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The aim of this study was to determine what visual cues are used to assess the time-to-contact (Tc) with a stationary target during locomotor positioning. For this purpose, we performed an experiment with restricted central visual cues and analysed the attention demand involved in use of visual cues. 11 subjects were asked to walk towards a target surface and stop in front of it. Two target sizes were used: the tau strategy, consisting of assessing Tc from the relative expansion rate of the target during locomotion, was possible only with the larger one. The same task was also performed in a dual-task situation wherein reaction time to auditory stimuli was recorded. Analysis showed that braking was initiated earlier only in the dual-task situation involving the small target. The secondary task reaction time also increased considerably and actually even sooner when the positioning was done on the small target. These results suggest that a strategy other than tau may be used to assess Tc: with this strategy, the braking distance and walking speed are taken into account. Furthermore, different processing levels must have been involved in these two different methods, since the attention demand differed from one positioning task to the other.
Bardy et al. (Sat,) studied this question.