Authors: Frank Tallis
Penfield seemed to have worked a miracle formerly necessitating opium, hypnosis, or insanity – he had liberated the contents of the unconscious. Moreover, unlike his romantic and psychoanalytic predecessors, Penfield seemed to have discovered where the stuff of dreams collects – in the deep crevices of the temporal lobe.
Was this, then, the physical location of the unconscious? Had Penfield shocked the unconscious out of its hiding place? And, more importantly, had Penfield finally demolished Cartesian dualism – reducing the mind (in all its complexity), to an electrochemical jelly?
Clearly, the tacit agenda of Penfield’s work was reductionist in nature. He had assumed that all mental experiences must be attributable to electrical and chemical activity in the brain. Over many years, the evidence he had gathered together was entirely consistent with this view; however, in later life he expressed a curious ambivalence concerning the value of his work, and cautioned others against simplistic interpretations. Moreover, the year before he died, he insisted that neuronal activity in the brain could never account for such human qualities as freewill, and argued for a distinction between brain and mind. Although there were some minor differences between his position and that of Descartes, such differences were only in the detail. Bizarrely, Wilder Penfield died a card-carrying Cartesian dualist.
The idea of a conscious mind exercising freewill is so powerful, and so central to the concept of being human, that even a reductionist like Wilder Penfield struggled to preserve its sanctity. It seemed – and still does seem -highly improbable that something as fickle, fleet, and spontaneous as thought can be the result of purely physical processes in the brain. Newtonian physics is mercilessly deterministic. So how can freewill be reconciled with the electrochemical foundations of behaviour – which must necessarily operate within the limits of unyielding physical laws? In the absence of a non-physical consciousness, each current brain state must be determined by its preceding brain state. Yet we all feel that we have choices.
Irrespective of the philosophical implications of Penfield’s work, the surgical and exploratory procedures that Penfield developed represented a considerable technical advance. He had demonstrated that it was possible to manipulate the brain of a conscious patient, and thereafter any surgical procedure that involved exposing the brain was also recognised as a potential opportunity to extend and develop Penfield’s work. Yet it was an opportunity that remained largely unexploited until the late 1950s. It was then that an astonishing research programme was initiated, one that would have profound implications for neuroscience in general and constitute a major chapter in the history of the unconscious.
In 1958 Beitram Feinstein, a surgeon working in San Francisco, was developing a procedure devised to treat the tremors, tics, and spasms that resulted from certain types of brain disease. A small hole was made in the skull through which electrodes could be inserted. A relatively strong current could then be passed through the electrodes, thus destroying the abnormal cells which Feinstein believed were causing the involuntary movements.
Feinstein was a friend of Benjamin Libet, a physiologist based at the University of California School of Medicine. Libet was already studying brain chemistry, and was very interested in Feinstein’s work. Moreover, he was keen to experiment with the living brain, replicating and perhaps building on Penfield’s studies.
The surgical procedure that Feinstein employed was more refined than Penfield’s, which meant that Libet had restricted access to the cortex. To conduct his operations, Penfield had removed the top of the skull completely. Feinstein, on the other hand, only intended to make a coin-sized hole. Even so, Libet could stimulate the cortex immediately below this small entry point, and other cells situated below the surface could be stimulated by pushing the electrode down into the brain.
The hole that Feinstein made in the skulls of his patients was immediately above the sensory and motor cortex – that is the part of the cortex subserving touch and movement. By stimulating the sensory and motor cortex Libet was able to reproduce some of Penfield’s findings. Patients reported a range of tactile sensations, some of which were vivid and detailed – for example a drop of water trickling down the back of the hand; however, Libet also noticed something very interesting. Most patients took approximately half a second to report sensations. Moreover, if Libet began stimulating the cortex but stopped before a half second had elapsed the patient would report nothing. These observations suggested that it took half a second for activity in the brain to reach awareness. Put another way, every experience is preceded by half a second of preconscious processing.
Although we experience the internal (mental) world and the external (physical) world simultaneously, this must be by virtue of some neurological trick. If the brain is a kind of biological machine, then it will take time to perform any of its functions – including the construction of consciousness.
In the 1840s Helmholtz had demonstrated that nerve impulses travel very quickly, but measurably (for example leg nerves carry signals at about 200 miles per hour). In fact, human nerve impulses travel at different speeds, mediated by numerous biological factors. Interestingly, although human beings have a tendency to self-aggrandise by citing ‘the speed of thought’ as a kind of competitor to ‘the speed of light’, nerve impulses in the brain rarely exceed relatively modest velocities. Consequently it must take time to build a mental representation of the world, and even longer to respond to that representation. Libet had simply confirmed what Helmholtz and common sense already suggested. Nevertheless, he had also advanced knowledge by establishing a specific temporal parameter, describing the half-second lag separating cortical stimulation and the entry of a tactile sensation into awareness.
In order to achieve a better understanding of the half-second of preconscious processing preceding awareness, Libet began to experiment with
evoked response potential
(or ERP) recording. The brain functions electro-chemically, and for many years, scientists had recorded ‘brain waves’ by fixing electrodes to the scalp. This form of measurement is known as electroencephalography (or EEG), and produces a continuous record of wavy or jagged lines. ERP recording is a special form of EEG and was relatively new when Libet decided to use it in his programme of research.
ERP recording involves exposing an individual to the same stimulus hundreds of times. If all of the subsequent EEG lines are averaged, the result is a single brain wave. This curve represents a relatively pure response to the stimulus. Essentially, ERP recording is a way of getting rid of EEC signals that have nothing to do with the brain’s specific response to the stimulus,
Libet recorded directly from the brain, placing his electrodes on the cortex in order to get a particularly good signal. The stimulus he used to provoke a response was a mild shock administered to the back of the hand. The subsequent brain wave showed a very early response, that is, within 10-20 milliseconds. However, the curve continued to rise, showing the brain in the process of building awareness. Libet had actually plotted preconscious processing on a graph – a shallow arc, climbing towards consciousness.
Libet’s research had now raised some disconcerting philosophical issues. If consciousness arrives late (some half a second after related brain activity begins), how can human beings exercise free will? The decision to stand up, for example, must be preceded by a half second of brain activity that eventually results in that decision entering awareness. Libet’s results suggested that all behaviour must – to a greater or lesser extent – be initiated unconsciously. Once again, the shadowy presence of a hidden intelligence seemed to be impressing itself on the debate, mischievously shaking the precocious pens of Libet’s EEG machine.
By the 1970s ERP research conducted by the German neurophysiologist Hans Kornhuber and his assistant Lüder Deecke had demonstrated that voluntary actions were preceded by considerable electrical activity in the motor cortex, sometimes beginning up to a second before the action takes place. They called the subsequent ERP curve a
readiness poíenüal
(as it showed electrical changes reflecting the brain’s readiness to perform an action).
The discovery of the readiness potential revived Libet’s interest in the relationship between volition and preconscious processing. Subsequently, he set about devising a task that could determine the exact moment at which experimental subjects decided to perform a simple action. Subjects were placed in front of a screen and asked to observe a circling spot of light, the trajectory of which followed a ‘clock face’. After at least one revolution, subjects were instructed to move a finger (or hand), but in their own time. The decision was entirely theirs. When subjects felt the urge to move arise, they were asked to follow it through with an action, but to note the position of the spot of light (i.e. when they first experienced the urge to move). This allowed the onset of the urge to move to be established with millisecond accuracy.
The results of these experiments were consistent with Libet’s earlier work. Subjects reported that they first became aware of the urge to move approximately 200 milliseconds before the action was executed; however, inspection of their ERP recordings showed that the motor cortex was busy planning the finger and wrist movements earlier. Indeed, the ERP recordings showed the now familiar half-second delay between the onset of brain activity and performance of the related action. The unconscious mind (or brain, at least) was definitely pulling the strings.
Although Libet’s collected studies were now looking like a formidable and persuasive body of research, there were still those who were disinclined to accept that preconscious processing was taking place. They took the view that, at best, Libet’s work merely showed that no significant processing could take place in the brain in under half a second. Subsequently Libet sought to demonstrate that subtle processing effects could be detected in periods of less than half a second, and to accomplish this he returned again to his early method of direct stimulation. Conveniently, by the 1990s there were people walking around who had already had electrodes permanently embedded in their brains. It was no longer necessary to spend hours in an operating theatre to undertake the experiment that Libet had in mind.
All Libet’s subjects suffered from chronic pain. Electrodes had been embedded in their brains which, when activated, blocked pain signals. These therapeutic bursts of electricity were delivered by the patients themselves on an as-required basis from a power source worn on their belts. When Libet activated the electrodes for more than half a second, all of his subjects reported experiencing ‘something’; however, activations of less than half a second were not noticed at all. Then, in a specific time interval communicated by a light display, Libet asked his subjects to guess whether or not their electrodes had been activated – even though they were activated for less than half a second (and therefore apparently undetectable). Remarkably, Libet’s subjects guessed accurately about 65 per cent of the time, and some subjects guessed accurately when given as little as 150 milliseconds of electrical stimulation. This work suggested that the brain was capable of processing information in very short periods of time; if this was the case, it was very likely that the half-second delay separating the onset of an ERP and the execution of a behaviour was filled largely by preconscious processing. The brain was not just waiting for consciousness to arrive but was engaged – among other things – in the task of making consciousness.
Libet’s studies span over thirty years and, taken together, represent an extraordinary and deeply perplexing contribution to neuroscience. His results seem to undermine completely the view that human beings can exercise free will. It would seem that everything a human being decides to do (be it tapping a finger or contemplating the existence of God) is determined by brain activity that precedes awareness by about half a second. Moreover, because it takes at least half a second to become aware of stimuli, consciousness is constantly trailing ‘real time’. We are all living in the past.
This latter proposition has been the source of considerable debate. We have no sense of a half-second delay as we transact with reality. It seems implausible that our affairs are conducted outside of real time. To address this criticism, Libet suggested that the brain is capable of glossing over inconsistencies between real time and neurological time. An illusion is created in which the delay is simply edited out. Although we are unaware of preconscious editing, the brain does this all the time. Think, for example, of the blind spot – the oval-shaped area of the retina where the optic nerve joins the eyeball. It creates a six-degree island of blindness in the visual field, which is large enough to accommodate six full moons. This handicap is never problematic because the brain simply fills in the gap. Contemporary neuroscientists have suggested that the blind spot represents a fundamental example of an interpolative process, of which there are probably many, designed to create an illusory though practical correspondence between phenomenal and real worlds.
Another robust example of the brain’s penchant for interpolation is the
phi phenomenon.
Two alternating light bulbs will produce the illusion of a single spot of light bouncing back and forth. (This principle is exploited to produce rotating displays around theatre billboards.) If two different coloured lights are used, for example red and green, and the alternation occurs within 50 milliseconds, the bouncing light appears to change colour halfway across. In other words, the brain perceives a colour change (in this case from red to green), before the green bulb has been lit.
Libet’s suggestion that the brain is constantly glossing over discrepancies between real time and neurological time becomes considerably more plausible when placed in a broader context. It would seem that the brain is equipped with numerous Orwellian mechanisms, continuously engaged in the process of distorting and editing phenomena, such that human beings can comfortably occupy their existential niches.