Are Our Brains Bayesian?

Significance MagazineIn a fascinating article published in Significance, author Robert Bain delves into the arguments for and against viewing human judgements and decisions in terms of Bayesian inference. We are grateful to Significance and the editor, Brian Tarran, for permission to publish the excerpt below. 


The human brain is made up of 90 billion neurons connected by more than 100 trillion synapses. It has been described as the most complicated thing in the world, but brain scientists say that is wrong: they think it is the most complicated thing in the known universe. Little wonder, then, that scientists have such trouble working out how our brain actually works. Not in a mechanical sense: we know, roughly speaking, how different areas of the brain control different aspects of our bodies and our emotions, and how these distinct regions interact. The questions that are more difficult to answer relate to the complex decision-making processes each of us experiences: how do we form beliefs, assess evidence, make judgments, and decide on a course of action?

Figuring that out would be a great achievement, in and of itself. But this has practical applications, too, not least for those artificial intelligence (AI) researchers who are looking to transpose the subtlety and adaptability of human thought from biological “wetware” to computing hardware.

In looking to replicate aspects of human cognition, AI researchers have made use of algorithms that learn from data through a process known as Bayesian inference. Bayesian inference is a method of updating beliefs in the light of new evidence, with the strength of those beliefs captured using probabilities. As such, it differs from frequentist inference, which focuses on how frequently we might expect to observe a given set of events under specific conditions.

In the field of AI, Bayesian inference has been found to be effective at helping machines approximate some human abilities, such as image recognition. But are there grounds for believing that this is how human thought processes work more generally? Do our beliefs, judgments, and decisions follow the rules of Bayesian inference?

Pros

For the clearest evidence of Bayesian reasoning in the brain, we must look past the high-level cognitive processes that govern how we think and assess evidence, and consider the unconscious processes that control perception and movement.

Professor Daniel Wolpert of the University of Cambridge’s neuroscience research centre believes we have our Bayesian brains to thank for allowing us to move our bodies gracefully and efficiently – by making reliable, quick-fire predictions about the result of every movement we make. Wolpert, who has conducted a number of studies on how people control their movements, believes that as we go through life our brains gather statistics for different movement tasks, and combine these in a Bayesian fashion with sensory data, together with estimates of the reliability of that data. “We really are Bayesian inference machines,” he says.

Other researchers have found indications of Bayesianism in higher-level cognition. A 2006 study by Tom Griffiths of the University of California, Berkeley, and Josh Tenenbaum of MIT asked people to make predictions of how long people would live, how much money films would make, and how long politicians would last in office. The only data they were given to work with was the running total so far: current age, money made so far, and years served in office to date. People’s predictions, the researchers found, were very close to those derived from Bayesian calculations.

Cons

Before we accept the Bayesian brain hypothesis wholeheartedly, there are a number of strong counter-arguments. For starters, it is fairly easy to come up with probability puzzles that should yield to Bayesian methods, but that regularly leave many people flummoxed. For instance, many people will tell you that if you toss a series of coins, getting all heads or all tails is less likely than getting, for instance, tails–tails–heads–tails–heads. It is not and Bayes’ theorem shows why: as the coin tosses are independent, there is no reason to expect one sequence is more likely than another.

“There’s considerable evidence that most people are dismally non-Bayesian when performing reasoning,” says Robert Matthews of Aston University, Birmingham, and author of Chancing It, about the challenges of probabilistic reasoning. “For example, people typically ignore base-rate effects and overlook the need to know both false positive and false negative rates when assessing predictive or diagnostic tests.”

Diagnostic test accuracy explained

How is it that a diagnostic test that claims to be 99% accurate can still give a wrong diagnosis 50% of the time? In testing for a rare condition, we scan 10 000 people. Only 1% (100 people) have the condition; 9900 do not. Of the 100 people who do have the disease, a 99% accurate test will detect 99 of the true cases, leaving one false negative. But a 99% accurate test will also produce false positives at the rate of 1%. So, of the 9900 people who do not have the condition, 1% (99 people) will be told erroneously that they do have it. The total number of positive tests is therefore 198, of which only half are genuine. Thus the probability that a positive test result from this “99% accurate” test is a true positive is only 50%.

Life’s hard problems

All in all, that is quite a bit of evidence in favour of the argument that our brains are non-Bayesian. But do not forget that we are dealing with the most complicated thing in the known universe, and these fascinating quirks and imperfections do not give a complete picture of how we think.

Eric Mandelbaum, a philosopher and cognitive scientist at the City University of New York’s Baruch College, says this kind of irrationality “is most striking because it arises against a backdrop of our extreme competence. For every heuristics-and-biases study that shows that we, for instance, cannot update base rates correctly, one can find instances where people do update correctly.”

So while our well-documented flaws may shed light on the limits of our capacity for probabilistic analysis, we should not write off the brain’s statistical abilities just yet. Perhaps what our failings really reveal is that life is full of really hard problems, which our brains must try and solve in a state of uncertainty and constant change, with scant information and no time.


We hope you enjoyed this excerpt. Go here to read the full article – free to access through August!


About the Author

Robert Bain

Robert Bain is a freelance journalist. He was previously editor of Lux magazine and deputy editor of Research magazine.

 

 

 


About the Magazine

Significance is published on behalf of the Royal Statistical Society and is a bimonthly magazine for anyone interested in statistics and the analysis and interpretation of data. Its aim is to communicate and demonstrate in an entertaining, thought-provoking and non-technical way the practical use of statistics in all walks of life and to show informatively and authoritatively how statistics benefit society.

Symposium on Martin & Le Corre, “Sensory Substitution Is Substitution”

TVSSS-PHONE-300x225If you haven’t gone over to The Brains Blog to view the Mind & Language symposium on Jean-Rémy Martin and François Le Corre’s “Sensory Substitution Is Substitution,” part of the journal’s April 2015 issue, you should. It’s an interesting perspective on what we normally think of as our other senses compensating for the lack of vision. Highly technical, this article and the subsequent symposium present the argument that the type of information processed by a SSD is metamodal and that the phenomenology is best described in terms of spatial phenomenology, only.

In addition, it also has been shown that they [the Vertical-Horizontal Illusion and the Ponzo Illusion] are equally and identically present in early blind people and that they usually depend on the same modulating factors as in vision (Gentaz and Hatwell, 2004).

If you’re interested in the entire article, you can view it on the Wiley Online Library, free through June 15th. Check out the symposium to get join the conversation on the topic.

The Atheist’s Guide to Reality

Source: Wikimedia Commons

Although it came out late last year, Alex Rosenberg’s book, The Atheist’s Guide to Reality: Enjoying Life Without Illusions hasn’t been getting the press it deserves. Indeed, the comparative attention lavished on Alain de Botton’s much less interesting Religion for Atheists seems downright unfair. Probably Rosenberg’s title is largely to blame. He has all but admitted choosing it as a marketing ploy. This was probably a mistake. The title does the book no justice, since one thing The Atheist’s Guide has relatively little to say about is atheism. This has led people like this Independent reviewer to focus on complaining that the book offers little to atheists (more sensitive to logical solecisms than de Botton, Rosenberg declines to offer them religion) while ignoring its real topic.

Continue reading “The Atheist’s Guide to Reality”

Art for Love’s Sake

Recent neurobiological research has shown that viewing art stimulates the brain in a way that mirrors the experience of romantic love. The study, conducted by Semir Zeki, Professor of Neuroaesthetics at University College London, scanned and mapped the brains of participants who had been asked to look at a variety of paintings from such artists as Botticelli, Turner, Monet and Cezanne. It was found that experiencing art releases into the orbito-frontal cortex of the brain a significant quantity of the neurotransmitter dopamine, a biochemical associated with love, happiness and sociability, as well as drug use and certain psychological disorders.

The result comes at an ideal time for the art world in Britain, which has felt itself to be targeted by the extensive cuts in public spending. The correlation between aesthetic experience and happiness gives extra leverage in justifying the arts according to standards of public interest, a justification which normally consists in pointing out the economic benefits of the revenue which art institutions can generate. Speaking to the Sunday Telegraph, Stephen Deuchar, director of the arts charity Art Fund, said:

I have always believed art matters profoundly so it is exciting to see some scientific evidence to support the view that life is enhanced by instantaneous contact with works of art

Professor Zeki’s work in neuroaesthetics also stands to be of high value to the philosophy of art. This latest link between art and love is just one of many discoveries made by Zeki which coincide almost seamlessly with what artists and theorist about art have said for centuries, perhaps even for thousands of years. Plato, in his dialogue The Symposium, recounts a speech in praise of Love (Eros) made by Socrates which describes a journey of ascent from sexual love, through aesthetic appreciation of the body, to a spiritual love of the soul, arriving finally at the contemplation of the Platonic Form of Beauty itself. Continue reading “Art for Love’s Sake”

Does Nirvana Have a Neural Signature?

What are we looking for when we look at neurological studies of spiritual figures?

We are used to thinking of Siddhartha Gautama as having spent his life seeking to attain wisdom and the release from unwanted thoughts and passions. We are less used to conceiving of Siddhartha as having been in pursuit of a specific brain state; and yet, we might be curious as to whether or not the mental state of nirvana is a physiological possibility for us, and if so, what characterises it? What was the Buddha’s brain doing at the moment of enlightenment; what does the brain of an advanced buddhist monk do whilst the monk is nearing what we might term a pre-nirvana state?

Despite the fact that East has met West so many times in the last few centuries that they must detest being continually reintroduced to one-another, NYU researcher Zoran Josipovic (as the BBC reports) is continuing, as he has been doing for the past decade, to use fmri in order to study the neurological states of Buddhist monks. Yet in the last few years, he has advanced a compelling hypothesis. The human mind, as we know from experience, vacillates between total or near-total involvement in the external world, a state where we experience little or no self-awareness, to states that are self-conscious and self-aware to the fact that we may have little recollection of what was happening around us. Josipovic refers to these alternate kinds of experience as intrinsic and extrinsic networks.

Buddhist monks, whilst meditating, display a kind of neural activity that few, if any, non-meditating subjects have ever demonstrated: dual, equal, and simultaneous activity in the intrinsic and extrinsic networks. Of course, the relationship between these networks is by no means one of two distinct, unrelated systems– they work together to provide our ordinary, everyday experience of the world (an experience that involves some side-commentary and some focus or attention on the external world). The interesting aspect of Josipovic’s results is that, during meditation, no one system is dominant, which leads Josipovic to conclude that the resulting first-person experience is permeated both by a feeling of total involvement and oneness with the environment, and also with a self-awareness of being so utterly involved. The apparent contradictoriness of the description appears to us only because we have rarely, if ever, occupied this median neurological state.

However, we might also wonder: do we tend to over-estimate the experiential exoticness of advanced states of meditation? Is it possible that the feelings of tranquility and oneness enjoyed by advanced meditators are not so experientially different from our own, but merely prolonged? Why do we expect Buddhist monks to feel and experience the tenets of their philosophy, rather than merely to affirm them and find comfort and resilience within them? Are we filling the heads of Buddhist monks with fictional, super-states of consciousness?

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Achieving Immortality.

HAL9000: A ready and easy way to become immortal.

I once read a quote by John Cottingham, a philosopher famous in the field, and perhaps out of the field, of philosophy, about philosophy and the meaning of life. To paraphrase, it went something like; “People are often drawn to philosophy to find answers to the big questions. If someone finds themselves reading philosophy hoping to find an answer to the question “What is the meaning of life?”, they will most likely be sorely disappointed.” I would have to agree.

It is this thought people should have in mind when reading about immortality and what philosophy may have to say about it. Philosophy does from time to time discuss the subject, though usually this discussion is about the morality of such a concept, rather than the ability to actually achieve it. Continue reading “Achieving Immortality.”

Mirror Neurons Keep Us Smoking

One might expect that in order for oneself to enjoy the pleasure and reward of smoking, one must be the person in question that is smoking a cigarette. Merely watching another person smoke keeps us as far away from the felt experience of nicotine as the distance between self and other. Necessarily, I cannot experience your experience (even if I choose to join you in a smoke).

Or up to a point. As The Journal of Neuroscience reports, researchers have discovered that (right-handed) smokers, when watching a (right-handed) smoker puff away in a film or television, undergo excitement in areas of the brain associated with pleasure and reward, in addition to those motor Continue reading “Mirror Neurons Keep Us Smoking”