Methods 48 

3.3.1.1 Subjects

Due to gender-specific differences in spatial navigation (Coluccia & Louse, 2004; Grön et al., 2000; Lawton & Morrin, 1999; Ohnishi, Matsuda, Hirakata, & Ugawa, 2006; Sakthivel, Patterson, & Cruz-Neira, 1999; Sandstrom, Kaufman, & Huettel, 1998; Shelton & Gabrieli, 2004) only male subjects participated in the experiment. Twenty male subjects of the Ludwig-Maximilians-University Munich, Germany, took part in Experiment 1 (age (M±SD) = 25.60 ± 3.42 years). Participants were either paid 8€ per hour or received course credit for taking part in the experiment. The experiment followed the American Psycho- logical Association's (APA's) Ethical Principles of Psychologists and Code of Conduct (American Association of Psychology, 2002). All subjects had normal or corrected to normal vision and reported no history of neurological disorder. All but one subject were right-handed.

3.3.1.2 Task, Materials and Procedure

Participants were seated in a dimmed and sound-attenuated black cubicle equipped with electromagnetic shielding in front of a 37.7 cm x 37.7 cm (ap- prox. 21 in. screen size) Eizo Flexscan F77s TCO 99 (resolution of 1280 x 1024 pixels, 102 Hz), placed 100 cm (approx. 39 in.) in front of the observers with the screen center being aligned with the participant’s horizontal straight ahead of sight. The visual angle of the presented tunnel passages was approximately 21°.

Categorization

Prior to the main experiment a categorization task was applied in order to classify the preferred spatial strategy of the subject (Nonturner vs. Turner). Participants traversed 30 tunnels with one single turn of varying angle. Tun- nels were composed of an initial straight segment followed by a curved seg- ment and two final straight segments. At the end of a passage through the tunnel two arrows were displayed that represented the correct homing re- sponse based on an allocentric or an egocentric reference frame (see Figure 3.2). In a forced-choice task, participants had to spontaneously decide which of the two arrows pointed back to the starting point of their passage. Within three blocks of 10 trials each turning angles gradually decreased, so that egocentric

and allocentric arrows converged, resulting in increasing task difficulty. In or- der to take part in the main experiment, participants had to consistently (i.e., more than 83% of the trials) select the allocentric or egocentric arrow to be categorized as Nonturner or Turner, respectively. All subjects selected for the main experiment demonstrated consistent choice of one or the other reference frame (Turners M±SD = 29.6 ± 0.7 categorization trials (99% ± 0.02%), Non- turners M±SD = 28.7 ± 1.3 trials (96% ± 0.04%; correlation between Preferred Strategy and strategy-specific arrow choice: r(19) = .997, p < .00001).

Figure 3.2: Cognitive headings of Turners and Nonturners differ with respect to the tunnel configuration. (A) Tunnels with one and two turns bending into the same direction result in distinct cognitive headings at the end of the passage. (B) By contrast, tunnels with two oppo- site turns of equal angularity result in identical final cognitive headings for Turners and Non- turners.

After categorization, subjects underwent a brief training block of approximate- ly 10 minutes traversing 16 tunnels (8 tunnels with one turn; 4 tunnels with two turns bending into the same direction; 4 tunnels with opposite turns of equal angularity). End positions differed from the ones used in the main expe- riment. Tunnels with one turn were generated of five segments, with the two initial and two final straight segments enclosing the turning segment. Tunnels with two turns had a total of six segments (turns in segments 2 and 5). In con- trast to the forced choice task of the categorization, subjects now had to adjust the homing arrow (initially in 180° position) so that it pointed back to the ori- gin of the tunnel passage. By pressing the right or left mouse button, the arrow was rotated continuously. When the adjustment of the subjectively correct

Chapter 3 – Behavioral Analyses homing vector was completed, the setting was confirmed by pressing the mid- dle mouse button. Participants were allowed to adjust the homing arrow with- out time limit. After subjects confirmed their response, the adjusted arrow was presented simultaneously with the expected strategy-specific response. Be- cause participants’ responses on tunnels with two turns were of special inter- est for the current study, feedback was provided only for tunnels with one turn. Main Experiment

The main experimental session included 21 blocks of 8 tunnels each and 1 fi- nal block with 12 tunnels with minor breaks between the blocks. During the experiment tunnels with one turn and two turns were presented. The task of the subjects was to keep up orientation during the tunnel passage and to ad- just the homing vector at the end of the passage. The time line of a single trial is depicted in Figure 3.3.

Figure 3.3: Experiment 1 – Time line of a single trial without feedback. After 500 ms presen- tation of a fixation cross, a static image of the tunnel entrance was displayed (200 ms). The- reafter, the tunnel passage was presented, lasting either 9400 ms for tunnels with one turns or 16920 ms for tunnels with two turns. A snapshot of the tunnel end was presented for another 500 ms, followed by a 500 ms blank screen pre-response interval. The arrow appeared, point- ing into the depth of the screen, which could be adjusted without time limit. Upon response confirmation, feedback could be provided (2000 ms). A consecutive 500 – 1500 ms blank screen announced the next trial.

Each trial started with a fixation cross for 500 ms, followed by the presentation of a static display of the first tunnel segment for 200 ms. Then, tunnel move- ment started. Length of tunnels and positions of turning segments were iden- tical to the training trials. Tunnels with one turn comprised five segments, with the two initial and two final straight segments enclosing the turning segment of varying angles. Tunnels with two turns had a total of nine segments (turns located in segments 3 and 7). After the passage along the outbound path par- ticipants had to adjust the homing vector so that it pointed back to the origin of the tunnel passage. In 10% of the trials and only for tunnels with one turn or two turns bending into the same direction strategy-specific feedback on the subject’s homing accuracy was displayed for 2000 ms. A blank screen (500 – 1000 ms) marked the onset of the next trial.

The placement of turns in segments 3 and 7 ensured that subjects could not determine if traversing a tunnel with five or with nine segments until either the end of the passage showed up or the tunnel continued with a second turn. Fur- ther, stimulus turns of tunnels with nine segments could bent into the same direction (2 turns, sd), or into opposite directions with equally angled turns (2 turns, od). The angle of turns was varied such that the eccentricity of end posi- tions of the outbound paths reached 15°, 30°, 45°, and 60° (± 2° deviation to avoid categorical reactions during arrow adjustment) to the left and right rela- tive to the starting position. Eccentricity of end position was conceptualized within an allocentric reference frame. For tunnels with one turn as well as tun- nels with two turns bending into the same direction, the egocentric eccentrici- ty could be retrieved by reversing signs. For tunnels with two opposite turns of equal angularity, end positions were identical within both reference systems. Due to geometric-mathematical constraints in the construction of the tunnel material (segments of uniform length, constant movement speed), it was not possible to program tunnels with two opposite turns in segments 3 and 7 (of equal angularity) that ended at the categorical eccentricity of 60°. This condi- tion only would have been possible with longer turning segments (as com- pared to straight ones). There were a total of 144 ‘experimental’ tunnels, ran- domly interfused by 36 ‘filler’ tunnels, consisting of tunnels with one and two turns ending up at the eccentricity of 60°, always bending into the same direc- tion. Participants received feedback on every tenth trial with one turn or two turns bending into the same direction.