In the present experiment, the effect of error attribution through instruction and body representation was investigated. The hypotheses regarding instruction could only be partially confirmed: The misinformation group, who were instructed that a random error was built in in addition to the visual displacement, did not show a significantly different magnitude, but a significantly smaller persistence than the control group.
In terms of magnitude, Welch's (1972) results could not be replicated. Although the same underlying mechanisms were assumed, there are many differences between these two experiments in terms of the manipulation. First, Welch's misinformation created the illusion of object identity between the luminous finger and one's own finger, and to achieve this, the visual environment was reduced to a minimum (target and feedback were shown in darkness). In the present experiment, misinformation was instead intended to break up this illusion within an immersive VR environment. Perhaps the immersion of the VR was too high to break up the illusion to such an extent that an effect on the magnitude could be shown here.
Second, Welch's misinformation manipulated the perceived reason for the displacement of the feedback. The informed group correctly believed that the luminous finger belonged to another person who was simply providing feedback offset from their own finger. The misinformed group falsely believed that the displacement was caused by prism glasses. In the present experiment, all participants were instructed that the virtual environment was visually displaced. The manipulation only affected the perceived accuracy of the feedback. The global displacement was therefore not questioned, only the local feedback within the displacement. Although this may have reduced the effect of instructions, it should also have led to a lower unity assumption, i.e. that the virtual hand is seen as less the same object as one's own hand.
In terms of persistence, the results were in accordance with the hypothesis. Besides the interpretation with regard to unity assumption, further explanations are possible. For example, false information of a random perturbation might have led to a higher perception of feedback variability. Studies have already shown that the actual additional trial-to-trial variability of a perturbation reduces the extent of adaptation during exposure (Fernandes et al. 2012; Havermann and Lappe 2010). According to Bayesian models, manipulating variability changes how much participants rely on new sensory information (Fernandes et al. 2012). Although the actual variability of the feedback was not manipulated, the illusion of higher variability may have led to a greater use of cognitive strategies. When the contribution of cognitive strategies is increased, aftereffects are reduced.Michel et al. (2007), for example, could show that a constant displacement, compared to a gradual displacement, led to greater awareness of the perturbation and was thus associated with a smaller aftereffect. Accordingly, the instruction that drew attention to a possible perturbation may also have increased awareness and thus led to less persistence of the aftereffect.
The hypotheses regarding body presentation could not be confirmed, neither in terms of magnitude nor persistence. One possible explanation could be that the difference between hand and arrow in terms of how much it affects object identity was too small. All studies that were able to show an effect of an abstract body representation on the aftereffect compared different media: direct view of the hand vs. a video of the hand (Norris et al. 2001). vs. computer-generated image of a cursor or a line (Aziz et al. 2020; Clower and Boussaoud 2000; Norris et al. 2001; Veilleux and Proteau 2015). In the present experiment, abstraction was varied within the same medium, namely within VR. Comparisons between media include other differences that can influence aftereffects, such as resolution. The effects within a medium may be much smaller, as seen in our experiment where only the representation is changed.
To reveal possible effects of representation within a medium, the difference between abstract and realistic representation should be further increased in terms of object identity. In order to further reduce object identity in abstract representation, other forms of representation than an arrow could be investigated, which are perceived as less realistic. Realism perception was significantly associated with magnitude within the arrow group. Those who considered arrow to be a realistic representation of their hand showed a higher magnitude of the aftereffect. Habit with computer-generated hand representations could be a contributing factor here. Thus, other forms of representation could be investigated that are not already used in computer-influenced everyday life, but rather those that are not yet associated with hand and pointing movements.
In order to increase the object identity in the realistic representation, a better virtual hand model should be used, one that corresponds as closely as possible to the users' hand. To meet the individuality of the users, customisations can be made, for example to their sex and the colour and size of their hands. Another aspect concerns the realism of the movement. In the present experiment, the index and the middle finger were fixated to enable finger tracking. This fixation suits an arrow with fixed components better than a hand with movable fingers. This could be the reason for the surprising opposite effect that the arrow group showed a more persistent aftereffect than the control group. In future studies, for example, a data glove could help to reduce this incompatibility and to transfer the mobility of the fingers into virtuality.
The hypotheses regarding error relevance could not be confirmed. Neither the misinformation group nor the arrow group showed an effect on perceived error relevance. Previous research has already revealed the influence of error relevance at the behavioural level (Wilke et al. 2013). The idea in this experiment was to measure the perception of error relevance through self-report. However, it is possible that the error relevance measurement was not sensitive enough. The formulation referred to a general movement error and was thus too unspecific. Individual interpretations of what the movement error refers to probably increased the variance, so that no effect could be found.
In addition to the hypothesis testing, further exploratory data analysis revealed an interesting effect: The target position led to a systematic movement error, namely a stronger outward deviation, i.e. right positions led to a stronger deviation to the right and left positions to a stronger deviation to the left. This target position effect can probably be attributed to an underestimation of the egocentric distances in VR. Since no simultaneous feedback was given about the movement, the movement planning had to be done on the basis of the target distance estimation. It is already known that distances are underestimated in VR (Renner et al. 2013). As can be seen in Fig. 8, the biased distance estimation may have led to biased movements as well. Due to the lack of simultaneous feedback about the movement, this bias was not corrected in the course of the movement and was reflected in a systematic movement error. This is relevant for the present study insofar as the magnitude was measured over a single target position, namely “down-right”. Since the aftereffect is defined as deviation to the left in this experiment (because of the visual deviation to the right), the magnitude of aftereffect was presumably reduced by the target position effect for all groups, which should be taken into account in the general evaluation about its size.