Source: Laboratory of Jonathan Flombaum—Johns Hopkins University
Reaching for objects, walking without hitting obstacles, landing on a chair as you sit (instead of falling to the floor), these and all our physical actions depend on an ability to perceive our own bodies in space, to know where our limbs are relative to one another and relative to the rest of the world. One way that the human brain encodes this information is called proprioception, the brain relies on its own control and feedback signals to keep track of limbs. Along with proprioceptive inputs, the human brain incorporates vision, touch, and even sound in order to represent the parts of the body in space. How does it combine all this information? In 1998, Botvinick and Cohen described a striking illusion, called the Rubber Hand Illusion, that has been used to investigate how the human brain integrates sensory and proprioceptive inputs to represent the body in space.1 This video will demonstrate how to induce the Rubber Hand Illusion and it will describe how it has been used by subsequent studies.
- This experiment requires three critical pieces of equipment: A rubber hand, two paintbrushes, and an occluder box.
- To build the occluder box, you will need a piece of cardboard that is about 1 ft high and 2 ft long. Draw a straight line down the middle, and then in the middle of each of the two squares, cut a tunnel large enough for a hand and arm to pass through. Figure 1 shows a schematic drawing of this portion of the box, as it would be seen from the point-of-view of the participant in the task.
Figure 1: Schematic drawing of occluder box seen from the point-of-view of the participant. The two holes in the cardboard wall are large enough for the participant to comfortably insert an arm. Please click here to view a larger version of this figure.
- Next, on one side of the cardboard box attach an opaque top along with an opaque divider between the two halves of the box. The participant's real arm will go into this part of the box, where it will be visually occluded from the participant. Figure 2 shows a schematic drawing of the completed box with this attachment on it, as it would be seen from the point-of-view of the person conducting the experiment.
Figure 2: Schematic drawing of occluder box seen from the point-of-view of the experimenter. The two holes in the cardboard wall are large enough for the participant to comfortably insert an arm. The side with the opaque top is the side in which the participant will insert her real arm, allowing the experimenter to brush it during the experiment. The other side will be where the rubber arm will sit during the experiment. Please click here to view a larger version of this figure.
- Finally, you will need to make survey in order to assess the extent to which your participant experienced the illusion. Figure 3 is such a survey, modelled directly on the methods used by Botvinick and Cohen.1
Figure 3: Survey questions with scales. The survey is used after the experiment to assess the extent to which the participant experienced the illusion. Please click here to view a larger version of this figure.
2. Inducing the illusion
- To induce the illusion in a participant, seat her at a table in front of the flat side of the occluder box. The box should be placed so that she can insert her left arm comfortably into the hole furthest from her left shoulder.
- Once the participant is seated comfortably and with her arm inserted, place the rubber arm so that it exits from the hole closer to the participant.
- Instruct the participant to move her left arm and fingers as little as possible.
- Ask the participant to look over the wall of the occluder. Her own arm will not be visible, as it will be inside of the box portion of the occluder box. But the rubber arm should be fully visible to her.
- Now sit in front of the participant, and slowly brush both her real hand and the rubber hand with the paintbrushes. Try to brush in synchrony. Continue brushing for 10 min.
- The participant may react during the brushing period, exclaiming that she feels like the rubber hand is her own. Let the participant know that such experiences are normal in the context of the experiment.
- After 10 min, remove the box and the rubber arm, and ask the participant to complete the survey.
Illusions are often used in psychology to test processes of perception; as it turns out, tricking the brain is rather easy.
Under normal circumstances, individuals walk around without bumping into any obstacles, because they know where their limbs are relative to items in the surroundings. This concept of body awareness is referred to as proprioception.
However, even with this understanding, the same person can be deceived into thinking that someone else’s arm—like the mannequin’s situated close by—is their own and react accordingly.
This video will demonstrate how to induce this body transfer trick, called the Rubber Hand Illusion—where a fake limb is perceived as being real—using methods originally devised by Botvinick and Cohen. It will also investigate how such an experience can be applied, for instance, to the treatment of phantom limb pain.
In this experiment, participants are asked to rest one arm on a table and a box is placed over it, occluding the limb from being visible. However, the other side is open, and a fake rubber hand is placed in direct sight.
As participants stare at the life-sized model, both appendages are lightly stroked with two paintbrushes in synchrony over a period of 10 min.
Afterwards, they are asked to complete a short survey about their experiences—rating how much they agree or disagree with different perceptual effects. Their responses on the sliding scale serve as the dependent variable and ultimately reveal whether or not the illusion was induced.
Participants are expected to feel like the rubber hand was their own during the brushing period. Yet, they are not expected to think that it looks similar to their own in appearance. Thus, vision plays an important role in our sense of touch and body position, but these do not influence visual representations in the same manner.
In preparation for the experiment, obtain the following materials: a rubber hand, two paint brushes, scissors, tape, and several pieces of cardboard that are 1 ft high by 2 ft long.
First build the occluder box: Take one piece of cardboard and draw a straight line down the middle of the longest side. At the bottom center of each half, cut a circle large enough for a hand and arm to pass through. Then, using tape, attach a second piece at the mid-point to create a divider. Finally, add the last section of cardboard across the top.
Before proceeding, create a survey, like the one used by Botvinick and Cohen, to extensively assess the participants’ subjective experiences.
Now, to begin the experiment, seat the participant at a table in front of the flat side of the occluder box. Have them insert their left arm into the hole directly ahead, and ask them to refrain from moving their arm and fingers as much as possible.
Next, place the rubber arm through the hole on their right side. Instruct the participant to look over the wall of the occluder box and focus on this artificial part.
Then, sit in front of the participant, and use the two paintbrushes to simultaneously touch their real and rubber hand for 10 min. If they react during the brushing period, inform them that such experiences are normal for this experiment.
Following the tactile phase, remove the box and rubber arm from the table, and ask them to complete the survey, rating nine statements on a scale of 'strongly disagree' to 'strongly agree'.
For each participant, determine whether or not the illusion was induced. To accomplish this, examine the surveys individually and initially focus on the first three items.
Notice that the participant shown here strongly agreed that they could feel the brushing on the rubber hand as if it were their own, indicating that their brain was tricked.
To see if proprioception was affected, look at the next four items: Questions 4 through 6 and 8. Note that responses were made towards 'strongly disagree', which suggests that they were still very aware of their own arms in space.
Furthermore, from the responses on the remaining questions—7 and 9—the participant also disagreed that the rubber hand began to look like their own in appearance. Overall, these results suggest that although vision influences our sense of touch and body position, the converse does not necessarily hold true.
Now that you are familiar with how to conduct the rubber hand illusion, let’s look at some other ways researchers use it to better understand how the brain integrates information related to vision, touch, and proprioception.
To understand what’s going on in the brain during the illusion, researchers exposed participants to the task while undergoing functional MRI. In this case, the premotor cortex—an area used to control motor actions—was the region of focus.
Activity from the synchronous condition was compared to an asynchronous one—where brushing doesn’t induce the illusion. They found that when the brain was tricked, there was greater activation relative to when it was not deceived.
Such observations suggest that neural activity in the premotor cortex is associated with one’s sense of their own body. Anatomically, this makes sense: the region is connected to visual and somatosensory areas, particularly the posterior parietal cortex, providing an anchor between visual, tactile, and proprioceptive information.
Understanding the neural underpinnings of the rubber hand illusion can also help to treat disorders where body ownership is distorted, as is the case in schizophrenia. In these patients, the illusion is stronger, with faster induction and increased perceptual reports, even during sensory asynchrony.
Interestingly, these effects can be mimicked in healthy individuals by administering drugs like ketamine or amphetamine, providing another approach for studying the neural mechanisms behind body ownership.
Finally, under certain circumstances, the illusion can be used therapeutically to treat individuals with Phantom Limb Pain, which occurs when amputees still have feelings in the body part that no longer exists.
Using mirrors, their brains can be tricked into seeing two complete limbs. This approach could ultimately help to reorganize the connections within the related multi-sensory pathways and alleviate pain.
You’ve just watched JoVE’s video on the Rubber Hand Illusion. Now you should have a good understanding of how to conduct this experiment to investigate the brain’s perception of the body in space, as well as how to interpret survey results from the participants’ experiences. In addition, you should also know more about the brain regions related to body ownership and the complexity involved in multisensory integration.
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Figure 4 shows typical survey results for one participant. In the first three items, a participant tends to strongly agree that the rubber hand felt like her own and that it felt like she could feel the brushing on the rubber hand. These results suggest that the visual perception of the rubber hand-in the place where her actual hand should have been-induced her brain to assimilate the rubber hand into its representation of her body. Moreover, she experienced brushing although the rubber hand obviously has no touch receptors. Thus the visual seeing of brushing, in this context, is sufficient to induce the brain to produce sensations of brushing. That is an important part of the effect-touch can be felt without actual touching of the skin, at least under some conditions. Visual inputs play a surprisingly strong role in our sense of our bodies.
Figure 4: Typical survey responses.
The remaining items in the survey demonstrate that the opposite is not true. People tend to disagree with statements that suggest that their visual representation of the rubber hand began to change. In other words, feeling it to be their own does not make it look like their own in appearance. So vision plays an important role in our sense of touch and body position, but touch and body position do not influence vision in the same way.
Applications and Summary
The rubber hand is a strange and striking illusion that has begun to play an important role in our understanding of how the brain integrates information from multiple sensory systems. An important study by Ehrsson and colleagues (2004), for example, induced the rubber hand illusion in much the same way just described, but with participants simultaneously undergoing fMRI.2 For a point of comparison, the researchers used a condition in which they brushed the rubber and actual hands of their participants asynchronously. This does not usually produce an experience of the illusion. They could then compare brain activity in this condition to brain activity during the usual, synchronous stroking condition. The result was that the synchronous condition produced greater activity in the premotor cortex. The premotor cortex is a part of the brain that is used to control motor actions. Activity is usually found in this area before someone executes an action. This led the authors to conclude that because the premotor cortex is the site of action planning, in some sense, it is the main site of representation for one's sense of their own body. As a result, it is also the site where information about one's body from different sources becomes integrated.
- Botvinick, M., Cohen, J. Rubber hands 'feel' touch that eyes see. Nature. (1998).
- Ehrsson, H.H., Spence, C., Passingham, R.E. That's my hand! Activity in premotor cortex reflects feeling of ownership of a limb. Science. (2004).