Introduction How do we perceive where our limbs are in space without having to look at them continuously? Although this question is fundamental to the understanding of both perception and motor control, it is far from resolved. Investigations of the neurophysiological basis of kinesthesis have produced many relevant findings, but they have also highlighted the need for psychophysical concepts that pertain to the role of mechanical, in particular kinetic, information in kinesthetic experiences, as is illustrated by a brief overview of the pertinent literature. 1969 1975 1977 1996 2000 1972a b 2003 1980 1985 g 2004 2004 2005 2006 1994 1981 1996 1995 1996 1998 e i e 1 e 2 e 3 e 1 e 2 e 3 e i e 3 e i e i e i 1995 1996 1998 1998 1998 2000 2001 2003 2005 2005 e 3 2002 2003 2005 e 3 1996 1998 e 3 e 3 e 3 CM CM e 3 e i CM g CM e i CM e 3 1996 1998  e i CM Experiment 1 1996 1998 e 3 e 3 CM e 3 e 3 CM Method Twenty healthy participants (11 female and 9 male; all right-handed; mean age 28.5 years, SD 6.0 years) participated voluntarily in the experiment. They were not familiar with the type of experiment or the rationale behind it. The experiment, which was conducted in accordance with the 1964 Declaration of Helsinki, was approved formally by the ethical committee of our faculty and carried out with the adequate understanding and written informed consent of all participants. e 3  CM 1 Fig. 1 dotted circle  1 e 3  CM 2 1 e 3 e 1 e 2 e 3 e 3 CM CM e 3 CM e 3  CM e 3  CM e 3 CM g g 1985 Fig. 2 e 3  CM CM e 3 a CM e 3 e 3  CM c CM e 3 c e 3 d c  CM e 3 d CM  Table 1 e 3 CM  Right forearm Left forearm  Load 1 Load 2 Load 1 Load 2 Condition Mass (g) x, y a  Mass (g) x, y Mass (g) x, y Mass (g) x, y e 3 b  \documentclass[12pt]{minimal}
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$$ \sphericalangle $$\end{document} 1 150 11, −18.8 150 33, 18.8 150 11, 18.8 150 33, −18.8 10° 0° 2 150 11, 18.8 150 33, −18.8 150 11, −18.8 150 33, 18.8 −10° 0° 3 300 11, 13.8 100 33, −13.8 300 11, −13.8 100 33, 13.8 0° 10° 4 300 11, −13.8 100 33, 13.8 300 11, 13.8 100 33, −13.8 0° −10° 5 150 11, 5.8 150 33, 12.8 150 11, −5.8 150 33, −12.8 10° 10° 6 150 11, −5.8 150 33, −12.8 150 11, 5.8 150 33, 12.8 −10° −10° 7 100 33, 18.8 100 33, −18.8 – – – – 0° 0° 8 – – – – 100 33, 18.8 100 33, −18.8 0° 0° a x y b e 3 e 3 e 3 e 3 c CM CM  CM CM Participants performed the nine conditions in nine corresponding trial blocks. Before each trial block, they assumed a position in which one arm was flexed at an elbow angle of 80°, which was achieved by letting the tip of the straw just touch the upper arm, and the other arm was fully extended. Note that, in this starting position, a perceptual reference was ensured both in the flexed arm (by the straw) and in the fully extended arm (by the end of the elbow’s movement range), preventing any drift effects over trials. From this position, one of the forearms (the target arm) was moved towards the other forearm until the experimenter called out “stop”. The experimenter ensured that this stop signal was given at a different arm orientation in each trial. The other arm (the matching arm) was then moved towards the target arm until the participant perceived the orientation of the two forearms to be identical. At this moment, the participant stopped the movement and called out “ja” (“yes”). After registering the orientation of both forearms (see below), the experimenter instructed the participant to assume the starting position for the next trial: The arm that was flexed at the start of the previous trial was now extended and vice versa. The target arm was alternately the left and the right arm. Participants were instructed to keep both arms and the crosspieces they enclosed with their hands in a vertical (i.e., sagittal) plane at all times. Each trial block was started with either the left or the right arm extended, and with either the extended or the flexed arm as the target arm. The four resulting starting configurations were counterbalanced across participants, with each individual participant starting all trial blocks from the same assigned configuration. Each trial block consisted of two series of eight matching trials. In one series the extended arm was the target arm and in the other the flexed arm was the target arm. All participants performed 144 matching trials in total (9 trial blocks; 16 trials per block). The duration of an experimental session was approximately 45 min. t  Results and discussion The average and the range of the elbow angles around which matching occurred in each of the nine conditions was 115.5° (SD over conditions 1.0°) and 26.9° (SD over conditions 1.3°), respectively. The low standard deviations indicate that both the average and the range of matching angles only differed marginally between conditions. t P 2 1985 1996 3 P 3 1982 Fig. 3 Matching errors of a representative participant in the control condition of Experiment 1, in which no loads were attached to the forearms. The alternating pattern of errors over trials reflects a tendency to overshoot the target arm with the matching arm  F P p 2 P P 4 e 3 P P CM t P 2 t P 2 e 3 CM e 3 CM t P 2 t P 2 CM e 3 CM t P 2 t P 2 4 g 1985 2004 2004 2005 CM e 3 1996 1998 CM e i Fig. 4 Error bars  e 3 CM e 3 CM e 3 CM g left  CM Experiment 2 e 3 CM CM e 3 CM Method Twenty healthy participants (13 female and 7 male; all right-handed; mean age 26.0 years, SD 4.3 years) participated voluntarily in the experiment. They were not familiar with the type of experiment or the rationale behind it. The experiment, which was conducted in accordance with the 1964 Declaration of Helsinki, was approved formally by the ethical committee of our faculty and carried out with the adequate understanding and written informed consent of all participants. 5 e 3  CM 2 Fig. 5 The experimental setup of Experiment 2. Participants sat on a stool besides a vertical wooden board. Their right arm and shoulder were placed through a circular hole in the board between two overlapping pieces of lycra cloth. Participants were wearing a T-shirt, of which the right sleeve was rolled up to uncover the shoulder. The stool was adjusted such that the projection of the right forearm on the vertical plane could coincide with line segments 1, 2 and 3 (15°, 0° and −15° relative to horizontal, respectively) and the projection of the upper arm with line segment 4 (−60° relative to horizontal). The experimental task consisted of matching the orientation of the unseen forearm alternately with line segments 1, 2 and 3, while maintaining the unsupported upper arm parallel to line segment 4. The carbon fiber frame, used to attach loads to the forearm (see text for a detailed description), was to remain parallel to the board at all times  1 e 3 e 3 CM e 3 CM e 3 CM The eight conditions were performed in eight corresponding trial blocks. Before each trial block, participants assumed a position in which the upper arm was oriented parallel to line segment 4, and the arm was either fully flexed or fully extended (counterbalanced across participants). The experimenter then called out one of the numbers 1 through 3 to indicate the target line segment for the forearm, upon which the participant rotated his or her forearm around the elbow towards the target line segment. Participants were instructed to stop moving the arm and push a button with the left hand when they perceived their forearm to be parallel to the target line segment and their upper arm to be parallel to line segment 4. If the arm was initially fully flexed (extended), participants subsequently fully extended (flexed) their arm. The experimenter then called out a new target line segment and the forearm was again rotated about the elbow (albeit in opposite direction) to match the orientation of the indicated line segment. Participants thus alternated between full elbow flexion and full elbow extension, and assumed the instructed arm configuration as a pause in each elbow flexion and extension movement. They were instructed to keep the crosspieces they enclosed with their right hand parallel to the wooden board at all times. In each trial block, participants matched the orientation of their forearm with each of the three target line segments four times, resulting in a total of 12 trials per block. Throughout each trial block, the upper arm was to remain parallel to line segment 4. All participants thus performed a total of 96 matching trials (8 trial blocks; 12 trials per block). The duration of an experimental session was approximately 30 min. 5 e 3 CM e 3 CM g e 3 CM t t Results and discussion F P p 2 F P p 2 P P F P p 2 t 6 P e 3 P CM t P 2 e 3 CM t P 2 CM e 3 CM CM Fig. 6 e 3 CM e 3  CM g e 3 CM upward g e 3 CM three leftmost bars g rightmost bar g Error bars  F P p 2 P P t CM e 3 P P e 3 CM P e 3  CM General discussion CM 1996 1998 CM e i CM 2 1985 1982 1999 g CM g g CM CM g g g 7 2 g g 7  CM 2 g 7 CM g CM g Fig. 7 g gray solid curve black dotted curve CM black solid curve CM CM  1982 1985 1987 1991 1999 CM g CM e i e i 1998 e i CM relative to the gravitational vertical e i e i 1998 CM e i e i  CM 2007 e i 1996 1998 g e i CM g CM e i e i CM e i CM CM g g g CM CM e 3 CM direction g magnitude g 7 2007 CM e 3 1995 CM e 3 CM CM CM e 3  1996 1998 CM CM CM 1996 1998 e 3 CM 2005 CM CM CM e 3 CM e 3 CM CM 2003 2004 2000 CM 1980 1994 2004 2004 2005 2006 2003 1981 1996 CM 2000 1980