Static and dynamic visual information about the size and passability of an aperture

Abstract

The role of static eyeheight-scaled information in perceiving the passability of and guiding locomotion through apertures is well established. However, eyeheight-scaled information is not the only source of visual information about size and passability. In this study we tested the sufficiency of two other sources of information, both of which are available only to moving observers (ie are dynamic) and specify aperture size in intrinsic body-scaled units. The experiment was conducted in an immersive virtual environment that was monocularly viewed through a head-mounted display. Subjects walked through narrow openings between obstacles, rotating their shoulders as necessary, while head and shoulder position were tracked. The task was performed in three virtual environments that differed in terms of the availability of eyeheight-scaled information and the two dynamic sources of information. Analyses focused on the timing and amplitude of shoulder rotation as subjects walked through apertures, as well as walking speed and the number of collisions. Subjects successfully timed and appropriately scaled the amplitude of shoulder rotation to fit through apertures in all three conditions. These findings suggest that visual information other than eyeheight-scaled information can be used to guide locomotion through apertures.

Figure 1

The geometry of eyeheight-scaling. G is the distance between the inside edges of the cylinders, E is eyeheight, W is shoulder width, α is the visual angle subtended by the inside edges of the cylinders, and γ is the angle of declination of the base of the cylinders.

Figure 2

The geometry of head-sway-scaled information. X_R and X_L are the lateral positions of the centers of the right and left obstacles, respectively. G is the distance between the inside edges of the obstacles (i.e., the size of the aperture), D is the diameter of each obstacle, x_h(t) is the lateral position of the head at time t, θ is the visual angle between the direction of locomotion and the center of the obstacle, and φ is the visual angle subtended by the edges of the obstacle.

Figure 3

The geometry of stride-length-scaled information. G is the distance between the inside edges of the cylinders, L is stride length (2× step length), and α is the visual angle subtended by the inside edges of the cylinders.

Figure 4

(A) Top down schematic view of subject walking through an aperture, showing locations of shoulder markers, and definition of Wx. (B) Data from sample trial showing Wx as a function of distance along the z-axis. Black and gray segments represent non-dropout and dropout frames, respectively. Dashed vertical line indicates location of aperture.

Figure 5

Sources of information available in each condition used in the experiment. EH = eyeheight; HS = head-sway; SL = stride-length. Screenshots from each condition are shown in the right column.

Figure 6

Method used to estimate W_x on dropout frames. See text for details.

Figure 7

(A) Shoulder rotation angle (deg) and (B) W_x (in units of shoulder width) at aperture crossing as a function of aperture size for the Post, Tall Post, and Wall conditions.

Figure 8

Signed spatial error as a function of aperture size for the Post, Tall Post, and Wall conditions.

Figure 9

Walking speed (in m/s) as a function of aperture size for the Post, Tall Post, and Wall conditions

Figure 10

(A) Percentage of trials with collisions as a function of aperture size for the Post, Tall Post, and Wall conditions. (B) Percentage of collisions as a function of absolute lateral deviation from the center of the aperture. Black squares, gray circles, and white diamonds represent the Post, Tall Post, and Wall conditions, respectively.