In the surface of the brain, there is an area called somatosensory cortex which contains a sort of mapping with the areas of the body. So stimulating a particular part of it, stimulates a particular part of the body.
Interestingly enough, if the motor cortex is stimulated, a part of the body moves and something similar happens when the visual cortex is stimulated.
Libet carried out electrical stimulations of different durations. When the stimulations were under half a second, patients felt nothing. But when they were over half a second, participants reported feeling as if someone was touching them. An answer for this might be that we need a minimum time of continuous electrical stimulation before we can become “conscious” of it. If this was true, it would seem to imply that awareness of sensory experiences comes after the electrical stimulation. In a more general view, this means that our awareness of sensory experience is lagging behind the real world. This makes us ask how useful is having a low response time in a faster moving world.
Decades later, Libet carried out some other experiments. He wanted to know what triggers the action when a person decides to flex their wrist. Participants were asked to flex their wrist 40 times at times of their own convenience.
He measured the time at which the flexion occurred (nicknamed “M” for “motor action”), the beginning of brain activity in the motor cortex (nicknamed “RP” for “readiness potential”) and the time at which participants decided to act (nicknamed “W” for “Will”).
He found out that:
a) The time at which the participants decided to act (‘W’) happened 200 milliseconds before the wrist flexion (M)
b) The beginning of brain activity (RP) happened 350 milliseconds before the participants decided to act (‘W’)
In other words, the brain activity started first followed by the participant’s decision to flex their wrist which was followed by the flexion of the wrist. The brain is a very capable organ that executes millions of operations in half a second so the lag is significant. The debate surrounding this study spins around the interpretation of this lag.
My own take is that this is evidence towards a deterministic and physical view of human behaviour and human will as opposed to a metaphysical free will-istic view of human behaviour and human will. Brain activity comes before “free willed decision” so the latter cannot be the cause of the wrist flexion. It might be the case that brain activity itself is not the prime cause but that case will have to be supported by evidence.