Seminar Series

Heading in the right direction

Heading in the right direction ? Analyses of the optic flow field from J.J. Gibson onwards have shown that there is information to specify the point of impact in the visual scene if locomotion were to continue along the same path. Locomotion direction can be modified if there is a discrepancy between the actual and desired points of impact. Use of these optic flow characteristics would be highly desirable if there are forces, such as side winds or cross currents, which result in there being a difference between the observer's direction of heading (with respect to a body or vehicle axis) and their actual locomotor path. If there are no such forces, observers could use the visual direction of the intended target (with respect to a body or vehicle axis) to alter their locomotor direction either directly, by their changing walking or swimming direction, or more indirectly by turning a steering wheel. Studies by Rushton et al (1998) and Rogers and Dalton (1999) suggest that observers rely more on visual direction than optic flow. Providing richer optic flow information through motion parallax or the addition of road markings produces locomotor paths which are more consistent with the use of optic flow information but these results are also consistent with the greater salience of static (rather than dynamic) alignment cues in these situations. Thus it remains to be shown if optic flow per se plays any role in the control of locomotor direction. Rushton, SK, Harris, JM, Lloyd, MR and Wann, JP (1998) Guidance of locomotion on foot uses perceived target location rather than optic flow, Current Biology 8, 1191-94. Rogers, BJ and Dalton, C (1999) The role of (i) perceived direction and (ii) optic flow in the control of locomotion and for estimating the point of impact, Investigative Ophthalmology and Visual Science, 40 S764.