Expected and unexpected head yaw movements result in different modifications of gait and whole body coordination strategies.

During locomotion we routinely make voluntary head movements, similar to those made during steering tasks, in order to scan our environment and obtain information about objects in the environment and our proximity to these objects. Given the importance that head segment orientation during locomotion has received in the recent literature, two studies were designed to investigate responses following a voluntarily generated and an unexpected, externally applied head turn. During a voluntary head turn, an efferent copy of the head movement could cancel the sensory effects of the head turn, effectively isolating the movement response to that segment. Alternatively, if the steering synergy is a part of our motor repertoire, as has been suggested, movement of the head could automatically release a steering "synergy" of segmental control and coordination. A unique head mounted air-jet apparatus, designed and developed at the University of Waterloo, was used for both studies to ensure that auditory stimuli and the physical presence of the apparatus on the head were similar for participants of the two experiments. During certain points in the gait cycle, this device was triggered and a short burst of compressed air (350 ms) was released to cue participants to make a voluntary head turn (Experiment 1). The same device was triggered in Experiment 2; however, in this experiment compressed air was released for a longer duration (1,500 ms) which resulted in an unexpected and quick turn of the participants' head to either the left or right. In these experiments, vision was also manipulated in certain trials with liquid crystal display glasses that occluded vision for the duration of the head turn. Data from the first experiment indicates that a subset of the steering synergy previously observed is released following the voluntary head movement; however, the travel trajectory path is preserved, suggesting that sensory input resulting from the head movement is partially nullified by the central nervous system. Overall safety is ensured by maintaining the same travel path. In the second experiment, an unexpected perturbation was applied to the head during locomotion to determine how the absence of an efferent copy of the movement pattern influences the level of control over body segments during locomotion. Whole body responses similar to those observed during steering tasks were observed following application of this unexpected head perturbation. It is proposed that the CNS interprets an unexpected yaw movement of the head as a change in the frame of reference, and global modifications of the walking trajectory, similar to that observed during steering tasks, are made in the perceived new direction of travel. Collectively this work extends our understanding of how the CNS establishes a head based orientation frame for locomotion. The CNS interprets and integrates anticipated and unexpected changes in sensory information from the head segment and subsequently modifies locomotion patterns according to the perceived whole body orientation in space. The sequence of control following these head movements appears to be part of a movement repertoire that is not immutable; maintaining whole body stability during locomotion is paramount.