Aping Mankind

Raymond Tallis

• Publisher: Routledge
• Year: 2016
• ISBN:
  9781317234623
• Edition: 1
• Topic: Philosophy
• Subtopic: Philosophy History & Theory

Neuroscience has made astounding progress in the understanding of the brain. What should we make of its claims to go beyond the brain and explain consciousness, behaviour and culture? Where should we draw the line? In this brilliant critique Raymond Tallis dismantles "Neuromania", arising out of the idea that we are reducible to our brains and "Darwinitis" according to which, since the brain is an evolved organ, we are entirely explicable within an evolutionary framework. With precision and acuity he argues that the belief that human beings can be understood in biological terms is a serious obstacle to clear thinking about what we are and what we might become. Neuromania and Darwinitis deny human uniqueness, minimise the differences between us and our nearest animal kin and offer a grotesquely simplified account of humanity. We are, argues Tallis, infinitely more interesting and complex than we appear in the mirror of biology. Combative, fearless and thought-provoking, Aping Mankind is an important book and one that scientists, cultural commentators and policy-makers cannot ignore.This Routledge Classics edition includes a new preface by the Author.

Chapter One
Science and Scientism

Neuroscience: The Queen of the (Natural) Sciences

There are two reasons for starting with a brief sketch of some of the central ideas of neuroscience. First, I want to make clear that what I am attacking is not science but scientism: the mistaken belief that the natural sciences (physics, chemistry, biology and their derivatives) can or will give a complete description and even explanation of everything, including human life. The body of knowledge and understanding, and the panoply of techniques, that go under the name “neuroscience” are some of the greatest intellectual achievements of mankind. Every element is a double triumph: over the opacity of nature; and over the presuppositions with which we approach our own bodies and those of the other living creatures whose bodies we use to cast light on our own. Neuroscience is the queen of the natural sciences. As someone who has contributed in a minor way to this discipline, adding my ant’s load to the ant-heap, with some 200 or more scientific papers that have filled in one or two lacunae, but have reported no spectacular breakthroughs, I have a very clear idea of the scale of this achievement.

Second, it will be difficult to follow the arguments against neuroscientism without an inkling of the fundamental concepts of neuroscience. What follows, addressed primarily to readers who are not familiar with neuroscience, is the barest outline of a few key notions, and certainly not a history of the subject, which is immensely complex. For a start, there are many neurosciences: neuroanatomy, neurochemistry, neuroendocrinology, neurogenetics, neuroimaging, neuroimmunology, neuropathology, neuropharmacology, neurophysics, neurophysiology, neuropsychiatry, neuropsychology, molecular neuroscience, various clinical neurosciences such as neurology and neurosurgery, and so on. In addition, these sciences pitch their investigations at many levels: examining the molecular architecture of nervous tissue; recording activity in single nerve cells; tracing various structures within the brain that are visible to the naked eye; examining the activity of large populations of neurons; seeing how the brain interacts with other systems in the body; and examining the behavioural neuroscience of whole organisms. What’s more, many other sciences are mobilized in the endeavour to cast light on the amazing organ that occupies the intracranial darkness and is wired to our senses, our muscles, our viscera and our glands. Neuroscientists draw on the expertise of physicists, chemists, biochemists, pharmacologists, immunologists and molecular biologists, to name only a few.¹ So what follows does no justice to the queen of the natural sciences: it simply provides enough to make the arguments comprehensible.

The nerve impulse

The obvious place to begin is with the atom of neural activity: the nerve impulse.² As we shall see below (“Why there can never be a brain science of consciousness” in Chapter 3), nerve impulses are not as conceptually straightforward as Neuromania would have us believe, but they are the key processes in the brain. A nerve impulse is a wave of physical and chemical excitation passing along a neuron, analogous to (although quite different from) an electric current going along a wire. The neurons are the micro-anatomical elements of the nervous system: their trunks (or “axons”) are often as little as a few thousandths of a millimetre in diameter. At any given point on the axon, the impulse, which usually occurs in response to an external stimulus, consists of a transient alteration in the electrical potential across the membrane of the neuron. When neurons are inactive they are negatively charged on the inside compared with the outside. When they get excited there is a change in that potential difference and that change propagates along the axon. Excitation consists of an influx of positively charged sodium ions across the membrane that constitutes the boundary of the axon, separating the fluid within it from the extracellular fluid outside. As a result, there is a reversal of the negative charge inside the neuron. This change is called “depolarization”. It is followed by a restoration of the resting state: “repolarization”. Repolarization is due to an efflux of positively charged potassium ions and resumption of active transport of sodium ions out of the axon. The cycle of depolarization followed by repolarization is called the “action potential”. In a typical neuron in the human brain, the cycle lasts about one millisecond at any given point in the membrane, and when it is displayed on an oscilloscope it looks like a spike. The passage of this spike along the axon – which we may think of as an electrical cable, although unlike a cable it generates the changes that propagate along it – is the nerve impulse. Spikes are kept distinct by a period, called the “refractory period”, in which the nerve membrane is effectively inactivated.

Unpacking the action potential depended, among other things, on finding species in which the individual neurons were large enough for recordings of minute changes in electrical charge – a few tens of millivolts – to be made. The squid axon, which is a gigantic half millimetre wide (compared with a few thousandths of a millimetre in most human neurons), proved to be the perfect model. Even so, it was still necessary to develop minute recording electrodes that would not kill the axons when they were inserted into them. For this purpose, fine glass electrodes, like microscopically thin pipettes, were manufactured. By this means it was possible not only to record the changes in electrical charge, but also to track the passage of sodium and other ions across the membrane.

Teasing out the ionic movements that caused the spike was only the first step. What was it that prompted the sodium ions to flood in at the beginning of the impulse and potassium ions to flood out at the end? The clue to that was found in the idea of “voltage-dependent gates” in the membrane. The gates – which are smart pores or minute holes in the membrane – are open or closed depending on the voltage difference between the inside and the outside. At rest, as I have already mentioned, there is usually a relative negative potential inside the axon because there the concentration of positively charged sodium ions is lower than in the fluid bathing it. The difference is maintained by the pumping out of sodium ions and the pumping in of potassium ions, in a ratio of 3 to 2. This “active transport” against the grain requires energy, which is provided by the transport of a phosphate group from adenosine triphosphate, or ATP.

Depolarization at any given point in the axon causes a reduction in the potential difference across the neighbouring part of the membrane. This opens the voltage- dependent sodium gates there: sodium floods into the axon, reducing the potential difference further so that the sodium gates are opened even wider. This self- regenerative process, like a controlled explosion, eventually causes the sodium gates to close, so that no more ions enter. It also opens the potassium gates and these (positively charged) ions leave the cell and the status quo is restored.

So there we have it: the nerve impulse is a wave or spike of electrochemical disturbance and recovery propagating along the axon. The beauty of the mechanism and the ingenuity of the research that unravelled it is breathtaking. In the 1970s, a couple of decades after Hodgkin and Huxley’s original brilliant work on the nerve impulse, the invention of the patch clamp method,³ which allowed sodium and potassium channels in the axon membrane to be examined individually, was made possible by the availability of ultrasensitive electronic amplifiers. The technique, in which tiny bits of the membrane were attached to ultra- thin pipettes, was used to examine the effect of molecules such as neurotransmitters on the behaviour of the membranes and hence neurons, as well as to peer more closely at the signalling inside the axon itself and to investigate the roles of “second messengers” in conveying and amplifying ionic and other changes that take place when neurotransmitters touch the surface of the axon.

This sketch leaves many unfilled gaps. For example, there has been work on the stereochemistry of the proteins involved in the transport of ions across the axon membrane. There has also been intense investigation of the properties of the insulating material around the axons, the myelin sheath, which dips down at intervals at the so- called “nodes of Ranvier”. This interrupted insulation allows nerve impulses to jump from point to point – called “saltatory” conduction – thus speeding up their passage along the axons. And there has been very detailed research into how nerve impulses are initiated at the beginning of the axon: how light, or sound, or touch, or pressure is translated into a “generator potential” that opens the sodium channels in the axon and in turn triggers the action potential. The manner in which the intensity and size of the stimulus is translated into the frequency of the firing of individual axons and the number of axons recruited has also been closely studied, as has the phenomenon of adaptation, in accordance with which a constant stimulus will invoke a diminishing response. What I have said, however, should be sufficient to make clear what “neural activity” is: and (to anticipate) enough to understand how, close up, it does not look like the kind of stuff that can explain human consciousness. Nerve impulses as revealed by neuroscience are, essentially, the passage of basic ions through smart membranes and that is about as physical as you can get.

The circuitry

Most of the work in brain science relevant to our present interest has focused on looking at how very large numbers of neurons work together and, centrally for the theme of this book, how different parts of the nervous system support different functions. Equally important is how the locations of these functions may vary over time, during the course of development towards adulthood and in association with the learning of new facts, the acquisition of the skills of movement, of perception and interpretation, and new ways of being. We need, therefore, to move on from the events that occur in the individual wires of the circuitry of the brain to the circuitry itself.

Beautifully detailed pictures, using electron microscopy, have been obtained of individual neurons, their cellular powerhouses and the multitudinous “dendrites” into which axons branch. One of the fundamental achievements of neuroscience was the establishment of the neuron doctrine, according to which neurons are discrete cells, not parts of a single fused network. Before the work of pioneers such as Ramón y Cajal, using silver staining techniques that picked out individual neurons, it was thought that the nervous system was a “reticulum”, or connected network. Cajal’s neuron doctrine has been modified in many ways, but it forms the basis of our current understanding of brain activity as being located and shaped in discrete circuits.⁴

Equally important is our understanding of the way nerve impulses pass from one neuron to another via joins that have been called “synapses”, from the Greek word meaning a “clasp”. These are not just blobs of dumb solder gluing neurons together, but complex way stations where activity may be added up, subtracted or modulated before it passes on to the next neuron. The heart of the synapse is typically a minute gap between neurons, which is crossed chemically. The impulses in one axon arrives at the “presynaptic” terminal and chemicals called “neurotransmitters” are released. They spread across the gap and influence the neuron on the far side. Some neurotransmitters are excitatory, facilitating neural activity, and some are inhibitory, damping down neural activity. This enables the synapse to add or subtract inputs from two or more neural pathways that converge on it. Summation and subtraction may be very complex indeed, with several sources of excitation or inhibition acting on either side of the synapse and coming from many different directions. The altered behaviour of synapses in response to the activity passing through them is thought to be the basis of the brain changes associated with learning and memory; and variable levels of neurotransmitters over large parts of the brain – in particular the cerebral cortex – have been linked with mood and behaviour. For those who believe that we are our brains, synapses – being the key to the endless wiring and rewiring that takes place in response to experience – determine what we become in response to experience. That is why there has been so much research into the distribution and effects of neurotransmitters.

The synapse, at any rate, is the means by which the discrete activity of neurons is brought together. It is the physical basis of what Charles Sherrington (1857–1952), perhaps the greatest neurophysiologist of all time, termed the “integrative action” of the nervous system,⁵ in virtue of which inputs from different sources, such as sight and sound and touch, and awareness of one’s own body and feedback from muscles engaged in movements, can all be taken account of in performing complex actions (and all actions, even simple ones, are complex). It is also a means by which an input can sensitize (“up- regulate”) or desensitize (“down-regulate”) the response to a stimulus when another is also being received or in response to the previous history of stimuli received by that particular part of the brain. Regulation of responsiveness has been the subject of much ingenious study, using recordings from individual pre-synaptic and post-synaptic neurons, as well as from groups of neurons or nerve tracts.

The way the different parts of the nervous system respond to different kinds of stimuli – so- called “localization” – has generated hundreds of thousands of papers. It is obvious that, for example, nerves wired into the ear are responsive to sound and those into the eyes to light, and so on. This was the “doctrine of specific energies” first advanced by Johannes Peter Müller in the nineteenth century. We have, however, moved on since then. Single- cell recordings over the past half century have shown that groups of neurons are tuned in unexpected ways to particular kinds of stimuli that are of importance to the organism. In the visual cortex, for example, there are neurons that preferentially detect lines placed at a particular angle or that respond to motion, depth and colour: in sh...

Table of contents

• Cover
• Title
• Copyright
• Dedication
• Contents
• PREFACE TO THE ROUTLEDGE CLASSICS EDITION
• ACKNOWLEDGEMENTS
• Introduction: The Strange Case of Professor Gray and Other Provocations
• 1 Science and Scientism
• 2 Consequences
• 3 Neuromania: A Castle Built on Sand
• 4 From Darwinism to Darwinitis
• 5 Bewitched by Language
• 6 The Sighted Watchmaker
• 7 Reaffirming our Humanity
• 8 Defending the Humanities
• 9 Back to the Drawing Board
• REFERENCES
• INDEX