In this Ted Talk Ramachandran sais that mirror neurons can make a person with an amputated arm feel sensation, is it true? And if so, why doesn't the brain feel a sensation by looking at someone else's arm when your arm is intact?
Mirror neurons exist and have been recorded from in primates and humans - these are cells that respond to the actions and inferred perceptions of others.
In papers by Ramachandran and others, they have shown that patients who have their arm anesthetized report experiencing a sensation of touch when they see someone else's arm touched:
Case, L. K., Abrams, R. A., & Ramachandran, V. S. (2010). Immediate interpersonal and intermanual referral of sensations following anesthetic block of one arm. Archives of neurology, 67(12), 1521-1523.
Case, L. K., Gosavi, R., & Ramachandran, V. S. (2013). Heightened motor and sensory (mirror-touch) referral induced by nerve block or topical anesthetic. Neuropsychologia, 51(10), 1823-1828.
These papers of course strongly suggest, but do not necessarily prove, that these phenomena occur due to mirror neurons.
Ramachandran's original research can be found at:
Ramachandran V. S. (2005). Plasticity and functional recovery in neurology. Clinical medicine (London, England), 5(4), 368–373. https://doi.org/10.7861/clinmedicine.5-4-368
A replication study has subsequently been performed:
Daniëlle Ezendam, Raoul M. Bongers & Michiel J. A. Jannink (2009) Systematic review of the effectiveness of mirror therapy in upper extremity function, Disability and Rehabilitation, 31:26, 2135-2149, https://doi.org/10.3109/09638280902887768
The conclusion of the replication study was:
The present review showed a trend that mirror therapy is effective in upper limb treatment of stroke patients and patients with CRPS, whereas the effectiveness in other patient groups has yet to be determined.
The replication study indicates that the mirroring effect is true at least in some situations.
The Ted lecture you refer to (time = 4:45) suggests that we do feel a sensation in our own arm when we sense that someone else may be feeling pain in their arm. Perhaps this is the basis of empathy?
Ramachandran conducted subsequent research:
Ramachandran, V. S., & Hirstein, W. (1997). Three laws of qualia: What neurology tells us about the biological functions of consciousness. Journal of Consciousness Studies, 4(5-6), 429-457.
This subsequent research indicates that if the area of the brain that contain the mirror neurons is damaged, then the individual may suffer a loss of empathy.
Conversely, bilateral damage to the amygdala may lead to a loss of emotion and empathy.