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Hugo Bruggeman, William Warren; Stepping in the right direction: control and adaptation of walking from optic flow. Journal of Vision 2009;9(8):828. doi: 10.1167/9.8.828.
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© ARVO (1962-2015); The Authors (2016-present)
Previous research indicates that optic flow is used to adapt the direction of walking, when the heading specified by flow is displaced from the locomotor axis specified by proprioception (Bruggeman, Zosh, & Warren, Current Biology, 2007). This adaptation does not affect the perceived straight ahead (no transfer to throwing or kicking—Bruggeman & Warren, VSS 2008), but it might recalibrate the direction of thrust. Here we investigate the degree of adaptation as the displacement of optic flow is progressively increased up to 22.5°.
Participants repeatedly walked to a target in a richly textured virtual environment while wearing a head-mounted display (63° H x 53° V), and head position was recorded with a sonic/inertial tracker (70 ms latency). The heading direction specified by optic flow was displaced to the right of the actual walking direction by 2.5° at the beginning of the experiment, and increased by 2.5° every ten trials to a maximum of 22.5° at the end of the experiment. Every tenth trial was a “catch trial” with a normal alignment (0° displacement), to measure the negative aftereffect of adaptation. If there is a limit to the recalibration of thrust direction (e.g. due to biomechanical constraints), adaptation and its aftereffect should saturate at some point; otherwise, they may increase monotonically.
Surprisingly, both the degree of adaptation and the negative aftereffect increased linearly with the flow displacement at a ratio of 1:2. In addition, detailed analysis of on-line steering showed a consistent pattern of adjustment based on optic flow across adaptation trials. These findings imply that optic flow adapts the walking direction by about 50% of the displacement, while the remaining discrepancy is reduced via on-line steering control. Thus, the visual control of steering from optic flow cannot be explained by adaptation.
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