Patla and Vickers (1997) recorded the gaze of participants as they approached and stepped over obstacles of varying height (no obstacle, 5 cm, 10 cm, 30 cm), as shown in figure 8.6. When people walk at a normal speed, they let their gaze travel along the pathway in front and use travel fixations to monitor where they are going. These fixations are anchored about 1 to 2 m in front of the feet and are carried along at the rate of locomotion.
Travel fixations monitor optic flow (Gibson, 1979); therefore, both the focal and ambient systems function to orient the person within the environment. Gibson (1979, p. 229) explains that during optic flow humans “see where they are going without having to look where they are going.” When people walk, the changing optic array and the changes in angular position of locations in the environment are registered continuously on the retina. As the size, orientation, and rate of expansion of the object change, the locomotor system adjusts automatically without conscious effort or awareness. Travel fixations have the qualities shown for the expert performers described in Tenenbaum’s model in that the gaze is often positioned in the center of the visual field and is used to subconsciously monitor events. During a travel fixation, the gaze is maintained in frontal space and acts as a visual pivot that aids ongoing locomotion.
Object fixations are the second type of gaze used during locomotion. These fixations are used to attend to objects, even as the feet continue walking or running. In order for the body to move effectively, the gaze dwells on specific objects even as the feet continue to move. Object fixations must be of sufficient duration to allow people to navigate safely and solve tactical problems even when moving at a high rate of speed. Patla and Vickers (1997) found that when participants approached obstacles of different heights, they fixated the obstacle well in advance before reaching it. The height of the obstacle affected both the frequency and duration of object fixations. Frequency of fixations increased as a function of obstacle height, with more fixations allocated to higher obstacles than to lower ones. Fixations were directed to the top of the obstacle during the approach and to the area on the floor where the takeoff foot landed. These gazes were used in a feed-forward or top-down manner and provided the locomotor system with the advance information needed to step correctly. During top-down feed-forward control, information is sent ahead from the higher neural centers and incorporated with incoming sensory information for use in motor planning.
How much time is needed to step over obstacles of different height? During normal locomotion, an object is identified at least two steps, or about 300 ms, before reaching the obstacle (Hollands, Patla, & Vickers, 2002; Patla & Vickers, 1997). If an obstacle is higher or more complex, then more object fixation time is needed. Well before the step is taken over the obstacle, the gaze is directed down the travel path and toward the next locations where stepping will occur. Since information gathered 300 ms in advance is needed to navigate an obstacle while walking, imagine the additional time needed by an athlete who is running, skating, speed skating, cycling, or skiing at high speeds. It is vital that athletes learn to look ahead and detect critical obstacles in the environment so that they can plan their steps well ahead of time.
Patla and Vickers (1997) found that travel fixations were the most dominant gaze behavior, accounting for 60% of all fixations during locomotion. Later studies investigated the gaze of participants who walked toward one of five cued target lights (Hollands, Patla, & Vickers, 2002). The same result emerged even when specific information had to be fixated and walked toward or over. The most dominant gaze behavior was travel fixation, and object fixations were used only when a complex problem had to be solved. Other researchers have found similar gaze behaviors, even in cats avoiding small objects (Fowler & Sherk, 2003; Wilkinson & Sherk, 2005).