It’s so commonplace that we forget the wonderful complexity of our every move. Picking up a bottle, bringing it to your mouth to drink its contents, even with your eyes closed, presents no difficulty and amazes no one. And yet! This controlled movement is only possible thanks to specialized sensors, called proprioceptors, located in our muscles, tendons and joints. These sensors send information to the brain about the instantaneous position of the limb. Proprioception is a little-known sensory system, but it is essential to perceive where our limbs are and how they move in space", explains Assistant Professor Mario Prsa from the Department of Neuroscience and Movement Sciences of the University of Fribourg (UNIFR). He and his colleagues sought to identify which specific signals are perceived and encoded in the brain when the proprioceptive system is activated.
The body serves as a reference for the movements of its own limbs
To achieve this, neuroscientists at UNIFR studied the behavior of mice trained to perform a perceptual task. We developed a robotic system to deliver well-quantifiable proprioceptive stimuli to the forelimbs of the mice," explains Irina Scheer, a doctoral student in the team of Assistant Professor Prsa. By moving their limbs in different directions, we noticed that the mice make a surprising distinction between movements away from their body and those towards it.’ Mario Prsa’s team was also able to demonstrate that proprioceptive signals travel from the muscles of the mouse’s forelimbs to the cortex and identified the cortical regions that process the consciously perceived proprioceptive stimuli.
What do the neurons code for?
In a second set of experiments, the researchers used two-photon microscopy to visualize the activity of hundreds of neurons in the proprioceptive cortical areas previously identified. ’We observed that these neurons encode limb proprioception in terms of direction of movement rather than spatial position or posture,’ explains Ignacio Alonso, a PhD student and co-first author of the study. However, not all directions are equally represented and, surprisingly, this non-uniform representation is not organized along the limb axis but rather in terms of the body axis.’ These results, published in the journal Nature Communications, therefore suggest that mice do not perceive their limbs as flexed or extended, but rather as moving toward or away from their own bodies.
A model for bidirectional neuroprostheses
If a neuroprosthesis is to completely replace a paralyzed or amputated limb, it must be able to send sensory signals back to the brain to mimic proprioception. A key question is what properties of motion should these next-generation devices take into account when designing the stimulation parameters? ’Our findings suggest that stimulation paradigms should be based on how the artificial arm moves relative to relevant targets, such as the patient’s own body,’ explains Mario Prsa. ’Beyond more precise bidirectional neuroprostheses, this area of research can help us better understand the often bizarre nature of proprioceptive disorders and inspire new therapeutic approaches.’
> Alonso I, Scheer I, Palacio-Manzano M, Frézel-Jacob N, Philippides A, Prsa M:
Peripersonal encoding of forelimb proprioception in the mouse somatosensory cortex
