Obstacle Avoidance Behaviour during Locomotion: Strategy Changes as a Result of Visual Field Limitations

In order to walk through structured surroundings without colliding with any obstacles or parts of that environment, humans rely for the most part on the visual system. Therefore, impairment in the acquisition of visual information poses a threat to efficient and save locomotion through structured environments. One form of impairment is the limitation of the extent of the visual field, which is approximately 200° wide and 135° tall when unrestricted. Eye diseases and certain hardware devices may limit this field extensively. This thesis is concerned with the influence of visual field limitation on obstacle avoidance behaviour. In Chapters 2 and 3 it is investigated how limitations of the horizontal and vertical viewing angles affect obstacle avoidance behaviour. The results show that decreasing the size of the visual field causes impairment of performance on several obstacle avoidance tasks. The finding that the size of the visual field causes similar performance degradation for each of the tasks suggests that the effect is robust and not dependent on the nature of the actual movements required to complete the task. Also, a performance plateau was found between a width of 75° and 120° above which speed increases and below which speed deteriorates. Furthermore, an enlargement of the vertical angle yields a greater increase in speed compared to a similar enlargement of the horizontal angle. In chapter 4 it is investigated how the vertical viewing angle affects body kinematics and strategy changes during an obstacle crossing task. For all viewing limitations, participants choose to optimise safety at the cost of spending more energy. Moreover, it seems that walking with a preferred speed has priority over the minimisation of clearance to the obstacle since the latter was sacrificed first as a consequence of compromised safety and a decrease in speed was only observed after further reduction of the vertical viewing angle. Finally, Chapter 5 presents a study on strategy changes during a steering task. In addition, head movement and balancing are examined. During the steering task similar strategy changes were observed as with the crossing task. As the visual field size deteriorates, safety becomes increasingly threatened causing participants to first enlarge their clearance before decreasing their speed. Furthermore, speed of head movement decreases as the visual field gets smaller while magnitude of head rotation remains unaltered. It seems that the spatiotemporal integration of small pieces of visual information requires additional time compared to the processing of larger pieces. In addition, the increased step width that is observed with a small visual field indicates the presence of balancing impairment. The influence of visual field limitation on obstacle avoidance behaviour as presented in this thesis may contribute to several application areas. First, these insights may benefit people suffering from eye diseases such as retinitis pigmentosa and glaucoma in traversing their environment in a safe and efficient way. Second, it is possible to formulate guidelines for the procurement and design of visual field limiting devices such as Head-Mounted Displays and Night-Vision Goggle