A guide to why your world is a hallucination
By Anil Ananthaswamy
What’s your brain doing when you process information? Could it be producing a “controlled online hallucination”?
Welcome to one of the more provocative-sounding explanations of how the brain works, outlined in a set of twenty six original papers, the 2nd part of a unique online compendium updating us on current thinking in neuroscience and the philosophy of mind.
In 2015, the MIND group founded by philosopher Thomas Metzinger of the Johannes Gutenberg University of Mainz, Germany, set up the Open MIND project to publish papers by leading researchers. Unusually, the papers were published in open access electronic formats, as an experiment in creating a cutting edge online resource – and it was free. The very first volume, spanning everything from the nature of consciousness to lucid dreaming, was a qualified success.
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The 2nd volume, Philosophy and Predictive Processing, concentrates entirely on the influential theory in its title, which argues that our brains are permanently making predictions about what’s out there (a flower, a tiger, a person) and these predictions are what we perceive.
Read more: You are hallucinating right now to make sense of the world
To make more accurate predictions, our brains modify their internal models of the world or force our figures to stir, so that the outward environment comes in line with predictions. This idea unifies perception, activity and cognition into a single framework.
Some of the titles of the papers are playful, and maybe a tad over-the-top: “How to entrain your evil demon”, “How to knit your own Markov blanket” or “Of Bayes and Bullets”. But despite the titles, the content is serious and heavy-going: it’s written by some well-known proponents of predictive processing, including Andy Clark, based at the University of Edinburgh, UK, and Jakob Hohwy, at Monash University, Australia.
Perception isn’t passive
Lay readers will do well to begin leisurely, with the introduction to the field by Metzinger and Wanja Wiese, also based at Johannes Gutenberg, before dipping their toes into the deeper waters. Most of us will not get beyond sampling the introductory paragraphs in each paper, but even doing so can provide a flavour of the ideas they contain.
One of the keys to predictive processing is that it sets out to challenge our intuitive feeling that our brains passively receive information (via our senses) and create perceptions of what is actually out there – the so-called bottom up treatment.
Instead, predictive processing argues that perception, act and cognition are the outcome of computations in the brain involving both bottom-up and top-down processing – in which prior skill about the world and our own cognitive and emotional state influence perception.
As Metzinger and Wiese point out in their introduction, the idea of top-down processing is not fresh, but “dominant theories of perception have for a long time marginalized” its role. The novel contribution of predictive processing, they write, is that it emphasises the importance of top-down processing and prior skill as a feature of perception, one which is present all the time – not only when sensory input is noisy or ambiguous.
Predicting sensations
In a nutshell, the brain builds models of the environment and the assets, which it uses to make hypotheses about the source of sensations. The hypothesis that is deemed most likely becomes a perception of outward reality. Of course, the prediction could be accurate or awry, and it is the brain’s job to correct for any errors – after making a mistake it can modify its models to account better for similar situations in the future.
But some models cannot be switched willy-nilly, for example, those of our internal organs. Our assets needs to remain in a narrow temperature range around 37°C, so predictive processing achieves such control by predicting that, say, the sensations on our skin should be in line with normal assets temperature. When the sensations deviate, the brain doesn’t switch its internal model, but rather coerces us to stir towards warmth or cold, so that the predictions fall in line with the required physiological state.
If you do get through enough of the papers to understand the computational principles behind predictive processing then, as Metzinger and Wiese put it, you get closer to understanding why “it is only a petite step towards describing processing in the brain as a managed online hallucination”.
Everything we perceive, including ourselves, are simulacrums of reality. The takeaway here is this wild thought: we are always hallucinating.
