Participants were asked to lift and place one of their feet as quickly as possible onto a target located 16cm in front of them. On a quarter of 96 trials, the target jumped unexpectedly 21cm to the left or right. Imagine reaching your foot out onto a stepping stone that suddenly moved. Raymond Reynolds and Brian Day found that in response, participants were able to adjust their reaching movement incredibly quickly - within 120ms - that's just an eighth of a second. The incredible speed of this adjustment suggests the existence of a sub-cortical visuomotor pathway for control of the foot (i.e. one not involving the cerebral cortex, which is associated with conscious thought).
Moreover, when they repeated the experiment without participants being able to grasp hand-rails for balance, they found the same rapid adjustments were made and, surprising the researchers, nobody fell over.
"Normally, foot-placement is pre-planned at the beginning of a step, and tightly coupled to the throw of the body that occurs before foot-off", the scientists explained. "But our results show some mid-swing alteration in foot placement can occur without balance being compromised...the central nervous system can alter foot trajectory quickly while simultaneously ensuring balance is not threatened".
The authors said such control systems could aid walking over uneven terrain, or even a football player's rapid interception of the ball.
Reynolds, R.F. & Day, B.L. (2005). Rapid visuo-motor processes drive the leg regardless of balance contraints. Current Biology, 15, R48-R49.
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