ゴム手の幻想

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.

Thanks for watching!