Effects of visual information on perceived posture of an experimental phantom foot

Our previous studies showed that a fully extended finger, wrist, and elbow became a flexed phantom hand and arm with ischemic anesthesia, and vice versa (Inui et al. in J Physiol 589:5775-5784, 2011, Exp Brain Res 221:369-375, 2012a, Exp Brain Res 218:487-494, 2012b). It was anticipated that if the ankle and knee were fixed in full extension or flexion before and during ischemic anesthesia, the perceived positions would move in the opposite direction. The present study examined what happened when participants looked at their fixed foot and leg at the end of the anesthesia. Using the left ankle and knee, ten healthy participants demonstrated the perceived postures of the right joints during an ischemic block of the right thigh (40 min) and after they looked at the right joints at the end of the block. When the right ankle and knee were fully extended before and during the block, the final joints were perceived as flexed by all participants, and vice versa. Although there was no significant difference between joints for the magnitude of the perceived changes in flexion, the magnitude in the knee was larger than that in the ankle in extension. At the end of the experiment, when participants were allowed to see their foot, its perceived position reverted to that indicated by them earlier, during the first 25 min of cuff inflation. This new finding suggests that the position of limbs is coded by visual input more dominantly than by proprioceptive input in the brain.