Microneurography and Intraneural Microstimulation for Upper Limb Sensory Investigation in Healthy and Amputee Subjects (MICROAMP)

Participants will attend the laboratory where the entire research protocol will be conducted in a single visit lasting up to 4 hours.

Medical history and a physical examination will be collected. In participants with amputation, additional clinical information from previous neurological assessments will be reviewed (including medical records, nerve conduction velocity data, and ultrasound or MRI examinations required to verify inclusion/exclusion criteria). Psychometric tests will also be administered to assess language abilities, visuospatial skills, and general intelligence, ensuring that participants are able to understand and perform the experimental tasks. If inclusion and exclusion criteria are met, participants will undergo microneurography and/or intraneural microstimulation procedures. Participants will be seated comfortably, with the dominant arm (healthy volunteers) or the residual limb (participants with amputation) supported. A trained medical operator will identify the median nerve using external stimulation and/or ultrasound guidance to assist electrode insertion. The active electrode will be inserted into the nerve, while a reference electrode will be placed in the subcutaneous tissue a few centimeters away. Correct electrode placement will then be refined using intraneural stimulation. Once the nerve fascicle is reached, tactile stimulation of the hand will be used to identify single mechanoreceptive units. Final electrode placement will be confirmed by visual and auditory inspection of the neural signal. The median nerve activity must be clearly distinguishable from background activity when mechanical stimuli are applied to the skin. During electrode insertion, participants will be instructed to remain still until the procedure is completed. The intraneural microstimulation session, performed in both healthy volunteers and participants with upper-limb amputation, is preceded by peripheral neural recordings using microneurography during the presentation of tactile stimuli (e.g., Von Frey filaments). This step is used for accurate nerve localization and preliminary assessment of the receptors and their receptive fields. Once the neural target is identified, participants undergo intraneural stimulation involving the perception and recognition of artificially generated tactile stimuli. These stimuli are delivered via intraneural microstimulation and are derived from mechanoneural transduction, using a sensorized artificial finger interacting with different surfaces to encode surface-dependent tactile information. Participants will be asked to recognize different properties of the presented stimuli. During the tests, electroencephalographic (EEG) activity may be recorded using a 128-channel EEG system, synchronized with the onset of stimulation.