Astonishing Experiments

Libet's demonstrations of a delay between decision and consciousness; the 'split brain' commisurotomy experiments of Sperry and others, and the remarkable phenomenon of blindsight: these experiments have such obvious implications for consciousness that they cannot be ignored here, even though the main concern of these pages is philosophical, rather than strictly scientific.

Pre-empted decisions home

Some of the most-discussed experiments in the field are those carried out by Benjamin Libet, which appear to demonstrate that supposedly conscious decisions are already settled before we become aware of them.

Libet asked his experimental subjects to move one hand at an arbitrary moment decided by them, and to report when they made the decision (they timed the decision by noticing the position of a dot circling a clock face).  At the same time the electrical activity of their brain was monitored. Now it had already been established by much earlier research that consciously-chosen actions are preceded by a pattern of activity known as a Readiness Potential (or RP).  The surprising result was that the reported time of each decision was consistently a short period (some tenths of a second)after the RP appeared. This seems to prove that the supposedly conscious decisions had actually been determined unconsciously beforehand. This seems to lend strong experimental support both to the idea that free will is an illusion (at most, it would seem, there is scope for a last-minute veto by the conscious mind - a possibility which has been much debated since) and to a form of epiphenomenalism

There are many different ways of challenging Libet's conclusions. We could still question whether RPs really have the significance attributed to them. We could question whether the unusual circumstances of the experiment, with subjects thinking in advance about making a decision, and then making one for no reason whatever, properly represent normal thought processes. We could take the view that the experiments involve at least two mental reporting processes, one to do with the occurence of the decision, one to do with the state of the clock, which makes any judgement of simultaneity highly problematic .  

Libet himself proposed in 1994 that the diverse neural events which contribute to consciousness are drawn together by a conscious mental field (CMF): a phenomenon which emerges from certain kinds of brain activity. The experiments he proposed to test this idea have not been carried out, and arguably it raises more problems than it solves. Libet's research, in any case, remains challenging and interesting.


Blandula If you ask me, the key point is that consciousness is not a simple, unitary phenomenon. There's a difference, for example, between just saying what you think, and carefully planning in advance the words you're going to say. But nobody would claim that just saying what you think wasn't a conscious activity. Libet's experiment requires two different levels of consciousness. First you have to make an unreflecting decision to move. Then you have to consciously think about having made the decision - second-order consciousness - in order to notice the time. It's not at all surprising that the conscious decision represented by the RP comes first, and the second-order thought about that decision a fraction of a second later. In no way does this show the decision wasn't conscious, or any of the rest of it. More generally, it's obvious that the simpler processes which together constitute conscious thought can't themselves be conscious, or we should be stuck with an infinite regress. There's nothing in that which ought to worry us.

A fuller account of Libet's experiments and theory.


Split brains home

It happens that the human brain comes in two well-defined halves. No-one seems to know why, though it has been argued that switching from symmetrical to asymmetrical functioning gave the human brain a sudden increase in power by cutting out duplication. But since consciousness is unitary, the dual structure of the brain raises some interesting problems. In Moby Dick , Herman Melville pointed out that since the eyes of whales are widely separated, they must either, unimaginably, perceive two entirely separate visual fields at once, or in effect, look at only one side at a time. If problems like that arise from having two separate eyes, what can we expect from two different centres of consciousness? Descartes chose the pineal gland as the centre of interaction between mind and body partly because it was one of the few brain structures which wasn't duplicated.

The bifurcation of the brain also means that, unlike the impossible experiments often considered by philosopers (gradually replacing neurons with silicon chips, for example), it is surgically perfectly feasible to cut the main connection between the two hemispheres of the cortex and, in effect, cut a living brain in half. This extraordinary possibility became reality when the operation was performed as a last-ditch attempt to relieve the symptoms of certain patients suffering severe epilepsy.

Perhaps the most important consequence of the operation was that it worked - it did stop epileptic activity propagating from one hemisphere to another, and in that respect improved the lives of the subjects. In other ways, it had no obvious effect: subjects were able to continue their normal lives without handicap or difficulty. However, a series of ingenious experiments carried out by Sperry and others showed that in many respects the two hemispheres were operating independently. The dominant hemisphere could report what it saw with the eye connected directly to it, or felt with 'its own' hand, but not what the other eye was seeing or the other hand was feeling. The other hemisphere, without the main language centres, could not report anything verbally but by pointing with 'its own' hand and other means it could indicate that it too had access only to one eye and one half of the body.

It is tempting to conclude that the operation had created two separate spheres of consciousness - put bluntly, two different people. It is reasonable to conclude that each hemisphere is capable on its own of supporting a person: the loss of either hemisphere, even the dominant one, causes severe damage but not, as Eccles puts it, annihilation.

However, some caution is required. All the experimental subjects were severely epileptic to begin with. The unusual effects appeared only under special experimental circumstances. The scientists involved were actively looking for this kind of effect, rather than noticing something which forced itself on their attention. Most important of all, the subjects themselves did not feel that they had been split into two people: they continued to lead a normal life without, for example, one leg trying to walk off in a different direction (though apparently a tendency to favour one side could sometimes be observed). The hypothesis that there were, in effect, two people in the subjects' heads is not really justified by the evidence: a more economical and perhaps more accurate description might be that they were normal people with the exception of an unusual mental deficit in the ability to report impressions from either side of the body.


Blindsight home
Damage to certain areas of the brain may leave a patient blind in one eye, or in one area of their visual field. Although they have normal vision elsewhere, such patients are unable to report what is going on in the 'blind spot' and say that they see nothing there. However, experiments have shown that when asked to guess the location of an object or a light in the blind patch, some of them perform well. Although they continue to say they cannot see the object in question, they consistently point in right direction. This phenomenon is known as 'blindsight'.

Blindsight appears to challenge the idea that we know what's going on in our minds and what motivates our behaviour, and it can therefore be seen as evidence in favour of some variety of epiphenomenalism. The experiments which reveal it can perhaps be seen as related to others which show that subliminal or unnoticed details can nevertheless influence our subsequent behaviour, as can post-hypnotic suggestions or information given only to one side of a 'split-brain' patient. A striking fact about all these cases is the patient's failure to be aware of what is going on. They may say they are acting at random, or they may invent reasons for their behaviour after the fact. This does suggest that we do less by way of controlling our behaviour than we think, and more by way of rationalising it.

Ramachandran has suggested that blindsight arises from the fact that there are two pathways from the eye into the brain - one to the cortex, assumed to be the home of consciousness, one to the 'older' instinctive parts. It's conceivable that blindsight represents survival of the 'old' pathway after destruction of the 'new' one to conscious functions. But this solution would be specific to vision.

A more general explanation might come from the idea that perception involves distributed activity in several different parts of the brain. Suppose, let's say, that seeing object A over there involves the activation of neurons (or neural groups) 1, 3, 7, and 9. Now suppose that the residual pathways of vision, the subliminal stimulus, the post-hypnotic suggestion, or whatever, are enough to activate 1 and 3, but not 7 or 9. As a result, there is no conscious experience of object A. But when it comes to pointing at random, or choosing what to do next, the fact that 1 and 3 are still 'reverberating' from the earlier stimulus predisposes us towards the choice of A over there, rather than say, B down there which would require activation of 2, 4, 6 and 8.