Spatial Updating of Self-position and Orientation during Real, Imagined, and Virtual Locomotion

Two studies investigated updating of self-position and heading during real, imagined, and simulated locomotion. Subjects were exposed to a two-segment path with a turn between segments; they responded by turning to face the origin as they would i f they had walked the path and were at the end of the second segment. The conditions of pathway exposure included physical walking, imagined walking from a verbal description, watching another person walk, and experiencing optic jlow that simulated walking, with or without aphysical turn between the path segments. If subjects failed to update an internal representation of heading, but did encode the pathway trajectory, they should have overturned by the magnitude of the turn between the path segments. Such systematic overturning was found in the description and watching conditions, but not with physical walking. Simulated optic flow was not by itselfsujjicient to induce spatial updating that supported correct turn responses. An important component of navigation is updating knowledge of one's spatial position and orientation. People navigating on foot receive multiple cues for updating. Vision signals self-motion by the changing positions of distal landmarks and by the optic flow field. Proprioception (including vestibular sensing as well as kinesthetic feedback from muscles, tendons, and joints) provides cues to the navigator's velocity and acceleration. In the research reported here, we asked how well people update their internal representation of location and orientation as they travel in space under conditions in which these cues are reduced or unavailable, including conditions in which they do not physically move at all. The conditions examined included walking without vision (proprioceptive cues), imagining oneself walking along a verbally described path (neither proprioceptive nor visual cues), watching someone else walk and trying to take that person's perspective (visual cues not coupled with self-locomotion), and watching optical flow fields generated by a virtual display to correspond to a physical walk (visual cues typically coupled with self-locomotion). Past research indicated that updating of position and orientation is not equivalent across these conditions. When a subject moves physically along a pathway without vision, he or she can update by path integration, the process of monitoring one's position in space from velocity or acceleration signals provided by proprioception. Many lower organisms are capable of path integration from nonvisual cues (see Etienne, Maurer, & SCguinot, 1996; Gallistel, 1990; Maurer & SCguinot, 1995). Studies testing human path integration on simple pathways have indicated that responses such as pointing or returning to an origin of travel Address correspondence to Roberta L. Klatzky, Department of Psychology, Camegie Mellon University, Pittsburgh, PA 15213-3890; e-mail: [email protected]. are performed well above chance. Similar measures indicate that after learning the locations of landmarks by visual exposure or nonvisually guided travel from a source location, individuals can update their position and orientation relative to those landmarks during locomotion without vision (Ivanenko, Grasso, Israel, & Berthoz, 1997; Loomis, Da Silva, Fujita, & Fukusima, 1992; Loomis et al., 1993; Mittelstaedt & Glasauer, 1991; Rieser, 1989; Rieser, Guth, & Hill, 1986; Sholl, 1989). Updating position and orientation over the course of imagined movement, as is required when encoding from a verbal description, appears to be considerably more difficult than updating from proprioceptive cues. In one paradigm (Rieser et a]., 1986; see also Loomis et al., 1993), subjects were exposed to a set of objects by walking to them from an initial position without vision. They were then asked to point to a target object after moving to a new location by either physical or imagined locomotion. Performance was worse in the imagination condition (particularly for sighted subjects). Rieser (1989; Rieser et al., 1986) has suggested that during physical translation or rotation, even without vision, updating of the distances and relative bearings of objects occurs through automatic perceptual processes. Updating after imagined rotations, and in at least some cases imagined translations (Easton & Sholl, 1995), in contrast, apparently requires effortful cognitive processing. The difficulty of updating orientation through imagination is apparent when imagined rotations and translations are compared. Rotations produce relatively long response times, and errors tend to increase with the angular difference between the physical and imagined orientation (Easton & Sholl, 1995; Farrell & Robertson, 1998; May, 1996; Presson & Montello, 1994; Rieser, 1989). The cognitive effort involved in imagining rotation can also be seen from the difficulty people have when using a map that is not aligned with their orientation in space (Levine, Jankovic, & Palij, 1982; Presson & Hazelrigg, 1984; Roskos-Ewoldsen, McNarnara, Shelton, & Can; 1998). To determine how various conditions affect spatial updating, we used a phenomenon that can be demonstrated as follows. Suppose you ask a colleague to stand with eyes closed and take an imaginary walk that you describe-without physically moving. At the end of the walk, the colleague is immediately to make the physical turn that a real walker, having traveled along the same path, would make in order to face the initial origin of travel. The imagined pathway to be walked is as follows: "Go forward 3 m, turn clockwise 90°, then go forward 3 m. Now face the origin." (The reader is invited to take the imaginary walk and make the turn before reading further.) If your colleague is like the subjects described here (and like many colleagues we have induced to try our demonstration), he or she will make a turn of about 225", or turn toward the southwest if the initial heading were north. The correct response, however, is a turn of 135", or toward the southeast! As the experiments reported here demonstrate, a person who had physically walked the same pathway, without vision, would correctly turn 135'. The situation is illustrated in Figure 1. VOL. 9, NO. 4, JULY 1998 Copyright O 1998 American Psychological Society PSYCHOLOGICAL SCIENCE Updating Spatial Representations Erroneous heading response Comect heading response Fig. 1. Schematic of the triangle-completion task. The subject (indi cated by unshaded head) is presented with the path consisting of Lej 1, Turn 1, Leg 2, and is then to turn and face the origin. Subjects whc do not update heading (indicated by shaded head) will erroneousl! overturn by the value of Turn 1. Further discussion requlres defining terms used in describing spa tial relations. The bearing from a navigator (or other object) to a targe object is the angle between a reference direction (e.g., north) and a linc originating at the navigator and directed toward the target. If an objec has an angular orientation, as defined by an intrinsic axis such as thc sagittal plane of humans, its heading is its direction of orientation rela tive to some reference direction. An object within the same space as ; navigator has an egocentric (or relative) bearing, which is the direc tion of the object relative to the navigator's axis of orientation (equiva lent to the difference between the navigator's heading and the bearin) from the navigator to the object). If a navigator wishes to face a~ object, the required turn angle (i.e., degrees of rotation of the body) i equal to the value of the egocentric bearing. An object's physical heading is what can be objectively measurec with respect to the reference direction. People's movements in space however, are governed by their internal representation of heading. Thl research described earlier indicates, in fact, a distinction between twc internal representations. Perceived heading results from automatic processes (e.g., during physical locomotion) and is what one believe to be one's orientation in a space. In addition, one can use effortfu cognitive processes to take an imagined heading, which may or ma! not be discrepant with the perceived heading. An important issue i, whether taking an imagined heading results in updating of the per ceived heading; if not, a person will be aware of any discrepancy. In these terms, people who have imagined walking two legs of a tri angle in our task shouId make a turn equal to the egocentric bearinj from the end of the second leg to the origin, from the perspective o someone who has physically walked and hence has updated perceived heading at the initial turning point in the pathway. But instead of doing so, people typically make the turn necessary to face the imagined origin of travel from their current physical heading, as aligned with the first leg. People may appear to ignore the tum in the stimulus path, but this is clearly not the case, for the response turn varies predictably with that turn. This means that people have encoded the trajectory along the path. It appears, however, that the internal heading that governs the response at the end of the imagined path is not aligned with the second leg; it is instead the initial heading as defined by the first kg. Accordingly, people turn the egocentric bearing corresponding to their physical heading, thus overturning by the angle between the first two legs of the path (in our example, 90'). We propose that the response in our task is governed by the automatically updated perceived heading rather than the cognitively effortful imagined heading, and furthermore, that people can encode the trajectory along the designated path w~thout changing their perceived heading. This claim is consistent with the literature on navigation in lower organisms (especially rodents), which indicates the existence of distinct neural systems for updating position (e.g., O'Keefe, 1976; O'Keefe & Dostrovsky, L971; O'Keefe & Nadel, 1978) and heading (e.g., Blair & Sharp, 1995; Taube, Muller, & Ranck, 1990a, 1990b). It is also consistent with theoretical proposals that navigating organisms have multiple reference systems potentially available for spatial updating (Gallistel, 1990; Hart & Mqore, 1973; Levinson, 1996; Pick & Lockman, 1981). According tb one frequently made distinction, an egocentric reference system represents the current distances and bearings of points in space relative to the navigator, and an allocentric reference system represents the relative positions of points in an environment external to the navigqtor. (These are akin to what Gibson, 1979, called perspective structure and invariant structure, respectively.)