The Spark of Life: Electricity in the Human Body (31 page)

Sight is a collaboration between our eyes and brain, for sensory experience does not arise from our sense organs alone. Open your eyes and you see a three-dimensional technicoloured world, but what the retina actually detects is a colourless, distorted, upside-down image that it translates into a myriad electrical signals. Some processing of these signals takes place in the eye, and multiple messages are then transmitted via the optic nerve to the brain, where different regions act as relay and processing stations. Ultimately, the information ends up as electrical impulses in the neurones of your visual cortex, which sits at the back of your brain.

Here the electrical signals are pieced together to create images and meaning. Nerve cells that respond to the same type of visual signal are localized to the same area of the visual cortex. Specific neurones are assigned different tasks. Some nerve cells seem to be specialized for detecting movement, others fire only when a human face is detected and some, known as mirror neurones, fire both when an animal acts and when it observes the same action performed by another. Once the image is recognized, signals are sent to the amygdala, the emotional core of the brain, where its significance is evaluated. Is this a lover about to embrace you, or are you in fact about to be mugged? Is this the bus you have been waiting for? Or are you just looking at a beautiful landscape?

You must then decide if what you see requires action. This involves signals being sent to the prefrontal cortex, the executive region of the brain, where you decide, for example, whether it is worth sticking out your hand to signal the bus to stop. If so, then more signals pass to the motor cortex, which instructs the necessary muscles to move your arm. Those signals that came in from the eyes via the optic nerve thus result in a multitude of complex messages that whizz back and forth around the brain. Bear in mind that we have not yet even considered how such visual information is integrated with that coming from other senses to build up a complete sensory picture of the world, or how that picture may be laid down as memory.

Our vision, of course, cannot be trusted. We do not always see what we think we do, as many optical illusions testify and many artists have exploited. Seeing is not believing because of the way our brains process information. We constantly make predictions about the world – anticipating, for example, where a squash ball will bounce, so that we can move to meet it before it lands. Illusions happen when the models we construct inside our heads, against which we unconsciously measure sensory information, do not match reality. In the left panel below, we see a non-existent inverted triangle, as our brain unconsciously fills in the missing lines. In the central panel, perspective tells us that the railway lines are receding into the distance and thus we see the light bars as a different size – even when we are told they are identical. The right-hand panel is seen either as two faces or as a candlestick, but never both; clearly, the image does not change – it is the way in which the brain interprets it. As such illusions reveal, our perception of reality is a construct of both the brain and the sense organs.

This is particularly easy to demonstrate in the case of colour vision. White paper looks white even in yellow light because we are used to it being white. Further, as Patrick Heron’s wonderful paintings show us, we judge colours by the company they keep; the same yellow, for example, looks different to us when it sits next to different colours. Early painters used this phenomenon to create the illusion of a colour for which there was no pigment then available (such as mauve). We can even perceive colour that is not there: a black-and-white image appears in colour when rotated rapidly. Conversely, if blood flow to the visual cortex is restricted the world turns grey – as can happen to boxers with head injuries.

Similarly, blindness does not always result from injury to the eyes. It can also be caused by damage to the visual processing regions of the brain; for example, by a stroke. Remarkably, some people who can see nothing and believe they are blind are able to ‘guess’ correctly what is sitting on a table in front of them, or pick up the right object when asked to do so. Such ‘blindsight’ demonstrates that we can see things without being consciously aware of doing so. Thus there appear to be at least two pathways by which visual information is processed in the brain, one that is linked to conscious awareness and one that is not.

Pay Attention Now!

The brain constantly filters the information it receives. Consider. Only the centre of our visual field is actually in focus, yet we see the whole of it in sharp definition. This happens because our eyes are constantly moving, focusing on different parts of the visual field, and the brain pieces the bits together into a coherent picture. We are blissfully unaware of what is happening because the brain ignores any visual inputs during the time our eyes are moving. This explains why if you stare in a mirror you will not see that your own eyes are constantly flicking from side to side – but another person will do so. Likewise, we ‘tune out’ background conversations and hear only the person we are talking to – unless we hear our name spoken, at which point our attention suddenly shifts. Our ability to attend to the most important information and disregard the irrelevant is very valuable, but it can also fool us.

I vividly recall one evening when I and a bunch of other scientists were asked to watch a film of a ball game between two teams, one dressed in blue and one in red. This film is now familiar to many people, but at the time it was new to me. We were instructed to count the number of times each team touched the ball. I was mortified when at the end of the film the lecturer said the number was of no consequence, what he really wanted to know was how many of us had seen the gorilla. Gorilla?! I had seen nothing, but to my surprise four people claimed to have done so, and when the film was replayed it was obvious – a man dressed in a gorilla suit walked into the centre of the screen, thumped his chest several times, and then strode off. How could I have missed it? It was an impressive demonstration that by focusing my attention elsewhere, my brain had ignored other information it had received.

The Gift of Coloured Hearing

Imagine seeing sound and hearing colours; a common experience when taking certain psychedelic drugs perhaps, but some individuals have this gift without recourse to pharmacology. One of the most famous was physicist Richard Feynman, who wrote, ‘When I see equations, I see the letters in colours -– I don’t know why. As I’m talking, I see vague pictures of Bessel functions from Jahnke and Emde’s book, with light-tan j’s, slightly violet-bluish n’s, and dark brown x’s flying around. And I wonder what the hell it must look like to the students.’ Another synaesthete with the gift of ‘coloured hearing’ was Vladimir Nabokov, who painted a vivid picture of the colours of the alphabet, with the green letters including the ‘alder-leaf f, the unripe apple of p, and pistachio t’ and the blue group the ‘steely x, thundercloud z, and huckleberry k’. As is clear from these descriptions, the synaesthetic experience is specific to the individual and words and letters do not always take the same colour – x, for example, was seen as blue by Nabokov and dark brown by Feynman.

Nor is the phenomenon confined to coloured letters. The great jazz musician Duke Ellington fused timbre and colour, writing, ‘I hear a note by one of the fellows in the band and it’s one color. I hear the same note played by someone else and it’s a different color. When I hear sustained musical tones, I see just about the same colors that you do, but I see them in textures. If Harry Carney is playing, D is dark blue burlap. If Johnny Hodges is playing, G becomes light blue satin.’ And Liszt astonished the Weimar orchestra when he requested, ‘O please, gentlemen, a little bluer, if you please! This tone type requires it!’ Other individuals may taste words or musical keys. They have an extra dimension to experience which most of us lack.

In synaesthesia, the palette of the senses becomes mixed. This melding of the senses does not occur in the sense organs, but in the brain, although quite how and where it is achieved is unclear. Brain scans have shown that the region of the brain concerned with processing colour information (the fusiform gyrus) sits right next door to that involved in number processing, suggesting that cross-wiring may be the reason why Nabokov, Feynman and others like them see coloured numbers. Presumably something similar happens when other senses get so gloriously muddled.

Synaesthetes may not be aware of their unusual abilities for some time, as they do not expect that others see the world a different way. Only when they discover, for example, that their friends do not remember phone numbers by their colour do they find out. In Nabokov’s case it came up at the age of seven when he was building a tower of alphabet blocks and casually remarked to his mother that the colours of the letters were all wrong. They then discovered that she too saw tinted letters; synaesthesia, it turns out, often runs in families.

Migraine

I am no synaesthete, alas, but I too occasionally have unusual visual experiences. In my case, it generally starts with the world becoming blotchy, rather as if I were looking through a car window during a rain shower when the windscreen wipers are not working. At other times, I see brightly coloured stars or shimmering zigzags that march across my visual field screaming ‘Wow! Zap!’ like something out of a comic book or a Roy Lichtenstein painting. Much as I may enjoy this spectacular lightshow I fear its sequel, for it heralds the onset of a migraine. Soon I am feeling extremely nauseous, usually vomit, become acutely sensitive to light and then follows the accursed one-sided headache – so severe I feel unbelievably ill. The only thing to do is to hide away in a darkened room and wait it out.

I am not alone. Many others struggle with this most malign of headaches although not all experience the preceding aura. The remarkable colours and distorted visual perceptions have been described by, perhaps inspired, many writers and painters, among them Virginia Woolf and Lewis Carroll. But as Virginia Woolf once said, ‘English, which can express the thoughts of Hamlet and the tragedy of Lear, has no words for the shiver and the headache.’ It is an unaccountably nasty experience. Hildegard von Bingen’s paintings and descriptions of visions of intense points of light and extinguished stars suggest that she too was a migraine sufferer.

One explanation of the strange visual phenomena associated with a migraine aura is that they occur because the electrical activity of the visual cortex is stimulated and that as this wave of excitation spreads across the cortex it generates illusory colours and perceptions. But this idea, like the origin of the headache itself, remains controversial. What is clear is that some unlucky families have severe forms of migraine that are caused by mutations in ion channels, including both calcium and potassium channels, that lead to enhanced electrical activity. In some people this activity is so intense that it eventually damages the nerve cells themselves so that ultimately they may even be unable to walk. There is evidence that enhanced electrical activity is also at the root of the problem in more common types of migraine. It is far from pleasant when suffering from a horrendous headache to reflect that it may also be damaging your brain.

The Balance of Power

Modern science, as we have seen, can now provide us with a crude map of the brain. In broad terms we know which regions are involved in processing different types of information. We can eavesdrop on the living brain while it is performing various functions, and see which bits are activated or suppressed. But what happens at the levels of the nerve cells themselves? How are they wired together and how do they talk to one another? The crucial element in the giant jigsaw that is the brain are the synapses, where, in Cajal’s words, nerve cells exchange ‘protoplasmic kisses, the intercellular articulations, which appear to constitute the final ecstasy of an epic love story’.

For synapses are not confined to the nerve–muscle junction. They are also found between nerve cells and gland cells and, very importantly, between nerve cells and nerve cells. There are several hundred trillion synapses in the brain, and millions more in the spinal cord. Typically each brain neurone will make contact with several thousand other cells. It is this intricate tapestry of connections that enables the complex behaviours of higher animals, including you and I.

Some of these synapses are excitatory, with the action of the transmitter being to excite the next cell, stimulating it to fire an impulse. Others, however, are inhibitory and the transmitter they release turns off the next cell in the chain, damping down its activity. Most nerve cells receive multiple excitatory and inhibitory inputs and their output reflects a balance between the strengths of these competing signals. In such a system, timing is crucial. An inhibitory signal will be ineffective if it arrives after an excitatory one, and an excitatory signal may find its passage blocked if it arrives at the same time as an inhibitory one. Further complexity is added by the fact that synapses present on the pre-synaptic nerve terminals may prevent the release of a transmitter. Therefore it is no easy matter to predict the response of even a single cell in an electric circuit.