A step over the horse race model: study of inhibitory control of gait initiation by a stop signal task

Walking towards desired locations in space supports the interaction with the environment. Sometimes, unpredictable changes require the cancellation of a planned step. Gait initiation triggers a chain of events leading our body to leave a stable position for an unstable one, in which the body's center of mass (CoM) moves from the center of the base of support (BoS) toward its perimeter, until it leaves the perimeter completely. When the distance between the COM and the centre of BoS in medio-lateral and anterior-posterior increases during the 500 ms of anticipatory postural adjustments (APA) that precede the step onset, a set of muscles is recruited both to lift the foot and to stabilise the body. Sometimes these adjustments need to be revised since external (environmental) changes will require to inhibit the prepared step and maintain the body in the initial position. The study of this form of motor inhibition, intensively analysed in action control of simple movements as a button press tasks, only recently has been extended to more complex actions. A tool used widely for studying the physiological correlates of this form of motor inhibition is the Stop Signal Task (SST). SST is a test requiring executing a movement in response to a go signal (Go trials) and inhibiting it when a stop signal suddenly occurs after the go (Stop trials). The outcome of the task is accounted for by a theoretical race between the Go and Stop processes, triggered by Go and Stop signals respectively. In a stop trial, movements are executed if the Go process runs sufficiently fast. This formulation of the model, mainly developed by studying movements completed in a very short period, such as saccades or finger (key press) movements, approximates the occurrence of movements at the end of the race. However, things can be different for movements evolving on longer timescales, such as gait initiation (GI). As a result, in this thesis we used a version of SST for GI, combined to the overtime recording of the CoM, CoP and muscle activity, to ...