Libet’s Mind Time

Benjamin Libet

In his introductory chapter, Libet admits that his views on subjective phenomena have altered somewhat since he was a young scientist. He says that he started with a full belief in determinsitic materialism, but has come round to the view that the subjective cannot be derived from neuronal function. He now instead highlights the distinction between being clever in the sense of solving complex problems and being conscious. In contrast to many 20th century philosophers and scientists, he stresses the importance of introspection. He thinks that any evidence that does not involve introspection cannot be taken as evidence of consciousness.

Large amounts of brain damage can occur without loss of consciousness, but on the other hand small amounts of damage to the brain stem or the intralaminar nuclei of the thalamus can cause loss of consciousness. Penfield, who performed many of the original experiments in the mapping of the brain, proposed that consciousness was located in these structures. However, Libet argues that although the brain stem is necessary for consciousness, it is not sufficient for consciouness as experieinced in large brains. For instance the functioning of the heart is necessary for consciousness in the brain, but it is certainly not sufficient for it. Penfield’s  experiments showed that while much of the cortex produced no conscious reponses, there were conscious reponses from the primary sensory somatic areas, the visual cortex and the temporal lobe. The other areas of the brain are assummed to mediate neural activity unconsciously.

Libet’s own experiments showed that the brain needed about half a second of activation before anything could come into consciousness. Nevertheless, subjects feel as if we are aware of a stimuli almost immediately. In Libet’s experiments, he placed electrodes on the somatosensory cortex, the area of the cortex that receives sensory input from the body. Sensory input from the skin ascends to the brain by several different spinal pathways. Repetitive pulses had to continue for about 0.5 seconds to come into consciousness, which is surprisingly long for any neural function. In the experiments there was no conscious sensation, if the train of pulses persisted for less than 0.5 seconds, even if a shorter pulse was of high intensity.

The 0.5 second delay for sensations to come into consciousness was nothing to do with the unusual process of directly stimulating the cortex. The nerves that carry sensory information from most of the body are carried up the spine and terminate on nuclei in the medulla oblongata in the lower part of the brain. From here nerve fibres run to the thalamus and from there to the somatosensory cortex.  This is known as the specific projection pathway.

Libet discusses the electrical response of the cortex to a single stimulus to the skin that is itself shorter than 0.5 seconds. A single pulse gives rise to a sequence of cortical electrical changes called evoked potentials (EP), known to represent nerve cell reponses. A primary EP arises in the relevant part of the somatosensory cortex within tens of milliseconds of the original skin stimulus. This can produce further neural activity lasting for more than 0.5 seconds, so that the short impulse to the skin can get into consciousness. These so-called late responses are necessary for achieving consciousness. Although normally involved with conscious processing, the EP is neither necessary nor sufficient to consciousness. It is shown to be inessential, because direct stimuli to the cortex by electrodes can produce conscious experience without producing an EP.

Libet discusses the question of ‘backward masking’ of stimuli, a phenomenon that was already known before his experiments. Experiments show that if a small pulse of light is followed by a stronger pulse within 100ms or more, the subject may not be aware of the first pulse. This can also apply to stimuli to the skin. However, if the delay is greater than 500ms, the subject will be aware of the first pulse. Libet argues that with the brain needing 500ms of activation for a stimuli to come into consciousness, another pulse during this period may interfere with the completion of the activation. Where the first stimulus was applied to the skin but the second stronger pulse directly to the cortex, the first stimulus was masked even for a delay of between 200 and 500ms. This experiment was seen as giving further support to the notion that 500ms of neural activation is needed for anything to come into consciousness. It was also found that on occassions a second impulse enhanced rather than masked the perception of the first impulse. This was seen as significant, as there had been some suggestions that it was the memory of the first impulse that was disrupted by the second impulse.

Libet stresses that it is possible to detect and respond to unconscious stimuli in a much shorter period of time than 500ms, and that it is only access to consciousness that requires such a long period. Therefore consciousness itself is argued to be a separate property from the mental events of detecting and responding. Libet does not think the access to consciousness is some sort of energetic build up, otherwise a single very strong pulse might go straight into consciousness. Rather the duration of activation is itself a code for access to consciousness.

Libet discusses the relationship between the 500ms access to consciousness and short-term or working memory. Dennett has suggested the 500ms is the time it takes to produce a short-term memory trace. There are two possible arguments here, either that the production of the memory trace is itself the code for consciousness, or that consciousness is not significantly delayed, but the memory trace needed to report consciousness does not emerge for 500ms.

Libet, however, argues against both these possibilities. He points out that patients who have lost the ability to form new memories retain normal consciousness. There is also trace conditioning, where one stimulus is associated with another, but there is a gap between the two. Amnesiacs, who have damage in the hippocampal region, cannot acquire trace conditioning. However, these patients are conscious suggesting that the ability to lay down memories and access to consciousness are separate functions.

Libet discusses a debate he had with Dennett at a conference in London. Dennett argued that the consciousness delay did not refer to consciousness, but the time needed to lay down a memory and then report it. Libet argues that delayed input of a second stimulus interfers with the consciousness of the first input suggesting that it is not merely a matter of memory recall, but Dennett suggested that the second input disrupted the laying down of memory for the first. Libet now counters that if there is a third input, consciousness of the second stimulus does not emerge, but the first stimulus re-emerges, which would not be the case if its emergence into consciousness was dependent on a memory trace erased by the second stimulus.

This still leaves the question of what the primary evoked potential actually does given that it is shown to be neither necessary nor sufficient for consciousness. In practise, it appears to be a timing marker, allowing the actual entry into conscious to feel as if it happened at the time of the stimulus. If, as in the case of some stroke patients, EPs no longer occur in certain parts of the somatosensory cortex, then stimuli to the relevant parts of the body will be expereinced as taking 500ms to come into consciousness. The loss of the EP deletes the ability to refer consciousness backward in time to the actual time of the stimulus.

Further experiments by Libet showed that readiness potentials arise in the brain 550ms before a voluntary act, but awareness of the intention to make the conscious act appears only 150-200ms before. So the supposedly voluntary act is initiated before the subject becomes aware of the wish to do it.

This experiment looked at free volntary acts performed without time restriction. The prescribed action was a sudden flexing of the wrist. The subjects were asked not to pre-plan when they would flex their wrists, but some insisted on doing this, so Libet gives results for planned and pre-planned action. With the planned responses the RP was 800-1000ms before the act. With no pre-planning the RP was narrowed down to 550ms, although Libet suggests the activity in the brain could start before there is a recordable RP. The awareness of the wish to act averaged 200ms in both the planned and unplanned instances. This awareness was in turn found to precede actual muscle activation by 150-200ms. Thus the will to act appears 350ms after the unplanned RP emerges.

Libet proposes the idea of a conscious veto. He points out that vetoing of an urge to act is a common experience. Tests on planned acts showed that the veto could be achieved in the last 100-200ms.

Libet attempts to discuss whether more complicated acts than a wrist flexion are governed in the same way. He points out that RPs have been found to precede more complex acts, such as starting to speak or write. This rather evades the point, since the act of beginning itself is an essentially simple mechanical act, such as moving a key on a key board, as distinct from the deliberative or strategic process of, for instance, deciding to write an article.

Libet gets round really discussing this by splitting off the deliberation about an act from the actual final action, and arguing that one can deliberate indefinitely and not act. This doesn’t really answer the question of what happens when we deliberate and do act, and the drive behind this is associated with the deliberation rather than a trivial mechanical act. P In the latter part of his book, Libet goes on to propose ‘a conscious mental field theory.’ He puts the familiar question as to how subjective experience can arise from nerve cells in the brain. He admits that finding the correlates of consciousness does not solve the problem, of how physical structures produce subjective sensation.

Libet finds himself forced back on the emergent property concept, but admits that it is a property that it is not directly observable or measurable. He likens this emergence of consciousness to the emergence of mass etc. from the fundamental particles, but apparently misses the point that neurons and other brain structures are far from fundamental. However, he regards conscious subjective experience as a fundamental property in nature. The binding problem is also supposed to be accounted for by the proposed emergent mental sphere, with no single cell or group of cells likely to be the site of consciouness. Consciousness is claimed to be globally distributed but only some cells are involved.

Libet wants to view conscious subjective experience as a field. Such a field would provide communication with the cerebral cortex outside of the neural pathways. The field would mediate between the physical activities of nerve cells and the emergent subjective experience. It solves the binding problem. The field would have the ability to effect or alter some neuronal functions. The field would not be in any category of physical fields or describable by any existing physical theory. In fact, it is detectable only in terms of subjective experience, and would not be amenable to observation in the way that all physical fields are.

It is, however, suggested to be a testable feature of brain function, although the proposed test, which is to create conscious experience in isolated bits of brain tissue, does not make it clear how consciousness would be communicated to the experimenters, and is in any case debatable because of the observed lack of ‘self starting’ qualities in actual isolated biological tissue. P The theory seems to suffer from all the disadvantages of dualism, without the advantage of not have to justify itself in terms of complex brain mechanisms.



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