Temporal Dynamics of Proprioceptive Hand Model Recalibration 1 On the Temporal Dynamics of Proprioceptive Hand Model Recalibration and the Interaction with Pre-existing Hand Representations 460375766 The University of Sydney Temporal Dynamics of Proprioceptive Hand Model Recalibration 2 Abstract Performing specific actions with precision requires an exact knowledge of the bodies current shape and location. To do this, the brain creates as ongoing mental representation of the body using two mains sources of information, vision and proprioception. If these sources of information come into conflict and a body part is not seen to be where it is felt the brain must create a new representation of this body part, and the new representation is predominately based off vision. This paper aimed to determine the temporal dynamics of this process as well as investigate how this new representation interacts with the prior, long-term representation of the body that the brain has. To do so, participants wore a VR headset through which their hand appeared to be roughly 10cm offset from its actual position. We then measured whether participants altered their hand movements to be more in line with this virtual hand and over what time period this change took place. We found that over the first five trials participants adjusted their hand representation to be significantly more similar to the virtual hand, however over the complete twenty trials this effect disappeared. This suggests that while the brain is capable of creating a new representation of the hand very rapidly it is in conflict with the long-term hand representation. This results in neither body representation being successfully maintained. Temporal Dynamics of Proprioceptive Hand Model Recalibration 3 Performing precise action requires a specific and accurate representation of the body’s shape and location (Henriques & Cressman, 2012). Often however, the representation needed to accurately perceive the body changes, such as in situations of a lost limb or when using a tool, causing the brain to recalibrate its representation of the body. Research has since focused on in what specific situations recalibration takes place, the mechanisms behind it and what conclusions can be drawn from this process surrounding brain plasticity. When trying to calibrate where a body part is, the brain mainly relies on visual and proprioceptive processes (Tuthill & Azim, 2018). These two processes show neural convergence in the brain, such that the same neurons in the premotor cortex respond to both visual and proprioceptive cues to determine where a body part is (Graziano, 1999). When these two processes are in conflict (i.e. a body part is seen to be in a different position to what is felt) the visual process has been shown to dominate the proprioceptive process (Martel, Cardinali, Roy & Farné, 2016). This results in the brain representing the body part where it is seen, not where it is felt through proprioception (Cressman & Henriques, 2010). This has been demonstrated in embodiment illusions such as the rubber hand illusion in which participants report taking ownership of and having feeling in a false arm while their real arm is hidden from sight (Botvinik & Cohen, 1998). Since then many studies have examined this phenomenon, generally following a similar method. Usually, the participant’s real hand is obscured, and they are led to believe that a fake hand is their real hand, called the adaptation period. Experiments then measure how strong the representation of the fake hand is, and in what contexts this recalibration can occur (Zbib, Henriques & Cressman, 2016; Keys, Rich & Zopf, 2018). A relatively unresearched aspect of embodiment extension is the temporal dynamics concerning how the brain forms a new representation of an altered limb. A time-resolved model would be an invaluable tool in the continuing research of body perception and Temporal Dynamics of Proprioceptive Hand Model Recalibration 4 peripersonal space (Martel et al.,2016). Specifically, this model would be influential in determining the brain’s plasticity toward new body representations as well as helping to determine how new body representations interact with the long-term representations. The small literature there is on this topic however is conflicting. Zbib et al. (2016) found that proprioceptive recalibration was a slow process, taking over 70 trials for participants to demonstrate a sensory change. This experiment however was examining proprioceptive adaptation specifically to a change in hand orientation, rather than a shift in an entire body part. While never having specifically developed a model of the temporal dynamics for complete hand movements, previous research suggests it to be a rapid process (Ganesh, Yoshioka , Osu, R & Ikegami, 2014). Ganesh et al. (2014) found that when using tools, a new representation of the arm with the tool is developed in only several trials, and that this process was qualitatively different to the representations made after repeated tool use over many trials. Rather than incorporate the tool into the body representation, the arm representation was shortened so that the tool ended where the arm previously did. After examining the rate at which adaptation occurs, it can then be used to examine the interactions between new representations of the body and pre-existing representations. Kitago, Ryan, Mazzoni, Krarkauer and Haith (2013) examined this, specifically looking at how participants returned to their normal body representation after adaptation to a new representation. They suggested that after the adaptation process, rather than simply forgetting the learned representation the original body representation actively replaces it with the long-term representation as the brain is aware this is again the most accurate representation of the body. This study has two main aims. Firstly, we aim to clearly demonstrate how quickly the brain is able to recalibrate proprioceptive models of the hand. While the process has been shown to take a relatively long time (over 70 trials) to recalibrate hand orientation, the speed to taken to recalibrate the absolute position of the hand in space has not been researched Temporal Dynamics of Proprioceptive Hand Model Recalibration 5 (Martel et al., 2016; Zbib et al., 2016). Secondly, the study aims to examine the way in which new proprioceptive models interact with a pre-existing, long term model of the body. Kitago et al. (2013) demonstrated that when de-adapting from a recalibration (reverting back to your normal body representation) it is an active process that involves replacement of the learned model with the original view of the body, rather than merely the passive extinction of the recalibration once adaptation is no longer occurring. This study looks to expand on that research and examine the causal processes for this active replacement. We are looking to determine whether the new hand representation competes with, integrates with or acts independently of the long-term body representation. To do so, participants will wear a VR headset and be asked to track a marker while they cannot see their real or virtual hand over several consecutive trials. Between each trial a period of recalibration will occur, in which the participant can view their virtual hand only, however this hand is offset from their real hand by roughly 10cm. We can then measure whether participants adapt to the virtual position of the hand while doing the tracking task or maintain the real position of their hand and measure the time this process takes. Firstly, we predict that recalibration of the model of the hand will occur such that throughout the 20 trials the distance between the virtual hand and the marker (Position Discrepancy) will decrease. We also predict this process will occur rapidly, such that a change can be detected within five trials. Despite previous research suggesting hand orientation adaptation takes much longer than this, the overwhelming majority of research suggests recalibration of hand position is a very rapid process (Ganesh et al., 2014; Martel et al., 2016). Following from this, we secondly predict that the virtual hand will become closer to the tracker than the real hand throughout the experiment. Finally, we predict that tracking in the second half of trials will be significantly more similar to the real hand than the virtual hand compared to the first half of trials. This stems from the fact that when adaptation is no Temporal Dynamics of Proprioceptive Hand Model Recalibration 6 longer taking place, the active process of reverting to the original body representation begins immediately. As we predict recalibration to occur so rapidly, it follows that de-adaptation will similarly occur very rapidly, potentially over the course of a single trial. Discussion Position discrepancy between the virtual hand and the marker did not decrease throughout the trials. This suggests that participants did not recalibrate their hand representation to be more similar to the virtual hand. Secondly however, it was found that over the first five trials alone there was a significant decrease in position discrepancy between the virtual hand and the marker. This suggests that in the early parts of the experiment recalibration of the hand representation did take place, but over the course of the complete 20 trials it was not sustained. Foremost, this shows that recalibration can occur very rapidly, in this case within five trials, corresponding to only 30 seconds of time in which the virtual hand was present for recalibration to take place (Ruttle, Hart & Henriques, 2018). This shows that the brains representation of the body is extremely plastic and can change very rapidly in order to accommodate changes in the environment (Ganesh et al., 2014). This also suggests that the representations for hand orientation and hand location are distinct, as orientation recalibration takes many more trials than hand location recalibration (Zbib et al., 2016). Furthermore, these results that over the course of the 20 trials the brain did recalibrate its body representation initially but shifted away from this in subsequent trials. When a new body representation is learned it is in conflict with the long-term representation of the body that the brain has, and it actively tries to replace the new representation with the long-term representation when active adaptation is not occurring i.e. when the virtual hand is not present (Kitago et al., 2013). In this experiment, as each adaptation period was much shorter Temporal Dynamics of Proprioceptive Hand Model Recalibration 7 than each trial (6 seconds vs 20 seconds), most time was spent trying to revert the hand model to the original representation. This resulted in the brain not being able to use a representation of the virtual hand, despite adaptation periods between each trial (Huberdeau, Krakauer & Haith, 2015). This suggests that the new models the brain builds of the body compete with the long-term representation of the body (Martel et al., 2016). Furthermore, it shows that to successfully change the representation of the body even only in the short term, recalibration must occur for longer time periods than six seconds per trial. Secondly, we found that over the course of the experiment participant’s virtual hand became comparatively closer to the target than participant’s real hand. However, as discussed above, the virtual hand did not get closer to the target in terms of absolute distance. This means that participant’s real hand moved further away from the target in successive trials. So, despite the virtual hand not moving closer to the target this suggests that participant’s representation of their hand did change. We suggest this may be due to the conflict between the adapted recalibration of the hand and the prior hand representation, resulting in failure to produce an accurately represent either representation (Warren & Cleaves, 1971; Fink et al., 1999). Finally, it was found that in the second half of each trial participant’s real hand was significantly closer to the target than the virtual hand compared to the first half of each trial (Note, no statistical test was run due to a lack of power but this trend can clearly be seen in the last results figure). This suggests that in this second half of each 20 second trial period participants hand representation had already begun to regress back toward their original, long-term hand representation (Huberdeau et al., 2015). In the adaptation period, participants relied on visual information to determine the location of the hand whereas while completing the trials they relied on proprioceptive information. Visual information has been shown to dominate proprioception if the two are directly in conflict (Scheidt, Conditt, Secco & Mussa- Temporal Dynamics of Proprioceptive Hand Model Recalibration 8 Ivaldi, 2005; Cressman & Henriques, 2010). However, this shows that if there is no current visual information being perceived, despite prior adaptation using vision the brain will update its body representation based on proprioception even if this conflicts with previous visual information. Secondly, this provides more evidence for the plasticity of the brain when building body representations, as it is able to both adapt and de-adapt from to changing representations very rapidly, even within the space of a twenty second trial (Ganesh et al., 2014). Lastly, this results also provides further evidence that, as discussed above, the new learned representation of the virtual hand competes with the long-term body representation, as participant’s reverted toward the long-term representation within a single trial (Kitago et al., 2013). A limitation of this paper was that recalibration was only tested over a short, 20 second period while the long-term effects of whether recalibration could be sustained was not tested. We suggest future research to determine the time period for adaptation needed to create longer lasting, sustained effects. A second limitation was that the virtual hand was not present while participants completed the tracking task, only during adaptation. This makes the findings less externally valid, as in real-world situations a change to the body would be present both when trying to recalibrate this change and when performing actions based on a recalibrated representation. Future research should examine what time period of adaptation with the offset virtual hand present is needed for robust, short-term recalibration to take effect. As a new representation is in conflict with the long-term body representation, future research should also examine whether the ratio between adaptation time and tracking time without adaptation effects the ability to recalibrate to a new representation of the hand. Lastly, we suggest researching whether other changes to body representation such as tool use occur with similar temporal dynamics to that found in this paper. Temporal Dynamics of Proprioceptive Hand Model Recalibration Overall, this paper provides strong evidence that the brain rapidly forms new representations of the body when visual changes are detected. However, this changed representation is in conflict with the prior, long-term representation of the body resulting in neither representation being successfully modelled in the short term. 9 Temporal Dynamics of Proprioceptive Hand Model Recalibration 10 References Botvinick, M., & Cohen, J. (1998). Rubber hands “feel” touch that eyes see. Nature, 391(6669), 756–756. https://doi.org/10.1038/35784 Cressman, E. K., & Henriques, D. Y. (2010). Reach adaptation and proprioceptive recalibration following exposure to misaligned sensory input. Journal of neurophysiology, 103(4), 1888-1895. Fink, G. R., Marshall, J. C., Halligan, P. 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