We thank Drs. Bromage, Melzack, and Gentili for their interest in our work. Both letters express concern about the relatively short duration of observation in our study.
Our study differs from that of Gentili et al. 1and Bromage and Melzack 2in two respects. (1) We exclusively studied lower extremity phantom proprioception after subarachnoid block. The difference in local anesthetic onset between subarachnoid block in the 3- to 8-min range 3and major peripheral and epidural blocks in the 10- to 30-min range, 4depending on local anesthetic used, or the potential for variable or incomplete anesthetic blockade with these nonsubarachnoid blocks may account for the delayed phantom phenomena emergence that these authors observed in their studies. Our observations clearly show that a phantom sensation can develop within 10 min, especially in the originally flexed limb. (2) Our subjects were forced to limit their limb position perception to either bent or straight. Limiting the subject response to a two-category variable of bent or straight may have resulted in a greater and possibly earlier yield of phantom perceptions in our study. These yields are clearly documented for each time point. In addition, it should be pointed out that the study of Drs. Bromage and Melzack and the study of Dr. Gentili involved phantom observations of affected limbs that necessitated surgery. Our study evaluated normal, healthy limbs that were not being operated on. The confounding influence of pathophysiologic pain or injury on the incidence of phantom sensations has yet to be determined and may result in different phantom yields.
We were well-aware of the work by Moriyama et al. and Gentili et al. ; however, they were not cited because full manuscripts have not been published to date. In developing our study design, our protocol attempted to balance time of onset of sensory and proprioceptive block with surgical time delay concerns. In addition, a surgical procedure scheduled for 45–60 min would not be able to tolerate plain lidocaine spinal anesthesia with a research delay longer than 12–15 min. However, neuroplastic phenomena do show change with time, as clearly demonstrated by numerous experimental works on hippocampal long-term potentiation. 5Proprioceptive phantom sensation as a manifestation of spinal cord neuroplasticity may exhibit time-dependent changes as well.
We do not claim a formal opposition between the existence of any classic body schema and the neuroplastic processes that we documented. Our observations suggest that a neuroplastic processes seems to override or dominate any existing default “position of rest” or “tetrapod” settings for the period of our observations. In this respect, a longer observation time using our protocol may yield more information either to confirm our initial observations or to reflect subsequent migration to any default settings observed at later times from the noted authors’ studies.