The Digital Hand Illusion

The brief:

This study looks for an answer to the question: Can people take ownership of a virtual limb which is not their own?

Our ability to adopt foreign body parts, as being part of our own body whether physical or virtual is a curious phenomenon, like many aspects of self-consciousness. We instinctively understand our hands as belonging to our own body, but the mechanisms behind the cognitive processes relating to this sense of body ownership and self-attribution are still not fully understood.

This study reproduces and builds upon the Rubber Hand Illusion (Botvinick & Cohen, 1998), where subjects report the curious sensation of feeling an artificial rubber hand as being their own, when watching the rubber hand being stroked with a paintbrush at the same time as their own hidden hand.

What I did:

I conducted a study where 62 volunteers were split into two groups of 32 (experiment 1) and 30 (experiment 2). During both experiments subjects sat upright, in front of a table, with a tape measure affixed to the nearside edge. A drift measure was recorded using the tape measure before each condition was applied to measure the drift toward or away from the fake limb. This drift measure, often referred to as proprioceptive drift (Ehrsson, et al., 2005) was recorded three times before and after each condition.

During each condition subjects observed a ‘false’ hand located in front of them for a period of 5 minutes, while their own ‘real’ left hand was situated palm face down on the table and hidden from view. During this period, the subject’s real hand was stoked with a paintbrush, at the same time as they observed the false hand being stroked with an identical paintbrush either at the same time (synchronous) or asynchronous control (out of time) depending on the chosen condition.

After 5 minutes had elapsed the drift measures were recorded once more, before subjects answered open questions about their experience. Lastly subjects completed a questionnaire. Two experiments were conducted as follows.

Experiment 1

Participants viewed a rubber hand in place of their own.

Experiment 2

Participants viewed a virtual hand in place of their own.

Virtual hand experiment 2

Stereoscopic virtual image

The outcome:

You can read the Digital Rubber Hand Illusion (PDF), but in summary I conducted an ‘Independent one-tailed t-test’ to provide the necessary inferential statistics using a standard alpha of 0.05, with the null hypothesis being there would be no significant difference in the drift toward the fake hand (rubber or virtual) for the control and experimental group.

Two experiments were conducted. The first investigating if the results from the original ‘Rubber Hand Illusion’ could be successfully reproduced using the same experimental design. The results corroborated the original findings with a mean drift result of 40mm toward the rubber hand for the experimental group; with t(30) = -2.58, p = .007, d = 0.94 (one-tailed t-test).

The second experiment investigated the constraints of the phenomena. Exploring if the illusion occurred using an anatomically correct virtual hand. Resolution of the virtual hand was set to 55% for both groups. The asynchronous stroking delay was set to 300ms for the control group. Results appear to show the minimal parameters applied were sufficient in eliciting the illusion using a virtual hand. Using a delay of 300ms in the asynchronous stroking is sufficient to break the illusion, with a mean drift of -10mm away from the virtual hand for the control group compared to a 29mm drift toward the virtual hand for the experimental group; with t(28) = -5.61, p < .001, d = 2.12 (one-tailed t-test).

Visit my Rubber Hand Illusion website and try it for yourself!

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