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David Peterzell; The psychophysics of phantom sensations evoked by Ramachandran's mirror: Temporal dynamics and individual differences explored using the phantom pulse effect in normal (non-amputee) observers. Journal of Vision 2011;11(11):787. doi: 10.1167/11.11.787.
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© ARVO (1962-2015); The Authors (2016-present)
Ramachandran, Altschuler and others have shown, in well-known studies, that viewing a mirror reflection of one's moving intact limb can reduce phantom limb pain in single-limb amputees, and can evoke phantom sensations in non-amputee normals. Peterzell and colleagues have found that these effects can be amplified in some observers by using stroboscopic self-motion (“the phantom pulse”) (VSS 2006, 2007, 2010). In these studies, Ramachandran's simple mirror is modified by using a real-time video image of the observer that flickers between a normal mirror image and a mirror reversed image. When normal observers view themselves and their movements in this way, they sometimes report tingling, numbness, tickling, pressure, heat, cold, or involuntary movement in their invisible, non-moving limb. In the present study, the temporal tuning of the phantom pulse effect was studied (in 8 normal observers, who previously reported experiencing the phantom pulse) by measuring the estimated magnitude of phantom sensations at 0, 0.5, 1, 2, 4, 8 and 12 Hz. In all observers, optimum perceptual effects were found to occur at 0 to 2 Hz. Only two individuals experienced peaks at 0 Hz (no temporal modulation, akin to Ramachandran's simple mirror). Four individuals peaked at 1 Hz, with the remaining two peaking at 2 Hz. At 4 and 8 Hz, all subjects reported zero or near-zero magnitudes. However, all subjects reported above zero magnitudes at 12 Hz. We speculate that neurons with similar transient temporal properties contribute to these profound perceptual effects. The present results imply that that the neural mechanisms underlying Ramachandran's mirror-based interventions for phantom limb pain are temporally tuned. Future use of a temporally-modulated stimulus may enable researchers to examine physiological correlates of these effects using EEG and FMRI.
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