andy_clark's picture
Professor of Cognitive Philosophy, Department of Philosophy and Department of Informatics, University of Sussex, Brighton, UK; Author, Surfing Uncertainty: Prediction, Action, and the Embodied Mind
Prediction Error Minimization

This sounds dry and technical. But it may well be the key thing that brains do that enables us to experience a world populated by things and events that matter to us. If so, it is a major part of the solution to the mind-body problem itself. It is also a concept that can change how we feel about our own daily experience. Brains like ours, if recent waves of scientific work using this concept are on track, are fundamentally trying to minimize errors concerning their own predictions of the incoming sensory stream.

Consider something as commonplace as it is potentially extremely puzzling—the capacity of humans and many other animals to find specific absences salient. A repeated series of notes, followed by an omitted note, results in a distinctive experience—it is an experience that presents a world in which that very note is strikingly absent. How can a very specific absence make such a strong impression on the mind?

The best explanation is that the incoming sensory stream is processed relative to a set of predictions about what should be happening at our sensory peripheries right now. These, mostly unconscious, expectations prepare us to deal rapidly and efficiently with the stream of signals coming from the world. If the sensory signal is as expected, we can launch responses that we have already started to prepare. If it is not as expected, then a distinctive signal results: a so-called "prediction-error" signal. These signals, calculated in every area and at every level of neuronal processing, highlight what we got wrong, and invite the brain to try again. Brains like this are forever trying to guess the shape and evolution of the current sensory signal, using what we know about the world.

Human experience here involves a delicate combination of what the brain expects and what the current waves of sensory evidence suggest. Thus, in the case of the unexpectedly omitted sound, there is evidence that the brain briefly starts to respond as if the missing sound were present, before the absence of the expected sensory "evidence" generates a large prediction error. We thus faintly hallucinate the onset of the expected sound, before suddenly becoming strikingly aware of its absence. And what we thus become aware of is not just any old absence but the absence of that specific sound. This explains why our experiential world often seems full of real absences.

When things go wrong, attention to prediction and prediction error can be illuminating too. Schizophrenic individuals have been shown to rely less heavily on their own sensory predictions than neuro-typical folk. Schizophrenic subjects outperformed neuro-typical ones in tests that involved tracking a dot that unexpectedly changed direction, but were worse at dealing with predictable effects. Autistic individuals are also impaired in the use of "top-down" predictions, so that the sensory flow seems continually surprising and hard to manage. Placebo and nocebo effects are also grist for the mill. For predictions can look inwards too, allowing how we expect to feel to make a strong and perfectly real contribution to how we actually do feel.

By seeing experience as a construct that merges prediction and sensory evidence, we begin to see how minds like ours reveal a world of human-relevant stuff. For the patterns of sensory stimulation that we most strongly predict are the patterns that matter most to us as both as and as individuals. The world revealed to the predictive brain is a world permeated with human mattering.