time after the onset of platform acceleration (loop time 40-70 ms). On the other hand, the amplitude of the postural resp onse tended to decrease with experience and practice, suggesting a lon g-term change in central set that may manifest as a reduction in senso rimotor gain.