A microexpression is a tiny facial expression that lasts less than a quarter of a second. They often occur involuntarily, and can reveal emotions people are trying to hide, or may not even be aware of themselves.
Most people do not seem to perceive microexpressions in themselves or others. However, in the Diogenes Project, Paul Ekman found that these tiny movements often can expose lying, and that a very, very small percentage of those he studied had a preternatural knack for detecting them. He now claims that anyone can be trained to see such microexpressions relatively easily.
The neurotoxin, Botulinum toxin, commonly known as Botox, is used in the medical cosmetic treatment of dystonia (involuntary muscle contractions) but it is also helpful in concealing unconscious facial microexpression. Neurotoxins operate by interfering with the normal signalling between nerve cells, in the case of Botox by inhibiting the release of a neurotransmitter. There are many different kinds of nerves, but the botulinum toxin attaches to nerves that control muscles (peripheral motor neurons). The botulinum toxin prevents the nerve from sending its chemical signal to the muscle, so in turn preventing the muscle from contracting. In the brain the difference between a conscious "lie" and a conscious "truth" is a difference in neurological pathways. These different pathways connect to different motor neurons, hence the twitching eye of the liar. Applied with the correct precision, Botulinum toxin can be used to specifically clog the "lying" pathways without hampering the "truthful" pathways.