Experiment 2: Tactile localization across posture changes of artificial hands

How can the RHI be used as a means of investigating the localization of tactile stimuli to the body using an external frame of reference? In the above study (Experiment 1; Begum Ali, Cowie & Bremner, 2014), it was found that vision of the hands interfered with tactile localization, especially in the uncrossed hands posture. I go onto test this idea further in an experiment where it was possible to more directly manipulate visual cues to hand position, via artificial hands.

As previously mentioned, the RHI is an illusion in which bodily sensations of touches to the hand are referred to a limb like object which is nonetheless external to the body. Botvinick and Cohen (1998) were the first to empirically investigate the RHI using a series of studies in a group of healthy adults. Here, participants were seated with their hands on a table, their left hand and arm hidden from view by a screen and an artificial hand and arm in the space their left hand would usually occupy.

Following this, the participant’s left hand was stroked with a paintbrush, in synchrony with a paintbrush that was used to stroke the artificial hand;

thus they were receiving tactile information from the felt touch to their real hand and visual information from the stroking of the fake hand they were fixating on. Both before and after this period of synchronous stroking, participants were asked to close their eyes and, using their right hand, underneath the table point to the location of the index finger of their

left hand. It was found that participants demonstrated a drift in their proprioceptive sensations of where their real hand was, in that after the illusion was induced, their judgements of where their index finger lay in space was displaced towards the artificial hand. Additionally, participants quickly began to gain a sense of ownership over this alien limb, often reporting that the artificial hand “felt like [their] own” (Botvinick &

Cohen, 1998, p. 756).

Interestingly, the intermanual reach displacement only occurred when participants’ viewed synchronous stroking of their real and the fake hand. If there was a disparity in the timings of the seen and the felt touch (i.e. an asynchrony between visual tactile information), the illusion was not induced and did not affect participant’s accuracy in locating the index finger of their left hand. Further conditions, that are a requisite in order to induce the illusion, include that the artificial limb should be in a similar orientation as participant’s real hand (Ehrsson, Spence & Passingham, 2004; Tsakiris & Haggard, 2005), the size of the artificial limb cannot be smaller than participant’s own hands (Pavani & Zampini, 2007; although see Bruno & Bertamini, 2010) and the distance between the real and artificial hand must not be greater than 30 cm (Lloyd, 2007).

The RHI has also been demonstrated in a variety of experimental situations, indicating the robust nature of the illusion in adult populations. For example, the illusion prevails even when there is not an alien limb, but an empty space receives a tactile stimulus (Guterstam, Gentile & Ehrsson, 2013), when the artificial limb is larger than an

individual’s own hand (Pavani & Zampini, 2007) or when the skin tone of the fake hand differs (Farmer, Tajadura-Jiménez & Tsakiris, 2012). The RHI demonstrates that intermodal correspondences (particularly between a seen and a felt touch, in this case) can lead to individuals accepting non-limb like objects within the body schema. However, there are a number of certain conditions that first need to be met (outlined above). These conditions relate to acceptable degrees of freedom of the body; for example, individuals can accept a larger artificial hand as their own, but not a smaller hand – your hand can grow in size, but it cannot shrink.

Ultimately, the RHI has illustrated the flexible nature of the way in which individuals construct the body schema.

As mentioned above, the majority of RHI studies have been conducted on an adult population. However, recently there has been much interest in the developmental trajectory of visual-tactile correspondences and how children use these to infer knowledge of their bodies. Cowie, Makin and Bremner (2013) conducted a rubber hand study with children aged 4-9 years. As expected, the authors found that children were susceptible to the rubber hand illusion when presented with contingent visual-tactile information, by way of proprioceptive drift towards, and a sense of ownership of, the fake hand. A particularly important conclusion drawn from this study was that children are more susceptible to the RHI and demonstrate more of the illusion than adults. From this, it could be understood that young children rely more on visual information to depict hand position than adults.

One way to assess the exact role of current vision of limbs in tactile localisation is to introduce a conflict between visual and proprioceptive information (as has been previously done in RHI studies).

Azañón and Soto-Faraco (2007) did just that; in their study participants’

hands were placed in either the uncrossed or crossed-hands posture whilst they viewed rubber hands in a posture that either corresponded with the posture of their real hands, or was incongruent. In a tactile TOJ task (Yamamoto & Kitazawa, 2001a; Shore et al., 2002; Pagel et al., 2009), participants then had to judge the location of the first of two tactile stimuli to the hands.

The researchers replicated previous findings showing that participants’ accuracy and response speed was impaired when their hands were crossed (relative to their performance in the uncrossed posture). The novel finding in this study was that participants demonstrated a decreased crossed-hands deficit (with improved accuracy and shorter response times in the crossed-hands posture) when individuals viewed a pair of uncrossed rubber hands, above their own hidden crossed hands.

A question to consider is if this pattern of findings would also emerge for developmental populations. As mentioned previously, from the findings of Azañón and Soto-Faraco (2007) and my findings in Experiment 1 (Begum Ali et al., 2014), two potential hypotheses arise. First, it is possible that the crossed-hands deficit will be eliminated due to better performance in the crossed posture when viewing uncrossed rubber hands (as in Azañón and Soto-Faraco, 2007). An alternative (and not necessarily

opposing) hypothesis is that visual information of the rubber limbs in the uncrossed-hands posture, whilst children’s hands are also in the uncrossed-hands posture, will impair localisation accuracy in this posture, eliminating the crossed-hands deficit in children as the difference in accuracy between postures would be reduced.

In order to investigate and disentangle potential explanations for the role of vision of current posture in localising tactile stimuli to the hands, I ran a variation of Experiment 1, this time with artificial hands in either a corresponding or an incongruous hands posture or without artificial hands at all. The first condition was a direct replication of the Hands Covered condition in Experiment 1 and I expected to replicate those findings (i.e. 4-year-olds would demonstrate a crossed-hands deficit, with a poorer localisation accuracy in the crossed-hands posture relative to the uncrossed-hands posture). Further to this, given the findings and conclusions of Experiment 1, I expected that when children had vision of uncrossed artificial hands, with their own hands in the corresponding posture, their tactile localisation accuracy would be impaired relative to the situation in which they could not see their own hands.

3.6 Methods