Text spelling out one of four colours (red, green, yellow and blue) presented at pseudorandom intervals using Powerpoint 2010 (Microsoft Corp., Redmond, WA, USA). The duration of each stimulus was between 600 and 1400ms. To avoid potential entrainment of temporal gait parameters and to encourage constant attention, stimulus duration was adjusted so that no more than two sequential stimuli had a duration within 200ms of that of the previous stimulus, although mean stimulus frequency was maintained at 1Hz over the trial
