If you study one neuron, you're a neuroscientist; if you study two neurons, you're a psychologist. Not a side-splitter, that one, but a joke that nicely describes old-fashioned attitudes to brain science: that the only real science is the hard science, and speculation about human meaning should be left to theorists in the psychology department or dreamers in the humanities. Up until about 20 years ago, if a neurologist started tinkering with the problem of consciousness, his colleagues could put it down to the onset of "philosopause".

Since then, the decade of the brain has come and gone, Oliver Sacks has been canonised and no self-respecting neuroscientist can acquire much of a name without publishing a book about the neural correlates of empathy, God, the nonexistent self or the disappearing-coin trick. Reinforced by big authors in cognitive and evolutionary psychology, this has become the busiest market for popular science writing. But amid the frenzied neuronal buzzing, it is often hard to distinguish the brilliant from the banal, or actual neuroscientific developments from roaming speculations.

The most vaunted speculations of the year have come from David Eagleman's Incognito (Canongate £20), which seeks to persuade us that contemporary neuroscience has transformed our understanding of every aspect of individual and social existence, revealing conscious thought to be a mere fragment of our mental processing and the self as an illusion that does not actually exist on any map of neuroanatomy.

He presents this as good news, in the sense that the cosmic circuitry of the brain has been revealed to be even more astonishing than previously imagined. A more cautious reading, however, will reveal that this is neither good nor bad, since it is not news at all. Philosophically, Eagleman moves us no further on from David Hume's "bundle theory" of the self, merely adding aspects of neuro-functioning to a well-trodden philosophical problem. Eagleman relies largely on familiar case histories, written up in a style of conceptual generalisation which left me nostalgic for the clinical observations of Oliver Sacks or Paul Broks.

A more substantial and original neuroscientist, VS Ramachandran, provides the best introduction to the field in The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human (Heinemann £20), a lucid account of what we can actually know about neuroanatomy, turbocharged by enjoyable flights of speculation into what it all may mean.

Ramachandran does best what neuroscience does best, which is to reveal the workings of the mind through the malfunctions of the brain. It's good to be reminded that, though fMRI scans are remarkable, neurologists still learn more about mental functioning by spending time with patients. Particular types of brain damage point to corresponding mental deficits from which the normal functions performed by those regions may be deduced.

Where Ramachandran really wants to fly is in the area he has made his own: the discovery of mirror neurons and the wild speculation that they form the foundation of empathy and, thus, the origin of distinctively human mental abilities arising some 150,000 years ago. But by taking the simple metaphor of "mirror neurons" and transforming it into the more complex one of "empathy neurons", Ramachandran inevitably strays into the philosopause. What is empathy? Is it sufficiently cogent as a conceptual entity to be ascribed distinct neural correlates? Do we empathise with a dying Nazi in the same way as we do with a dying child? The mental calculations required to take the imaginative leap into empathy surely require not just one constellation of neurons, but the wider galaxy of the "social brain".

Which is where Simon Baron-Cohen locates his signature deficit. A world-leading expert on autism, Baron-Cohen has been crucial in distinguishing it as a condition caused by flaws in parts of the brain involved with social interaction. But in Zero Degrees of Empathy (Allen Lane £20), he claims to be "drilling down into the brain-basis of empathy". He seeks a medical-scientific description of evil, defined as an absence of empathy. As an exploration of the brain, the book is characteristically brilliant. As an analysis of the problem of evil, it suffers from acute deficits in the region of moral philosophy.

Scientific explanation without philosophical understanding is no explanation at all. An accessible philosophical account of the problem of the self  its existence or nonexistence  comes in the form of Julian Baggini's The Ego Trick: What Does it Mean to Be You? (Granta £14.99), but a really, enjoyably, angry rebuttal of the "neurotrash" of speculative brain science comes from Raymond Tallis in his Aping Mankind: Neuromania, Darwinitis and the Misrepresentation of Humanity (Acumen £25). It does not detract from the work of serious neuroscience to have some of its contemporary pretensions punctured by one of its own practitioners. Sometimes, Tallis goes too far, disregarding the sharp gleams of fascination in the field, but he is both philosopher and neuroscientist and this is a necessary corrective.

When it comes to putting two neurons together to make a psychological theory, Daniel Kahneman's Thinking, Fast and Slow (Allen Lane £25) is a classic of modern psychological science and the first self-help book written by a Nobel laureate in economics; but there are still some out there who haven't gone down either the cognitive or neuroscientific routes, keeping alive what we must now call "psychodynamic" ideas. It's a relief to discover that someone can still take Freudian psychodynamics seriously, as Darian Leader does in What is Madness? (Hamish Hamilton £20)  though that also means persisting with some silly ideas too. And Lisa Appignanesi writes with beautiful psychological insight in All About Love: Anatomy of an Unruly Emotion (Virago £20)  not, thankfully, about the neural correlates of love, but the phenomenon itself. We don't, after all, hold hands among neurons; instead, we embrace each other in the phenomenal realm, which is our reality, even if it doesn't physiologically exist.

David Eagleman is much possessed by ideas of salvation. Sum, his bestselling collection of short fables ("You will not read a more dazzling book this year" tweeted Stephen Fry) asked us, in the most rigorous and stark manner, to be careful what we wish for in the afterlife. His exclusive iPad app, Why the Net Matters, is concerned with saving us from the kind of Jared Diamond-style collapse that has befallen every previous civilisation (the net is a worldwide alerting system, a mass data collector, a hedge against the loss of knowledge in an Alexandrian library-type disaster, and a subverter of tyranny). Finally, in his professional and academic capacity as a research neuroscientist, he believes that the new knowledge of the brain can help solve one of our most intractable problems: how to turn bad people into good  how to rehabilitate criminals.

If salvation is his goal, his method in both Sum and his new book, Incognito, is to ask us to cast off our lazy, commonplace assumptions. In one, he delineates, with remorseless logic and clarity, what any conceivable afterlife would actually entail. In the other: you think your brain and senses reveal the world as it is? He will show you the tricks they play on us.

These tricks (including an ingenious dust-jacket) make up the bulk of Incognito. The brain offers a seamless impression of reality, but that is an illusion. Eagleman gives countless examples to demonstrate that vision is not a passive process  the eye and brain a camera  as it appears to us. We see with the brain, not the eye, and the brain generates expectations of what is out there, which are then modulated by the signals coming in. This explains how a batsman can hit a 90mph ball or how a fielder sprints to catch a skyer falling many yards away from him. The brain is able to compute trajectories based on past experience. These are quite complicated ballistics equations, but we have no conscious control or knowledge of them, since most of the brain's work is unconscious.

Eagleman demonstrates these perceptual processes with an array of deceptive figures, some already very familiar, such as the vase-and-two-heads illusion, and case histories. Perhaps the most stunning example of the brain's surprising powers is sensory substitution. All brain activity is of the same kind  there are no special neurons for vision, hearing, touch etc. This means that it has proved possible for blind people to see, in the sense of perceiving the layout of a room, catching a ball etc, by receiving electronic signals from a video camera relayed to parts of the body other than the eye: the back and the tongue have been used (the latter is especially suited to this). Over time, the patterns made by the waves of input, combined with other sensory knowledge from touch and sound, start to build a visual space. So we don't see with our eyes: we see because the brain can actively interpret the flood of data from light waves.

To turn from analysing the brain to making action urgent and its nature clear, Eagleman, an assistant professor of neuroscience at Baylor College of Medicine in Texas, is the founder and director of the college's initiative on neuroscience and law. He details many cases in which brain lesions, resulting either from injury or disease, have dramatically changed a person's behaviour, and asks what this implies for our notions of moral responsibility. His basic premise is that brain chemistry is more implicated in bad behaviour, including criminal behaviour, than most of us, or the law, currently accept. He speaks of a shift from blame to biology.

Some readers will be thinking by now that Eagleman must be a reductionist, downgrading human volition and free will in favour of genes, hormones and automatic neuronal responses. But Eagleman is not a reductionist at all, and he explains why very cogently. Here, the fabulist in him gets to work again. A Kalahari bushman stumbles across a radio in the desert. By twiddling the knobs he induces the radio to produce voices and music. He sets out to discover its secret, and in fact learns a great deal about electrical circuitry. But he will never understand the sounds in terms of the circuitry because he cannot deduce that the voices were first recorded and then transmitted as radio waves. The big picture is real and cannot simply be reduced to molecular events.

You will learn a great deal that is fascinating from Incognito, but if you've read Sum, what you will miss is the searing moral insight, the dramatic denouements. Despite Eagleman's passionate advocacy, neuroscience cannot deliver that kind of certainty. He asserts that we are "tied to our molecules and proteins and neurons", but recognises that we are not "best described only as pieces and parts". Thus far, Eagleman's adventures in the Two Cultures demonstrate the ancient literary technique of moral fable trumping the massed programmes of expensively wired-up research.

Peter Forbes's Dazzled and Deceived: Mimicry and Camouflage (Yale) has been awarded the 2011 Warwick prize for writing.

David Eagleman's previous book, Sum: Forty Tales from the Afterlives, was a delightful collection of short fables, each offering a wish-fulfillment image of life after death in which the wish turns out to contain its own perverse consequences. The fable principle was grounded in a nicely ironic psychology, subtly underpinned by Eagleman's own profession, neuroscience. Using fiction, Eagleman found a neat way of revealing how the mind cannot escape the contradictions of its underlying construction.

With this new book, Eagleman dispenses with fiction. This is a straight account of his own neuroscientific beliefs. Belief is the appropriate term, because Incognito isn't precisely an examination of neuroanatomy or neurological case histories; nor is it an exploration of the philosophical struggle involved in explaining the relationship between brain and mind. It is, rather, a breathless account of possible implications opened up by the rise of neuroscience as a way of looking at the world.

What are these implications? First, the process of learning more about the brain has changed our idea of what it means to be human. Man's sense of self has been rocked by key scientific revolutions in our understanding of the universe: the discovery that earth was not its centre, that time is deep not shallow, that humans were not God-created but a product of evolution. Brain science, Eagleman believes, provides the final frontier in our understanding of our own littleness and contingency: the realisation that consciousness is not the centre of the mind but a limited and ambivalent function in a vast cosmological circuitry of non-conscious neurological functions. Hence, most of our mental operations occur "incognito".

We should not worry about all this "decentering", Eagleman concludes, because science shows us that brain and mind and life are even more wondrous and exciting than we thought.

This interpretation of modern intellectual development is ahistorical and incorrect. As an enthusiast of Freudian models of the unconscious, it should be perfectly apparent to Eagleman that the decentering of the conscious mind took place long before the rise of contemporary neuroscience. We haven't needed fMRI scans, or software metaphors of brain circuitry, to tell us that we are subject to non-conscious drives that override our limited rational faculties. We got that much not only from Freud but from romantic poetry and 19th-century Russian novels.

Nor have we needed the finer developments of functional neuroanatomy to tell us that brain damage causes changes in behaviour, thus undermining simplistic notions of free will or criminal culpability. Eagleman canters through various well-known neurological cases, none original to this book, in which criminal acts or radical changes in personality have been shown to be the result of brain damage or disease. Appearing not to notice the glaring chronological anomaly, he cites the case of Phineas Gage, the American railroad foreman whose brain was violently punctured by an iron rod. Amazingly, Gage survived and could still function. But he was so drastically altered as a personality that colleagues could scarcely recognise him. The basic elements of the mind-brain problem have been chewed over in this case ever since it occurred  in 1848.

This book belongs to a popular trend of neuro-hubris  wildly overstating the ramifications of a science that is still in its infancy. The true fascination of neuroscience lies not in bombastic philosophical claims but in the fine detail of brain function, illustrations of the mind-brain problem, and the human interest of case histories. There isn't even that much actual neuroscience in Incognito. Its illustrations are drawn just as much from the annals of evolutionary psychology, behavioural economics and more traditional forms of psychology.

The contrast with Sum could not be more vivid. Eagleman is the rarest kind of science writer: better at translating his knowledge into fiction than explaining it as fact.