The time course of attention shifts following perturbation of upright stance.

Recent work has revealed the specific time course of attention shifts associated with balance control in a seated model using a dual-task paradigm. This work highlighted an initial "automatic" and later "attention-demanding" phase of the evoked balance reaction. The objective of the present study was to determine if comparable influences would be observed for performance of a visuomotor tracking task when responding to perturbations of upright stability. Small-amplitude floor translations were applied in the forward or backward direction to evoke stabilizing postural reactions. Balance reactions were evoked with and without the concurrent performance of a visuomotor tracking task using the right hand. Results showed significant disruptions (pauses) in tracking that invariably occurred after onset of the earliest balance reaction measured in ankle muscles. On average, there was a delay of 345 ms between ankle-muscle activation (average onset 144 ms) and the pause in visuomotor tracking. The concurrent tracking led to modest change in later phases of the balance reaction, as measured by an increase in center-of-pressure excursions, but did not affect the earliest phase of the reaction. These results support the view that compensatory balance reactions, even those evoked by small perturbations, are characterized by an initial "automatic" phase and subsequent control that may be more dependent on cognitive resources.