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The present study investigated differences in homing accuracy based on ego- centric or allocentric reference frames by systematically varying the complexi- ty of virtual outbound paths. We asked whether visual path integration of the strategy groups was based on history-free updating (Müller & Wehner, 1988, 2007) or configural updating as initially proposed by the encoding-error model of Fujita and colleagues (1993). To the author’s knowledge, this was the first study on egocentric and allocentric path integration applying a virtual homing task with naturally bounding curves instead of on-spot rotations (Klatzky, 1998; Klatzky et al., 1990; Loomis et al., 1993; Riecke & Wiener, 2008; Wiener & Mal- lot, 2006). Additionally, this was the first study on complexity effects during visual path integration taking into consideration individual proclivities for cod- ing space within a (body-centered) egocentric, or an (environment-centered) allocentric reference frame. Since configural updating implies that the path integration process is independent of path complexity, the homing task should

Chapter 3 – Behavioral Analyses have been afflicted only with random error. In contrast, configural updating suggests that not only the configuration of the currently encoded path influ- ences homing responses but also that expectations based on the history of previously traversed outbound paths impacts sensory integration during spa- tial updating. These experiences are assumed to reside in the representational system and are retrieved for encoding. Therefore, encoding would be context- dependent with the history of presented outbound paths influencing homing responses and latencies.

3.6.1

Configural Updating of Primitive Parameters

Based on the current findings, path integration is unlikely based on history- free updating processes, even when path complexity is unpredictably varying. Rather, path integration during outbound pathways of varying complexity seems to be based on configural updating processes including reference-frame specific primitive parameters (Klatzky, 1998). The history of traversed path- ways serves as a basis for estimating the origin of the current passage (Peruch et al., 2005; von der Heyde, Riecke, Cunningham, & Bülthoff, 2000). Regarding the difference between the standard (Fujita et al., 1993) and the extended con- figural model (Klatzky et al., 1999), our results suggest that the different mod- els explain behavioral performance to different degrees dependent on the ref- erence frame used as well as the complexity of outbound paths.

The standard encoding-error model of Fujita and colleagues (1993) assumes errors to arise during sensory uptake and encoding of spatial information. Therefore, each outbound path is supposed to be encoded independently of other pathways in an ‘object-centered’ representation (Klatzky et al., 1999). By contrast, the extended configural model assumes that navigators build up a representation in which several pathways are interrelated, e.g., by assuming that all paths arise from a common starting point. In this case, encoding is me- diated by the comparison of the current path’s configuration with respect to the distances and turning angles of the previously traversed trajectories. Based on previous experience the navigator might build up expectations regarding the layout of the traversed pathway (Cornell, Heth, & Skoczylas, 1999). Naviga- tors refer to these prototypical expectations whenever navigation may become too challenging, resulting in ambiguities regarding the layout of the traversed trajectory. Klatzky and colleagues (1999) assumed that in these cases the navi- gator resorts to an internal path model, a simplified ‘average path’.

In Experiment 1, where participants were confronted with multiple complexity levels, these ambiguities were associated with increasing side errors as well as inaccurate homing responses for paths with turns into opposite directions. This pattern supports the assumption that a more elaborated representation of pathways was computed comprising significant characteristics such as the number and direction of subsequent heading changes along different path- ways. Fujita et al. (1993) suggested that participants’ angular responses might be characterized as compromise between presented and represented values

due to sensory noise, memory loss, and subjective awareness of lack of know- ledge. In this case, the regression tendency should have been more pro- nounced the less effective information was encoded. However, signed error scores provided evidence for an extended configural updating. Tunnels were not encoded in a discrete manner, but were categorized due to the respective complexity levels (Klatzky et al., 1999). Turners and Nonturners built up repre- sentation-based expectancies incorporating a complexity-dependent ‘average path’, resulting in an overestimation of under-average pathways and an unde- restimation of over-average pathways (Lathrop & Kaiser, 2005).

When variability of complexity was reduced to tunnels with one and two turns bending into the same direction, Turners and Nonturners displayed distinctive regression patterns. Whereas Turners generally overestimated end positions for tunnels with two turns, Nonturners’ angular responses were primarily un- derestimations. However, based on the premises of models based on configur- al updating (Fujita et al., 1993; Klatzky et al., 1999), Turners and Nonturners did not apply separate average paths for each complexity level but rather en- coded all tunnels with respect to their eccentricity of end position. Therefore, the response was primarily determined by the actual value of the end position (within an allocentric or an egocentric reference frame, respectively), and not by the number of turns. Interestingly, when turns bent into opposite directions, Turners and Nonturners again displayed comparable error patterns, resem- bling results of Experiment 1. Taken together, homing errors of both strategy groups were comparably affected by path complexity, mirroring and further extending results of previous tunnel studies (Gramann, 2002; Gramann et al., 2005; Gramann et al., 2006; Riccobon, 2007). Therefore, the present study ac- centuates the stability of the egocentric and allocentric reference frame even when confronted with multiple, unpredictably varying complexity levels.

3.6.2

Co-Existence of Egocentric and Allocentric Representa-

tions

Turners’ response latencies grew linearly with increasing pathway complexity, bearing resemblance to triangle completion studies with blindfolded subjects (e.g., Loomis et al., 1993) and recent path integration studies in virtual envi- ronments (e.g., Riecke & Wiener, 2007). These differences might be linked to the updating of distinct primitive parameters within different reference frame. However, although the number of elements that have to be stored within an egocentric and an allocentric reference frame might be comparable, the repre- sentations might differ when the objects (i.e., the locations of turns along the pathway) have to be computed relative to the position (and orientation) of the navigator. Within an egocentric reference frame, these egocentric bearings have to be updated until the very end of the passage, whereas within an allo- centric reference frame only the position (but not the orientation) of the navi- gator changes within stationary surroundings (Burgess, 2006; Klatzky, 1998; Klatzky & Wu, 2008). Therefore, Turners might have taken longer to initiate

Chapter 3 – Behavioral Analyses the homing response, whereas Nonturners responded equally fast at all com- plexity levels.

Due to the first-person perspective of the incoming information path integra- tion might be accomplished primarily based on an egocentric reference frame, with the primitive parameters of the egocentric locational representation sub- sequently transferred into an allocentric reference frame. Such a re- computation would be associated with increased response latencies as well as higher variability in homing responses. This was clearly not the case. Nonturn- ers responded as fast as Turners and produced no additional errors or higher variance in homing responses. Rather, the data support the assumption that allocentric and egocentric reference frames co-exist in parallel as implied by neuropsychological investigations (Aguirre & D'Esposito, 1999; Burgess, 2006; Burgess et al., 2004; Byrne et al., 2007; Hartley et al., 2003; Mou et al., 2004; Redish, 1999; Sholl, 2001) and, more specifically, by recent neuropsychologi- cal investigations using the tunnel paradigm (Gramann et al., 2006; Gramann et al., submitted; Seubert et al., 2008).

3.6.3

Conclusions and Upcoming Steps

The present results demonstrate that the complexity of an outbound path in- fluences homing accuracy of subjects using distinct reference frames beyond the current outbound trajectory. Importantly, the use of distinct reference frames to build up a spatial representation and to maintain action-relevant in- formation is influenced by path complexity to different extents. This demon- strates the necessity to incorporate individual strategies and the use of con- trolled paradigms to investigate the cognitive and neuropsychological basis of spatial navigation.

The following section will address if the comparable behavioral performance is also mirrored in terms of comparable cortical processes during encoding and retrieval of spatial information. Additionally, future research has to address how – additionally to directional estimations – distances are updated along the tested outbound pathways in order to gain further insights into mechanisms coding for the layout of the pathway. Also it would be of advantage if subjects were able to actively reproduce the traversed pathway, mirroring the retracing task of Loomis et al. (1993).

Chapter 4