Along with Karl Friston, Clark has been on the forefront of the effort to understand the human mind via the concept of predictive processing. This is a pop-neuroscience book intended to detail this concept to the layman. He begins with levels of processing, “However complex or high-level the predictions, it is prediction errors that must then carry the news, signaling differences from the expected and thereby keeping us in touch with a changing and sometimes surprising world…. Human brains seem to benefit from intelligent prediction strategies of just that kind, and they do so in an especially powerful way, thanks to the use of multiple “levels of processing.” In these multilevel contexts, simple predictions are nested under less simple, more abstract ones…. In this kind of multilevel arrangement all that flows forward (from the sensory edges ever deeper into the brain) is news—deviations from what is expected. This is efficient. Valuable bandwidth is not used sending well-predicted stuff upward…. Systems like that are wonderfully frugal in their use of the incoming stream of information. Instead of trying to deal with everything from scratch they effectively sift and filter the incoming data by highlighting only what was unexpected. This is the nugget of truth in the notion that human brains hallucinate reality. It means that the world we experience is to some degree the world we predict. Perception itself, far from being a simple window onto the world, is permeated from the get-go by our own predictions and expectations. It is permeated not simply in the sense that our own ideas and biases impact how we later judge things to be, but in some deeper, more primal, sense…. What we might think of as simple or “raw” sensory evidence is itself never experienced. Instead, experience always and everywhere reflects those rich webs of prior knowledge and here-and-now expectation…. Since all human experience is constructed from mixtures of expectation, attention, and sensory stimulation, it will never be possible to experience either the world or your own body “as it really is.” Indeed, it rapidly becomes unclear what this could even mean.”
Action is just as important as perception in the framework of predictive processing. “Ordinary daily actions (according to predictive processing) are caused by predictions of bodily sensation. They are caused, more precisely, by predictions of the flow of bodily sensations that would occur if that very action were to be performed…. Successful action involves a kind of self-fulfilling prophecy…. By making prediction the common root of both perception and action, predictive processing (active inference) reveals a hidden unity in the workings of the mind. Action and perception form a single whole, jointly orchestrated by the drive to eliminate errors in prediction…. Actions come about because we mentally represent the completed effects of the action…. This is sometimes said to reverse a commonsense notion of causality, since instead of the action causing the effect, it is the representation of the effect (the completed action) that causes the action itself to unfold…. This became known as the “ideomotor theory of action,” since the idea (or mental image) of the completed motor action is what brings the actual movements about…. By launching a cascade of sensory predictions, and then rendering them true by means of action (thus eliminating the resulting prediction errors), the brain creates the desired movements…. The deep unity (under predictive processing) of perception and action should now be apparent. There are two different, but equally effective, ways to minimize prediction errors during our encounters with the world. The first is by using prediction errors to help us discover the best guess about how things are out there in the world. But the second is to act so as to make the world fit some of our predictions.”
Clark references Lisa Feldman Barrett’s work on the role of perception and interpreting the signals of the interoceptive system in creating emotions. “The central idea is that a single kind of process combines inner and outer sources of information, generating a context-reflecting amalgam that is experienced as emotion…. The very same bodily information can thus feel very different according to how we represent the larger context in which the bodily signals arise…. According to interoceptive predictive processing, feelings and emotions are what result when we integrate basic information about bodily states and general arousal with higher-level predictions of their most probable causes…. Emotions and feelings reflect a process that combines interoceptive (inward-looking), proprioceptive (action-guiding), and exteroceptive (outward-looking) information with model-based predictions of all those signals as they are occurring…. The winning predictions will be the ones that best “make sense” of that large and varied body of information.”
Precision-weighting is integral to the system of predictive processing. “The most successful predictive model always sculpts the way the brain deals with the incoming signals. It alters the response of neurons at multiple cortical levels, amplifying and dampening them (courtesy of all that variable precision-weighting) in ways that reflect the brain’s best guess at the structure of the objects out there in the world…. Precision-weighted prediction (usually) serves our purposes by highlighting some things at the expense of others.”
Our personal histories shape our predictive models. Clark argues that perception is neither passive reception nor free fantasy, but a controlled hallucination continuously corrected by prediction error. “The most basic way that we actively construct our world is by selective sampling. We move our body and aim our gaze in ways that reflect what we expect to encounter…. Humans with different individual histories, will harvest different sets of stimulations from the very same world. But as we selectively harvest those simulations, our brains impose structure a second time, processing the sensory information in ways that amplify and dampen, extracting meaningful structure that itself reflects our own prior experience…. We cannot help but base our current waves of prediction on our own native tendencies and particular life histories. Where those predictions vary, so does human experience.”
Clark returns to the role of actions in shaping and creating our world models. Predictive processing thus becomes, for Clark, not merely a neuroscience theory but a general framework dissolving sharp distinctions between perception and action, mind and world, organism and environment. “Practical actions and epistemic actions are determined in exactly the same way, as the predictive brain makes counterfactual predictions about what kinds of futures will result if certain actions are launched. Actions are then chosen that deliver preferred outcomes directly (when possible) or else that probe and sample the environment to bring forth more information, reducing key uncertainties, and making the desired outcome more likely in the future…. Any predictive processing agent able to minimize error relative to future goals will discover both epistemic and practical actions, and how to mix them together. In all such scenarios, all the brain does is select the actions that best minimize future prediction error relative to goals.”
David Chalmers and Clark came up with the idea of the extended mind to describe the scaffolding outside the body that becomes enmeshed with the human mind, creating one seamless entity, “Extended minds arise because predictive brains are naturally expert at exploiting opportunities to use information-gathering action loops to help them achieve their goals…. Our constructed worlds can sometimes take over, transform, and augment functions once carried out by our brains…. I continue to believe that as the resulting weave between brain, body, and external resources tightens, it becomes less and less productive to think of mind as something locked neatly behind the barriers of skin and skull…. Brains are prediction machines that invoke external resources as easily (and for the same reasons) as they engage practical actions and activate different aspects of they dinner circuitry…. This creates a circular causal web in which mind is—at the very least—constantly porous to body and world.”
Finally, Clark returns to the importance of weighting in building world models, “Variable precision-weighting is the single most powerful tool in the predictive processing toolkit…. In the brain, precision-weighting alters patterns of post-synaptic influence (the strength of the signals passed on after the synapse “fires”). This means that specific signals can be selected for enhanced impact. The signals selected for this special treatment will be ones that are expected to be both reliable and important for the task at hand…. Precision variations of this kind underpin both conscious and automatic deployments of attention…. The brain learns how and when to vary its precision estimations as part and parcel of the process by which it acquires the generative models themselves. Those models and associated estimations of precision are learned by repeated exposure to flows of sensory information in the context of trying to act in the world.”