Conscious Experience and Brain Activity

extract from Rupert Sheldrake, The Science Delusion

“Many philosophers have speculated about the relationship between the mind and the brain, but the neuroscientist Benjamin Libet and his team in San Francisco investigated it experimentally by measuring changes in the brain and the timing of conscious experiences.

First, Libet’s group stimulated their human subjects either by flashes of light or by a rapid sequence of mild electric pulses applied to the back of the hand. If the stimulus was short, less than about half a second (500 milliseconds), the subjects were unconscious of it, even though the sensory cortex of their brains responded. But if the stimulus went on for more than 500 milliseconds, the subjects became consciously aware of it. So far, so good. The need for a minimum duration of stimulus is not in itself surprising. What is surprising is that the subjects’ conscious awareness of the stimulus began not after 500 milliseconds but when the stimulus started. In other words, it took half a second for the stimulus to be experienced subjectively, but this subjective experience moved backwards to when the stimulus was first applied. ‘There is an automatic subjective referral of the conscious experience backwards in time.. The sensory experience would be “antedated” from the actual delayed time at which the neuronal state becomes adequate to elicit it; and the experience would appear subjectively to occur with no significant delay’.

Second, Libet investigated what happened when people made free conscious choices. Eh measured the electrical activity of their brains by means of an electroencephalograph (EEG), with small electrodes placed on the surface of the head. The subjects sat quietly, and were asked to flex one of their fingers or push a button whenever they felt like doing so. They also noted when they decided or felt the wish to do so. This conscious decision occurred about 200 milliseconds before the finger movement. This seems straightforward – the choice preceded the action. What was remarkable was that electrical changes began in the brain about 300 milliseconds before any conscious decision was made. These changes were called the ‘readiness potential’.

Libet .. suggested that in the time between conscious awareness of the desire to act and the actual movement – a gap of 200 milliseconds – there was an opportunity for the person’s mind to veto the decision. Instead of free will, we have ‘free won’t’. This conscious decision depended on what Libet called a ‘conscious mental field’ (CMF), which emerged from brain activities but was not itself physically determined by them. The CMF acted on the activities of the brain, perhaps by influencing otherwise random or indeterminate events in the nerve cells. This field also helped integrate the activities of different parts of the brain and had the property of ‘referring back’ subjective experiences, and thus worked backwards in time.

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