Dynamic changes in the perceived posture of the hand during ischaemic anaesthesia of the arm

Abstract

Non-technical summary Even when the hand is stationary we know its position. This information is needed by the brain to plan movements. If the sensory input from a limb is removed through an accident, or an experiment with local anaesthesia, then a 'phantom' limb commonly develops. We used ischaemic anaesthesia of one arm to study the mechanisms which define the phantom hand. Surprisingly, if the wrist and fingers are held straight during anaesthesia, the perceived phantom hand becomes bent at the wrist and fingers, but if they are bent during anaesthesia, the final phantom is extended at the wrist and fingers. There is no 'default' posture for the phantom hand. Further, the hand appears to increase gradually in size as anaesthesia develops. The start of these perceptual changes occurs when input from large-diameter sensory nerve fibres is declining. These results provide new information about how the brain generates phantom limbs.Abstract Contorted 'phantom' limbs often form when sensory inputs are removed, but the neural mechanisms underlying their formation are poorly understood. We tracked the evolution of an experimental phantom hand during ischaemic anaesthesia of the arm. In the first study subjects showed the perceived posture of their hand and fingers using a model hand. Surprisingly, if the wrist and fingers were held straight before and during anaesthesia, the final phantom hand was bent at the wrist and fingers, but if the wrist and fingers were flexed before and during anaesthesia, the final phantom was extended at wrist and fingers. Hence, no 'default' posture existed for the phantom hand. The final perceived posture may depend on the initial and evolving sensory input during the block rather than the final sensory input (which should not differ for the two postures). In the second study subjects selected templates to indicate the perceived size of their hand. Perceived hand size increased by 34 ± 4% (mean ± 95% CI) during the block. Sensory changes were monitored. In all subjects, impairment of large-fibre cutaneous sensation began distally with von Frey thresholds increasing before cold detection thresholds (Aδ fibres) increased. Some C fibres subserving heat pain still conducted at the end of cuff inflation. These data suggest that changes in both perceived hand size and perceived position of the finger joints develop early when large-fibre cutaneous sensation is beginning to degrade. Hence it is unlikely that block of small-fibre afferents is critical for phantom formation in an ischaemic block.

Experimental orofacial pain and sensory deprivation lead to perceptual distortion of the face in healthy volunteers

“…It was not possible to prevent some movements during the experimental sessions although these, where attempted, were minimized by instructions to the participants. These inputs could indeed have an effect of the evoked perceptual distortions (Tsakiris et al 2006;Inui et al 2011).…”

Systematic changes in the perceived posture of the wrist and elbow during formation of a phantom hand and arm

“…While a phantom limb refers to a body part that is perceived when there is no sensory input from that body part (also see Gandevia et al 2006;Walsh et al 2010), the present study examining 'experimental' phantoms has used the term to mean a body part that was perceived when the large-diameter sensory input from that body part was completely gone, although some small-diameter sensory input may remain (see Inui et al 2011). Inui et al (2011) tracked changes in the perceived posture of the hand during ischemic anesthesia of the arm to determine whether the initial posture of the wrist and fingers before anesthesia systematically influenced the perceived posture of the hand. They found that if the wrist and fingers were extended fully before and during anesthesia, the final hand was perceived as bent at the wrist and fingers and vice versa.…”

“…Because a fully flexed or extended hand became perceived as an extended or flexed phantom hand sensation as the ischemic anesthesia progressed in our previous study (Inui et al 2011), we asked what would happen if the hand was held in the midposition before and during the anesthesia. Therefore, by holding the wrist in the fully extended, fully flexed, and midpositions before and during the anesthesia, the present study demonstrates that a fully flexed or extended position of the wrist is essential for systematic perceived changes in the posture of the wrist when anesthesia sets in.…”

The role of sensory signals in perception of the body

“…When afferent signals of a concealed upper limb are blocked by applying ischemic compression, the perceived posture of the wrist and fingers is changed 42,43) . The final perceived posture depends on the initial one: when the joints of the wrist and fingers are initially flexed, the joints appear to become extended; when the joints are initially extended, the joints appear to become flexed.…”