Predictive processing turns the traditional model of perception its head. Traditional neuroscience argues for the domination of forward flow of information and the visual cortex as detector of bottom up perception. It was a model of stimulus drivers and sensory inputs, accumulating structure in “lego-block fashion”.
In recent decades there is increasing scientific work on spontaneous neuronal activation. The brain is not just suddenly ‘turned on’. There is usually plenty of top-down influence (active prediction) in place even before a stimulus is presented.
According to Clark, information transfer in the brain consists of two streams: Bottom-up inputs are processed in the context of priors (beliefs/hypotheses) from layers higher up in the hierarchy.
The unpredicted parts of the input (errors) travel up the hierarchy, leading to the adjustment of subsequent predictions. As these two streams move through the brain they continually interface with each other. Each level receives the predictions from the level above it and the sense data from the level below it.
According to PP theory, we are not seeing the world as it is. We are seeing our predictions about the world and then shaped by the actual sensoring data.
To deal rapidly and fluently with an uncertain and noisy world, brains like ours have become masters of prediction – surfing the waves and noisy and ambiguous sensory stimulation by, in effect, trying to stay just ahead of them. A skilled surfer stays ‘in the pocket’: close to, yet just ahead of the place where the wave is breaking. This provides power and, when the wave breaks, it does not catch her. The brain’s task is not dissimilar. By constantly attempting to predict the incoming sensory signal we become able – in ways we shall soon explore in detail – to learn about the world around us and to engage that world in thought and action.
Andy Clark – Surfing Uncertainty