Introduction 2004 2001 1995 1992 2004 1995 1998 2002 2002 2001a 2001b 2002 2004 2001b 1995 2000 2002 2001 2005 2006 2007 2004 1992 1992 1993 1996 2004 1999 1992 1994 1987 1997 1995 2004 1992 2002 2004 Method Participants 1 Table 1 Participant information Part M/F Age (year) Percep Pro: supi comf:uncomf Strategy 1. F 23 0.90 100:125 225:0 Comfortable ender 2. F 22 0.92 24:201 225:0 Comfortable ender 3. F 25 0.97 101:124 223:2 Comfortable ender 4. F 20 0.82 108:117 225:0 Comfortable ender 5. F 21 × 52:173 224:1 Comfortable ender 6. M 25 0.90 82:143 222:3 Comfortable ender 7. F 27 0.79 87:138 225:0 Comfortable ender 8. F 20 0.90 123:102 225:0 Comfortable ender 9. M 26 0.80 146:79 225:0 Comfortable ender 10. M 27 × 138:87 225:0 Comfortable ender 11. F 18 0.90 225:0 38:187 Pronation starter 12. F 19 0.74 225:0 144:81 Pronation starter 13. F 19 0.70 224:1 56:169 Pronation starter Part: participant number; M/F male/female; age: age in years; Percep: Perception task—percentage correct answers; Pro:supi: Start posture—number of pronated start postures: number of supinated start postures; Comf:uncomf: End posture—number of comfortable end postures: number of uncomfortable end postures; Strategy: strategy used in the action task (see text for description) Experimental set-up and apparatus 1 Fig. 1 a b  Procedure The study consisted of two experimental sessions that were conducted in succession. First, an action task was performed, second we performed a perception task to assess participants’ perceptual sensitivity for the illusion [these tasks are denoted as (1) Action task and (2) Perception task in what follows]. Standard rest breaks were present between sessions, and on participants’ demands. Action task The action task consisted of a pre-measurement and the main experiment. The procedure for both was as follows. A trial started when the participant pressed the button on the button-box with the index finger of the preferred (right) hand. Subsequently, the goggles were closed and the second experimenter manually changed the rod and frame orientation. When ready (i.e., within 2 s) the goggles opened, which was the start-signal for participants to grasp the rod as quickly as possible and place it vertically with the marker facing upwards in a hole of a tight fitting box that was located in front of them, slightly to the right of the body midline. Participants were asked to grasp the rod with a power grip, i.e., with the thumb on one side of the rod and the fingers on the other side. Once the rod was grasped, participants were not allowed to change the grip type during rotation of the rod. This was necessary, because it urged participants to plan the task prior to grasping the rod. If this had not been the case and participants were allowed to manipulate the rod in-hand, then it would not have been strictly necessary for participants to plan the movement prior to grasping the rod. 2000 1992 2 Fig. 2 The grip type scoring system used to establish the grip type that participants used. Grip types were defined by the combination of the initial posture (pronated or supinated) and the end posture (comfortable or uncomfortable). Explanation, see text  With respect to the grip type, our primary interest was the rod orientation at which a switch into another grip type occurred. The rod orientation at which there was an equal chance to observe both grip types was denoted the “switch point”. Pre-measurement 1992 2000 3 Fig. 3 black color 1 2  The main experiment The rod orientations during the main experiment were normalized to the individual switch points, which allowed us to study the individual switch region into detail without overloading participants with too many trials. Measurements were performed in a range of 80° surrounding the individual switch point, separated by angles of 10°. This resulted in a total of nine rod orientations that were tested in the main experiment (−40°, −30°, −20°, −10°, 0°, 10°, 20°, 30°, 40° relative to individual switch point). Negative orientations are clockwise rod orientations compared with the individual switch point, whereas positive orientations are directed counterclockwise to the switch point. During the experiment we also manipulated the orientation angle of the frame, such that the “rod-and-frame” illusion was created. The frame was rotated in either a clockwise (CW) or a counterclockwise (CCW) direction. A total of five frame orientations were used (20° CCW, 10° CCW, 0°, 10° CW, 20° CW) yielding a total of 45 unique conditions. In each condition, five trials were performed in a completely randomized order. The main experiment, involving 225 trials, took about 45 min for each participant. Perception task We performed a perception task to assess participants’ perceptual sensitivity for the illusion. It was examined whether different rotations of the surrounding frame affected the perceived orientation of the rod. To that end, two rod-and-frame combinations were sequentially shown to the participant. First, a rod surrounded by a tilted frame was shown, followed by either the same or a different oriented rod surrounded by Frame 0°. In between presentations, the goggles were closed for less than 2 s. Participants had to report if the orientation of the rod was the same or different in the two displays. In the majority of the trials the rod orientation did not change between presentations (for example, when the first display was a combination of Rod −30° and Frame 20° CCW, the second display combined Rod −30° with Frame 0°). In this perception task, 4 frame rotations × 9 rod orientations × 3 repetitions were tested, yielding 108 trials. In addition, we also added 72 “catch trials” (4 Frame rotations × 9 rod orientations × 2 directions of rod changes), where the rod orientation actually did change between the two presentations, either 10° CW or 10° CCW. The main reason to add catch trials was to prevent that participants could anticipate that the two rods were the same in all trials. However, catch trials were not used in the analyses. The total of 180 trials was presented in a completely randomized order. The perception-task took about 45 min to be carried out. Data analysis Action task Analysis of pilot recordings revealed that participants used two strategies to perform the action task. Although all participants used grip type 1 in some of the trials, at the individual switch point differences in grip type choice appeared. While most of the participants switched to an underhand initial posture resulting in a comfortable end posture (grip type 2), some participants switched to an overhand initial posture resulting in an uncomfortable end posture (grip type 3). Consequently, two movement strategies could be delineated. One group of participants switched between grip type 1 and grip type 2 and always ended with a comfortable end posture (this strategy is denoted as “comfortable enders”), whereas the other group of participants switched between grip type 1 and grip type 3 and always started with a pronated initial posture (this strategy is denoted as “pronation starters”). 2007 y x c k \documentclass[12pt]{minimal}
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$$ y = 1/1 + {\text{e}}^{{ - k{\left( {x - c} \right)}}} . $$\end{document} 1971 1982 Perception task The number of errors per condition were analyzed using a 4 (frame: 20° CCW, 10° CCW, 10° CW, 20° CW) × 5 (rod: −20°, −10°, 0°, 10°, 20°) repeated measures ANOVA. Frame as a factor in the ANOVA denoted the first frame that is presented to the participant. The second frame was always the same, i.e., 0°. Results Action task n n 1 4 x y 5 F P P Fig. 4 x y  Fig. 5 Error bars x y  Perception task 1 F P F P P P Discussion The purpose of the work reported here was to evaluate the influence of visual context on the planning of a sequential object manipulation task. Earlier research on the effects of visual illusions on action was limited to simply grasping a target object without any further purpose. By contrast, in the present study we asked participants to grasp a target object to subsequently place it in a hole. This task requires anticipatory planning, in which constraints arising from the end posture prevail in initial grip choice. That is, the initial grip must accommodate the upcoming movements. As far as we know, no other study has scrutinized visual context effects in such a sequential, object manipulation task. In our study, a rod was embedded in a typical “rod-and-frame” illusion configuration. We used a wide range of rod orientations that would force participants to switch between different grip types if they were to reach a comfortable posture at the end of the task. The effect of visual context on anticipatory planning processes was investigated by measuring if the location of the switch point shifted when the surrounding frame was tilted. 1997 1987 1995 1971 1982 2004 2001b 2005 2007 1995 2004 2003 2001 2002 2002 2002 position orientation 1995 2002 2001 2001 2001 n