More than just one 'body representation' in our mind: Tohoku University overturns a theory that has been accepted for more than 100 years

Prof. Kazumichi Matsumiya of the Graduate School of Information Sciences, Tohoku University revealed that the traditional assumption of more than a century is incorrect and instead of just one "mental body (representation of our body in our brain)," our mind and brain actually have multiple representations of our body within them. He developed a method to assess the "mental body" using VR technology. He examined how subjects perceived their right hand with either their left hand or their eyes. Consequently, he could experimentally demonstrate that the right hand recognized by the left hand differs from that recognized by the eyes. These findings will likely modify the long-held theories of the mind and body. In addition, new diagnostic technology is expected to emerge for motor dysfunction to enhance the effect of rehabilitation. The results were published in the January 25^th issue of PNAS.

Japan has the world's most aged society and has entered a super-aging society. The increase in the number of patients with motor paralysis due to aging-related motor dysfunction and stroke has become a social problem. Patients with motor paralysis are thought to have a discrepancy between their "mental body" and their actual physical body. Generally, patients with motor paralysis show a certain degree of recovery after undergoing rehabilitation. However, after returning to their society, their physical function often deteriorates once again. This may occur because current rehabilitation does not consider the recovery of the "mental body". Moreover, abnormalities of the "mental body" are largely subjective impressions in the mind of patients. Hence, a complete understanding of these pathological conditions has been difficult.

Based on this background, Dr. Matsumiya developed a method to assess the "mental body" by performing multiple exercises using VR technology. Twelve healthy college students in their 20s participated as subjects. Subjects wore a head-mounted display with an eye tracker and placed their right hand under a transparent board with a position sensor. While unable to view their right hand, they were asked to demonstrate it using their left hand by positioning it on their right fingertips and joints. Next, they were asked to demonstrate it using their eyes. By plotting the points indicated by both the left hand and eyes, the "mental body shape of the right hand" for each movement was visualized and estimated.

To date, such "mental" measurements have been performed on a single exercise. However, Prof. Matsumiya reproduced and measured the state in which a person performs multiple exercises that are universally performed in daily life simultaneously. Hence, although the eyes and left hand were supposed to point to the same position, they actually indicated different positions, i.e., they showed different right-hand shapes. Both were compressed from the actual right hand. For example, the finger length was compressed by about 15% for the eyes and about 30% for the left hand.

These results indicate that at least the eyes and left hand are represented as separate bodies in the mind/brain, respectively. Each part (motor effector) may have its own "mental body" representation in the brain. The conception of the human body as having a corresponding "mental body" was proposed in 1911. Only one single representation, commonly used for all exercises, has been thought to be there in the brain. In contrast, the results of this study indicate that "the mental body" differs with different exercise effectors. This result overturns the conventional knowledge on this topic. Current rehabilitation practices do not consider the "mental body" and instead rely on experiential heuristics learned by physical therapists over time. If the abnormalities of the "mental body" in patients with motor dysfunction can be visualized and reflected in rehabilitation, an effective treatment is expected to emerge.