Supplemental materials:Untangling visual and proprioceptive contributions to body localization
Fig. S1: Experimental design
Initially the position of the participant’s hand was 13 cm away from the body midline. At the end of the adaptation procedure the left hand was 9 cm away from the body midline in all the conditions. In contrast, the shift of the right hand varied. In the Congruent condition the final position of the right hand was 9 cm from the midline (i.e. 4 cm inward; the ‘Inward’ condition) or 20 cm from the body midline (i.e. 7 cm outward; the ‘Outward’ condition). In the Incongruent condition, the right hand was 20 cm away from the midline (i.e. 7 cm rightwards from initial position), but its image was positioned at 9 cm from the midline (i.e. 4 cm leftwards from initial position). Thus, in the Incongruent condition the seen position was the same as it was in the Inward condition, but its real position was the same as it was in the Outward condition, leading to an incongruency between vision and proprioception. When the adaptation procedure was concluded, the right hand disappeared from view and the participants were either asked to keep their eyes closed for 20 sec or to keep fixating on where they felt their right hand to be. Then the localisation task started and the participants were asked to say stop when a superimposed moving red arrow was aligned with where they felt their middle fingertip to be.
Table S1:Conditions performed
Conditions / Eyes / Seen movement / Real movement / Congruency1 / Open / Inward / Inward / yes
2 / Closed / Inward / Inward / yes
3 / Open / Outward / Outward / yes
4 / Closed / Outward / Outward / yes
5 / Open / Inward / Outward / no
6 / Closed / Inward / Outward / no
Training procedure: practice protocol for the localization task.
Before starting the experiment, the participants underwent a training procedure in order to familiarise with the localisation task. We employed a standardised practice protocol, which consisted of three components:
(1)Self-paced, hand seen. The participants were able to see both of their hands, but they were asked to fixate on their right middle fingertip while the red arrow moved from the centre of the screen towards the right. The movement of the arrow across the screen was called ‘a trial’. The participants were instructed to say ‘stop’ when the arrow was aligned with their middle fingertip. The aim of this first component was to get the participants accustomed to stopping the arrow exactly when they wanted it to stop. There were no time constraints. After 5 trials, the participants were allowed more trials if either they, or the investigator, judged that they needed more practice. Once both participant and investigator were satisfied that they were accurate, we moved to the second component.
(2)Every 15”, hand seen. Even though the participants were still able to see their hand, they were not allowed to adjust the position of the arrow once they made their judgement. The aim of this second part was to become accustomed to the pace of the real experiment, in which a trial commenced every 15”. This also assured that the participants were sufficiently accurate in stopping the arrow exactly where they wanted it to in a time-constrained context. This step included 7 trials (i.e. 2 trials more than the first one) in order to get the participants gradually used to the experiment full duration (as the localisation task included 13 trials). Once both the participant and the investigator were satisfied that they were accurate, then we moved to the third component.
(3)Every 15”, hand covered. In these final practice trials, we covered the image of the right participants’ hand with a black square on the screen, to mimic the experiment. The participants were told that the experimenter was going to cover their right hand, but they had to fixate on where they felt their right middle fingertip to be and stop the arrow accordingly, just as they had in the previous trials. Again, a trial commenced every 15” and the participants were not allowed to adjust its position (i.e. once they stopped, the position was recorded and the trial was complete). There were 10 trials. Once both the participant and investigator were satisfied that they were performing the task consistently, then we proceeded to the experiment.
Table S2: Mean and standard deviation (SD) responses to each statement for each condition. The participants were asked to give a number between 0 and 10 in order to describe their degree of agreement with each statement (with 0 meaning ‘totally disagree’ and 10 ‘completely agree’). The right-most column shows significance values for each comparison. Note that α was set at 0.0167, and there were no significant differences between conditions for any of the items.
Questionnaire item / Cond / mean / SD / F / p (α=0.0167)My right hand was part of my body / CI EO
CI EC
CO EO
CO EC
INC EO
INC EC / 9.19
9.37
9.56
9.56
9.31
9.12 / 1.60
1.09
0.89
0.96
1.58
2.03 / 1.185 / 0.319
It seemed like I had no right hand / CI EO
CI EC
CO EO
CO EC
INC EO
INC EC / 1.89
0.75
0.69
1.12
1.44
1.25 / 2.53
1.34
1.14
2.55
2.87
2.69 / 0.879 / 0.386
It seemed like my right hand was no longer there / CI EO
CI EC
CO EO
CO EC
INC EO
INC EC / 2.25
2.06
2.25
2.12
2.06
2.31 / 3.53
3.71
3.73
3.61
3.60
3.70 / 0.324 / 0.767
It seemed that my right hand no longer belonged to me / CI EO
CI EC
CO EO
CO EC
INC EO
INC EC / 1.31
1.06
1.19
0.62
1.50
1.31 / 2.70
1.61
2.66
1.26
2.76
2.77 / 0.946 / 0.362
I had the sensation that my hand was numb / CI EO
CI EC
CO EO
CO EC
INC EO
INC EC / 2.06
1.50
1.31
1.81
1.37
1.06 / 2.77
2.19
1.96
3.10
2.30
1.95 / 1.119 / 0.331
It seemed like I had more than one right hand / CI EO
CI EC
CO EO
CO EC
INC EO
INC EC / 0.44
0.56
0.50
0.44
0.44
0.50 / 0.81
1.09
0.97
0.81
0.81
0.97 / 1.471 / 0.248
I couldn’t tell where my right hand was / CI EO
CI EC
CO EO
CO EC
INC EO
INC EC / 4.19
3.87
3.69
4.69
4.06
4.75 / 3.19
3.54
3.46
3.34
3.47
3.36 / 0.768 / 0.512
Table S3: Error scores by trials and conditions
Congruent Inner (mean±sd) / Congruent Outer (mean±sd) / Incongruent (mean±sd)Trial (time) / EO (cm) / EC (cm) / EO (cm) / EC (cm) / EO (cm) / EC (cm)
1 / 8.94±19.68 / 1.56±25.64 / 5.31±16.64 / -3.56±32.11 / -110.88±35.08 / -96.88±36.90
2 / 10.63±20.14 / 8.25±30.35 / 7.50±18.59 / -0.69±35.12 / -102.63±41.88 / -86.31±42.82
3 / 13.00±25.87 / 8.75±31.42 / 5.63±19.13 / 0.13±38.07 / -105.69±39.96 / -83.19±42.32
4 / 17.69±32.58 / 9.13±34.14 / 7.44±19.13 / -0.44±40.46 / -94.38±45.11 / -78.25±46.28
5 / 19.88±29.93 / 13.50±37.95 / 8.25±19.19 / -0.31±44.51 / -93.06±5.79 / -76.38±45.15
6 / 21.44±31.32 / 14.25±38.21 / 10.63±20.11 / 1.13±75.73 / -92.06±57.83 / -74.50±46.87
7 / 21.31±32.95 / 14.50±37.82 / 11.25±18.68 / 3.00±48.09 / -84.75±56.34 / -68.50±48.16
8 / 24.31±38.03 / 18.38±41.14 / 13.19±18.53 / 1.44±47.62 / -80.69±58.73 / -63.13±46.70
9 / 25.81±40.69 / 15.69±38.62 / 16.06±24.69 / 3.88±45.41 / -82.31±60.12 / -64.44±51.42
10 / 30.13±41.98 / 15.13±42.64 / 15.88±24.45 / 3.44±48.90 / -80.81±61.18 / -63.31±49.56
11 / 31.38±44.29 / 18.44±43.13 / 17.50±24.53 / 6.50±48.75 / -78.50±66.68 / -63.69±50.06
12 / 25.56±44.87 / 19.38±45.57 / 17.00±22.70 / 9.38±51.19 / -78.25±71.86 / -61.19±52.32
13 / 32.31±51.04 / 20.75±42.79 / 16.69±22.90 / 6.38±53.26 / -75.75±67.38 / -56.50±54.69
Video.In all experimental conditions (see Fig.S1), participants underwent an adaptation procedure during which the seen location of the hand could be decoupled from the real location without conscious awareness. For this procedure participants were instructed to hold their hands approximately 5 cm above the table surface and to maintain the position of their hands between moving blue bars that could be seen on either side of each hand. The bars served no real purpose other than to provide a guide for the hands to remain within. The seen image of the hands and the bounding blue bars could be controlled independently such that participants would have to move their hands sideways in order to make them appear stationary. For example, if the image of the right hand were moved incrementally to the left, then in order to keep the hand in the same place visually, the real hand must be moved to the right at the same speed and for the same distance as the seen image moved to the left. We refer to the incremental realignment of the hands relative to their seen position as the realignment process. The speed of the realignment process (25mm/s) was such that participants were not aware that they were moving their hands. The outcome of the adaptation was that, at the end of the process, participants would see their hands in the location that they had been concentrating on keep them in, whereas in reality they had moved their hands to another location. This process has been used successfully elsewhere, most notably as part of the Disappearing Hand Trick [39]. If vision is removed at this point and participants are asked to reach across and touch their adapted hand, they are universally surprised to find that it is not there [39]. For the current experiment, however, this secondary procedure was not employed.
The video shows respectively the seen online image of the participant hand (on the top) and the movement that the participant’s hands were actually performing (on the bottom) during the adaptation procedure. It can be easily noticed that, at the end of this procedure, the participant’s right hand (on the bottom) is not aligned with its own image (on the top). To clarify, the top and bottom video images are pasted together from two separate recordings of the same condition.