  The diversity of cognitive deficits and neuropathological processes
associated with dementias has encouraged divergence in pathophysiological
explanations of disease. Here, we review an alternative framework that
emphasises convergent critical features of pathophysiology, rather than the
loss of memory centres or language centres, or singular neurotransmitter
systems. Cognitive deficits are interpreted in the light of advances in
normative accounts of brain function, based on predictive coding in
hierarchical neural networks. The predicting coding rests on Bayesian
integration of beliefs and sensory evidence, with hierarchical predictions and
prediction errors, for memory, perception, speech and behaviour. We describe
how analogous impairments in predictive coding in parallel neurocognitive
systems can generate diverse clinical phenomena, in neurodegenerative
dementias. The review presents evidence from behavioural and neurophysiological
studies of perception, language, memory and decision-making. The re-formulation
of cognitive deficits in dementia in terms of predictive coding has several
advantages. It brings diverse clinical phenomena into a common framework, such
as linking cognitive and movement disorders; and it makes specific predictions
on cognitive physiology that support translational and experimental medicine
studies. The insights into complex human cognitive disorders from the
predictive coding model may therefore also inform future therapeutic
strategies.
