Prosthesis embodiment and attenuation of prosthetic touch in upper limb amputees – A proof-of-concept study

Sensory attenuation of self-touch, that is, the perceptual reduction of self-generated tactile stimuli, is considered a neurocognitive basis for self-other distinction. However, whether this effect can also be found in upper limb amputees using a prosthesis is unknown. Thirteen participants were asked to touch their foot sole with a) their intact hand (self-touch), b) their prosthesis (prosthesis-touch), or c) let it be touched by another person (other-touch). Intensity of touch was assessed with a questionnaire. In addition, prosthesis embodiment was assessed in nine participants. Self-touch as well as prosthesis-touch was characterized by significant perceptual attenuation compared to other-touch, while self- and prosthesis-touch did not differ. The more embodied the prosthesis was, the more similar was its elicited touch perception to actual self-touch. These findings – although preliminary – suggest that perceptually embodied prostheses can be represented as an actual limb by the users’ sensorimotor system.     

Two hands are better than one: a new assessment method and a new interpretation of the non-visual illusion of self-touch

A simple experimental paradigm creates the powerful illusion that one is touching one’s own hand even when the two hands are separated by 15 cm. The participant uses her right hand to administer stimulation to a prosthetic hand while the Examiner provides identical stimulation to the participant’s receptive left hand. Change in felt position of the receptive hand toward the prosthetic hand has previously led to the interpretation that the participant experiences self-touch at the location of the prosthetic hand, and experiences a sense of ownership of the prosthetic hand. Our results argue against this interpretation. We assessed change in felt position of the participant’s receptive hand but we also assessed change in felt position of the participant’s administering hand. Change in felt position of the administering hand was significantly greater than change in felt position of the receptive hand. Implications for theories of ownership are discussed.     

Spatial limits on the nonvisual self-touch illusion and the visual rubber hand illusion: Subjective experience of the illusion and proprioceptive drift

The nonvisual self-touch rubber hand paradigm elicits the compelling illusion that one is touching one’s own hand even though the two hands are not in contact. In four experiments, we investigated spatial limits of distance (15 cm, 30 cm, 45 cm, 60 cm) and alignment (0°, 90° anti-clockwise) on the nonvisual self-touch illusion and the well-known visual rubber hand illusion. Common procedures (synchronous and asynchronous stimulation administered for 60 s with the prosthetic hand at body midline) and common assessment methods were used. Subjective experience of the illusion was assessed by agreement ratings for statements on a questionnaire and time of illusion onset. The nonvisual self-touch illusion was diminished though never abolished by distance and alignment manipulations, whereas the visual rubber hand illusion was more robust against these manipulations. We assessed proprioceptive drift, and implications of a double dissociation between subjective experience of the illusion and proprioceptive drift are discussed.     

Attention focus and self-touch in toddlers: The moderating effect of attachment security

The superior self-regulation and attention-regulation abilities of securely attached children have been repeatedly demonstrated. However, the mechanisms that allow securely attached children to exhibit higher levels of attention focus than insecurely attached (anxious-ambivalent and anxious-avoidant) children need to be explored. One possible mechanism that has been hypothesized to play a role in focusing attention is self-touch. Previous research has shown that 10-year-old children exhibit more bilateral self-touch (i.e., both hands are simultaneously moving onto each other or on the body, and both hands are in contact with each other or with the body), but not lateral self-touch (i.e., one hand is moving on the other hand or on the body, and the hand is in contact with the other hand or with the body), when they focus attention on a task. Because bilateral coordination is still developing during childhood, we expected that lateral self-touch, instead of bilateral self-touch, may be associated with attention focus for toddlers. The objectives of the present study were to examine whether securely attached toddlers exhibit more self-touch, particularly lateral self-touch, while they focus on a task than while they do not focus on a task. We expected to find that the association between lateral self-touch and attention focus is not as strong for insecurely attached toddlers. Data from forty-nine mother-child dyads were employed for analyses. The attachment classification of the children was determined using the Strange Situation. The duration of attention focus and self-touch behavior during a reading task were coded. An association between lateral self-touch and attention focus was found for children of all attachment classifications. This association was particularly strong for securely attached children. We discuss the possibility that securely attached toddlers may use lateral self-touch to regulate attention.     

The spatial specificity of sensory attenuation for self-touch

Self-touch is sensed less intense as touch produced by another person. According to the standard explanation, if predicted and actual sensations match, the intensity of the touch will be reduced. Here, we asked whether sensory attenuation is spatially specific. To this end, participants laid their left hand under a metal arc on which a force sensor was mounted. Pressing the sensor caused one of two motors to rotate a lever that either touched the index or the ring finger. We took care that no cues would reveal in advance which of the motors would move in order to leave participants uninformed about the finger that will be stimulated. Any reduction in felt intensity of the touch could therefore be induced only by the efference copy of the touching movement. We found strong spatial specificity of sensory attenuation of self-touch, selecting between two fingers of the same hand.