Summary: A new study shows that rats, like humans, can experience a sense of physical ownership of a prosthetic limb, similar to the familiar illusion of a rubber hand. When their natural limb was hidden and the two limbs were stroked together, the rats visually registered a threat to the prosthetic limb, indicating a sense of ownership.
This discovery creates a powerful animal model for investigating the brain mechanisms responsible for body ownership perception and prosthetic integration. It could pave the way for advanced neuroprosthetics and treatments for conditions such as phantom limb pain or body dysmorphia.
Key data:
- Rubber hand illusion in rats: Rats responded to threats from fake limbs when tactile and visual cues matched.
- Visual tracking reveals integration: Rats focused their gaze on the threatened prosthetic limb if it matched their own limb and moved in sync.
- New model for research: This study provides the first practical rodent model to study body ownership and prosthetic limb integration.
Source: PLOS
According to a study published June 5 in the open access journal PLOS Biology by Luc Estebanez of the CNRS : Centre National de la Recherche Scientifique in France and his colleagues.
We see our limbs as part of ourselves and feel threatened by any threat that affects them. This sense of embodiment can be compromised by brain damage, causing us to no longer recognize body parts and even actively reject their representation.
In amputees, effective use of prosthetic limbs can be hindered by a lack of integration. This gradually leads to a decrease in daily use of the prosthesis, which ultimately leads to neglect.
In contrast, prosthetic limb placement is associated with a reduction in phantom sensations of the missing limb, including the sensation of pain.
In an experimental setting, it is possible to construct or alter the sense of physicality of a prosthetic limb.
In the rubber hand illusion experiment, a rubber limb is placed so that it is visible to the participant, while the real hand remains hidden. Both the hidden real hand and the visible prosthetic hand are touched simultaneously with a brush.
A large proportion of participants in these experiments indicate that the rubber hand they see after this stimulus is their real hand. To date, the physiological basis of the sensory perception of frontal shape remains unclear, partly due to the lack of animal models to study this phenomenon.
To address this challenge, Estebanez and his colleagues developed an analogous protocol for the rubber hand illusion in rats.
The researchers presented the mice with a static, 3D-printed, prosthetic replica of their right arm in an anatomically plausible position with their heads facing up. At the same time, their real limb remained hidden and was placed in place under the platform where the prosthetic leg was located.
During a two-minute mating session, the researchers applied mechanically controlled synchronous or asynchronous brush strokes to the real and prosthetic limbs. They then dropped a sharp object on the prosthetic forehead within the animals’ field of vision.
During this sequence, the rats’ gaze was tracked using high-speed videography. Consistent with the existing literature on rubber hand brum, rats kept their gaze focused on the threat for several seconds when the brush strokes were synchronous, but not when they were inconsistent.
Furthermore, the behavioral response was more intense when the prosthetic limb resembled a real limb compared to a white, cube-shaped object.
According to the authors, this work has practical implications for studying prefrontal cortex in a model that offers unprecedented experimental possibilities. For example, mapping the brain circuits of the cortex using genetic and optogenetic research methods could lead to new strategies for restoring cortex.
Furthermore, the study also supports the theory that rodents can exhibit behavioral correlates of embodiment in environments known to trigger it in humans. This is consistent with several recent findings suggesting that rodents, and rats in particular, can exhibit some of the skills associated with higher cognitive functions.
The authors added: “The rubber hand illusion is a fundamental element in the study of body representation in humans, but until now there was no equivalent to this test in the most practical model: rats.
“Here we have shown that the absorption of elements in organs can be effectively activated and measured in mice.
“This opens up many possibilities for better understanding and applying the fundamental mechanisms of body ownership, for example to improve prosthetics.”
Funding: This work was funded by LE PRC Hermin, ANR (https://anr.fr/) for LE, JCJC Mesobrain, ANR (https://anr.fr/) for LE, PRC Expect, ANR (https://anr.fr/) for DES, PRC PerBaCo, ANR (https://anr.fr/) for LE, PRC PerBaCo, ANR (https://anr.fr/) for LE, 80 RISE iNavigate (873178) and the Horizon 2020 Framework Programme. LE from https://cordis.europa.eu/project/id/873178 to DES, OI hCODE, Université Paris-Saclay, https://www.unversite-paris-saclay.fr/objets-interdisciplinarys/h-code.
The funders were not involved in the study design, data collection and analysis, preparation of the manuscript, or decision to publish.
About this neuroprosthetics and neuroscience research news
Author: Claire Turner
Source: PLOS
Contact: Claire Turner – PLOS
Image: The image is credited to StackZone Neuro
Original Research: Open access.
“Embodiment of an artificial limb in mice” by Luc Estebanez et al. PLOS Biology
Abstract
Implantation of artificial limbs in mice
Disturbances in body perception can be caused by disease or physical injury. To address these impairments, methods are needed that investigate the integration of organs within the body. This includes the development of neuroprosthetics that better align with the user’s body schema.
To this end, the rubber hand illusion protocol is an important behavioral method for powerful sculptural exploration that can be activated by coordinated somatosensory and visual limb inputs.
To date, the neurophysiology of limb-body perception is poorly understood. This is partly because translating the rubber hand illusion into animal models such as mice remains a challenge.
However, mapping the brain circuits of physicality using genetic and optogenetic research tools will provide an opportunity to propose new strategies for restoring physicality.
Here we demonstrate that the rubber hand illusion as described in humans can be transferred to a mouse forelimb model using an automated procedure based on videography.
We exposed mice to a visible, static, 3D-printed replica of their right and left limbs with their heads facing away from them, while their own forelimbs remained hidden. We brushed both the hidden limb and the replica simultaneously.
Following these visuotactile associations, the replica was visually threatened and we analyzed the mice’s responses using automatic pupil and facial expression tracking.
Rats focused their gaze significantly more toward the threatened forelimb counterpart after receiving synchronous tactile and visual input compared to asynchronous input.
In general, the mouse pupillary response, in both test and control conditions, was consistent with the apparent human response to the rubber hand illusion.
Thus, our results suggest that rats exhibit quantitative behavioral signatures capable of artificial forelimb integration.
