Anaesthesia changes perceived finger width but not finger length

The brain needs information about the size of the body to control our interactions with the environment. No receptor signals this information directly; the brain must determine body size from multiple sensory inputs and then store this information. This process is poorly understood, but somatosensory information is thought to play a role. In particular, anaesthetising a body part has been reported to make it feel bigger. Here, we report the first study to measure whether changes in body size following anaesthesia are uniform across dimensions (e.g. width and length). We blocked the digital nerves of ten human subjects with a clinical dose of local anaesthetic (1 % lignocaine) and again in separate sessions with a weaker dose (0.25 % lignocaine) and a saline control. Subjects reported the perceived size of their index finger by selecting templates from a set that varied in size and aspect ratio. We also measured changes in sensory signals that might contribute to the anaesthetic-induced changes using quantitative sensory testing. Subjects perceived their finger to be up to 32 % wider during anaesthesia when compared to during a saline control condition. However, changes in perceived length of the finger were much smaller (<5 %). Previous studies have shown a change in perceived body size with anaesthesia, but have assumed that the aspect ratio is preserved. Our data show that this is not the case. We suggest that nonuniform changes in perceived body size might be due to the brain increasing the body’s perimeter to protect it from further injury.