Alterations in perceived heaviness during digital anaesthesia

Area of Science:

• Neuroscience
• Human Physiology
• Sensory-Motor Control

Background:

• Previous research suggested altered motor commands or antagonist co-contraction explain sensory feedback changes during anesthesia.
• Gandevia & McCloskey (1977) and Marsden et al. (1979) proposed differing mechanisms for perceived weight changes with thumb anesthesia.

Purpose of the Study:

• To investigate the mechanisms underlying altered perceived heaviness of lifted weights during digital anesthesia.
• To differentiate between altered motor commands and antagonist co-contraction as causes of sensory feedback changes.

Main Methods:

• Digital nerve blocks to anesthetize digits and radial nerve blocks to paralyze extensors.
• Subjects lifted weights using thumb or middle finger flexion under various nerve block conditions.
• Co-contraction was manipulated by specific postures and simultaneous index finger contractions.

Main Results:

• Digital anesthesia of the thumb increased perceived weight during flexion, while extensor paralysis alone did not.
• Preventing extensor co-contraction did not alter the effect of digital anesthesia.
• Anesthesia of the middle finger also increased perceived weight during flexion when extensor co-contraction was impossible.

Conclusions:

• The perceived increase in weight with digital anesthesia is not due to antagonist co-contraction.
• Altered sensory feedback from the anesthetized digit is the primary cause of the increased sense of heaviness.
• Extensor co-contraction can influence perceived heaviness when unrelated to the primary task.